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See also Colorado Geology Overview, Colorado Rocks
Last modified 01/16/05
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Colorado in context |
A good understanding of Colorado's physical history and rocks requires a feel for large-scale geodynamics. This groundwork article attempts to provide just that context while supporting the photojournals and other groundwork articles that draw on such ideas.
Most importantly, this article seeks to provide a basis for appreciating if not answering the $64,000 geo-question about Colorado:
How do you get a long-enduring and still rising world-class mountain range in a deep intraplate setting subject for the last 1.4 Ga to at most the far-field effects of first-order plate interations?
As of 2004, no one even pretends to know the answer, but it sure is fun to think about. To many trained minds, anomalous upper mantle stirrings and enduring lithospheric weaknesses must enter the Colorado equation, but how and why remain as big a pair of mysteries as you'll find in geoscience.
Speaking of mysteries, you may wonder why marine processes receive so much attention in this article when the nearest true ocean is over 1,000 km away and has been for at least 1.4 Ga. There are two good reasons. First, much of Colorado's landscape came from the sea, however continental it may seem today. Its basement was largely cobbled together from a host of island arcs and backarc basins between 1.78 and 1.65 Ga, and marine deposits as young as ~70 Ma make up a good bit of its sedimentary cover. Together, these marine contributions account for much of the rock exposed in Colorado today. Second, subduction of ocean floor has driven the planet's surface dynamics since at least the onset of plate tectonics at ~2.0 Ga. Details of the subduction process and of ocean floor creation and anatomy become pertinent at many points in Colorado's story.
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Pangea, complete and during break-up |
Since its emergence in the early 1960s, the theory of plate tectonics has evolved rapidly. Today, it's a theory only in the sense that relativity and quantum mechanics are theories: The testable predictions having been thoroughly confirmed, no one seriously doubts the basic premises at this juncture, but significant challenges and shortfalls remain here and there. As a site of substantial and long-lived intraplate deformation, Colorado embodies more than its share of the remaining difficulties in plate tectonics, but the state's early evolution can only be understood within that framework.
Through the pioneering work of a small group of geophysicists and physical oceanographers, the fundamentals of the theory of plate tectonics
a lithosphere broken into strong, rigid moving plates that carry the continents on their backs
ocean basins that come and go at mid-ocean ridges and subduction zones, respectively
fell quickly into place in the 1960s. Once the fundamentals and their immediate implications were taken seriously, many important details of the theory quickly took shape. Among the founders were Harry Hess, Robert Dietz, J. Tuzo Wilson, Ron Mason, Fred Levine, Drummond Matthews, Lawrence Morley, Walter Pitman, Bill Menard, Bruce Bolt, Jack Oliver, Xavier Le Pinchon and Dan McKenzie, many of whom worked under essential facilitators like Maurice Ewing of Columbia's Lamont-Doherty Geological Observatory and Teddy Bullard of Cambridge.
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Spreading center and subduction zone, USGS |
To a large extent, the early work in plate tectonics was provoked by startling new geophysical observations coming out of
the U.S. Navy's newfound Cold War interest in the oceans, where the topographic and magnetic effects of plate interactions happen to be the clearest, and where some 80% of the planet's plate boundaries happen to reside, and
the first-ever world-wide seismographic network, sponsored by the U.S. Defense Department to monitor nuclear weapons tests but also quite capable of listening in on plate interactions and yielding important information on the structure and behavior of the plates and the mantle at depth.
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Magnetic stripes |
Once people started looking at the ocean floor in earnest, arresting patterns of topography, magnetism (right) and global seismicity quickly emerged. Never before seen or even imagined, these new data sets demanded explanations that theories of the earth based on fixed continents simply could not provide.
The earliest plate tectonic notions were easily summed up in grossly oversimplified cross-sectional cartoons showing plates spreading apart, subducting, colliding, and sliding past each other at homogeneous plate boundaries. The only possible driving force for plate motions anyone could think of at the time was simple pot-on-the-stove Rayleigh-Bénard convection within a homogeneous single-layer mantle.
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Northern Pacific floor with Hawaii-Emperor and many other volcanic chains |
Plate tectonics version 2.0 developed in the 1970s and 1980s with the recognition that
real plate boundaries vary considerably along strike (i.e., from point to point along their lengths)
mixed plate boundary types are common
plates aren't all that rigid after all
plate interactions should also explain, or at least jibe with, large continental features like mountain belts and long faults
intraplate deformations like the Rocky Mountains and Basin and Range provinces of the western United States and major midplate volcanic disturbances like the ^Hawaiian Islands had to fit in somehow
The simple 2D cross-sections of the early days were no longer adequate, and the role of the mantle had become at once more complex and less clear. Among the version 2.0 pioneers making lasting contributions were Tanya Atwater, Peter Molnar, Warren Hamilton and Bill Dickinson.
As attention turned to the continents and hot spots (excessive volcanic disturbances like Hawaii), the theory stretched to incorporate mantle processes not directly tied to plate motions. In 1971, W. J. Morgan introduced the idea of a narrow plume of heat rooted in the lower mantle to explain the long angled trace of the Hawaii-Emperor chain (above). Plumes were soon invoked to explain all kinds of hot spots, including midplate magmatism at Yellowstone, isolated ocean islands like the Galapagos, oceanic basaltic plateaus like Iceland and triple junctions like Afar. Suddenly plumes were everywhere, often in places that could be explained much more simply without them.
Note that version 1.0 and 2.0 ideas continue to dominate both professional and popular geologic accounts—particularly those in the displays and books offered at tourist attractions of geologic interest.
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Colorado in context |
By the late 1990s, geoscience found itself struggling to move beyond earlier formulations of plate tectonics, just as the founders of plate tectonics had struggled against the fixed-continent establishment of their day. Among the pioneers of plate tectonics version 3.0 are geophysicists Don Anderson and Warren Hamilton. This revision is still in progress.
It's been slow to die, but Morgan's plume model continues to lose credibility as ever more reliable and far-reaching geophysical observations tighten constraints on the workings of the plates and the mantle at depth. As the structure of the mantle and the importance of extensional stresses in locating hot spots became clearer, lower mantle convection, if it occurs at all, had to be decoupled from surface processes, and upper mantle convection had to be seen as a result far more often than a cause of plate interactions.
In plate tectonics, version 3.0, the lithosphere is viewed as a far-from-equilibrium open thermodynamic system of self-organizing semi-rigid plates acting to dissipate crust and mantle heat. The plates are weak in extension and permeable to melt from the underlying, barely solid asthenosphere. Top-down cooling and gravity are now seen as the exclusive driving forces behind plate motions. The largely passive upper mantle receives heat from the lower mantle only by conduction and cools primarily through seafloor spreading (60%) and subduction. Subduction-related hinge rollback and overriding plate extension have emerged as the fundamental shapers of the face of the earth. Hot spots are now seen as excessive volcanism focused by extensional plate failures or, in rare cases, by local and relatively shallow thermal disturbances related to upper mantle temperature variations induced by nearby plate motions.
This article presents the planetary structure and dynamics underpinning Colorado's physical evolution from a plate tectonics version 3.0 perspective. In this endeavor, I owe a great debt to Warren Hamilton, whose gift of an informal but incisive personal education in v.3 tectonics I'll always cherish.
The only thing better than a home planet full of beauty, power and fascination is one that's finally starting to make some sense.
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Colorado in context |
Since its initial Early Proterozoic assembly via subduction-related collisions at 1.78-1.65 Ga, Colorado has been far removed from active plate boundaries, but far-field effects from distant plate interactions to the west, south and east continued to play important roles in its evolution well into Tertiary time, both at the surface and at depth. Before launching into the planetary dynamics needed to put Colorado's physical evolution into a current plate tectonic framework, we'll need to spend some time getting to know the playing field and the players.
Having a hard time remembering the Cretaceous? To brush up on geologic time scales and nomenclature, try this deep time refresher. You might also want to keep a browser window open on the ^University of California (Berkeley) Museum of Paleontology Web Geological Time Machine. You'll find links to additional geologic timelines and glossaries on the search page.
The current planetary structure described below had quite literally fallen into place by ~4.4 Ga, but plate tectonics has operated on planet Earth only in the last ~2.0 Ga (from 13:20 hours on). Prior to that, a much hotter earth lacked the crustal and upper mantle strength to support discrete continents standing high above ocean basins, and very different surface processes applied. If you're interested in pre-plate details, take a side trip to the Earth before plate tectonics or a brief thermal history of the Earth.
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Structure of the Earth, courtesy USGS |
As every school kid and parent knows by now at some level, the earth (right) consists of
a very dense metallic core of iron-nickel alloy divided into inner solid and outer liquid shells, which together comprise about a third of the earth's mass and over half its 6,371 km mean radius
a solid but variably viscous rocky mantle composed of less dense high-pressure silicate minerals comprising over half the earth's mass and functionally divided into upper and lower mantles by a fundamental physicochemical barrier located globally at a depth of 660 km
a very thin (6-100 km) and complex rocky (silicate) crust at the surface, also divided functionally into brittle upper and ductile lower parts
a hydrosphere averaging ~5 km thick, including all surface waters but overwhelmingly dominated by the oceans, which contain 97% of the planet's unbound water
an atmosphere ~500 km thick
The layers have cooled, and the upper mantle and crust have stiffened up considerably since then, but this basic solid-earth structure has been in place since ~4.4 Ga.
This article focuses on the crust and the upper mantle, which together supply both the players and the playing field for plate tectonics. Via climate, the hydrosphere and atmosphere strongly influence surficial geologic processes like weathering, erosion, sediment transport, deposition and diagenesis, isostatic rebound and subsidence, but they're still passive players at the plate tectonic level. The lower mantle contributes only gravity and some heat to the process.
Largely isolated as it is from surface processes by the profound thermodynamic and compositional barrier at 660 km, we can safely ignore the workings of the lower mantle here. And since the core contributes little to our story beyond gravity and the geomagnetic field, we'll ignore it entirely.
You'll find a concise summary of ^Earth's physical attributes as a planet at Bill Arnett's ^The Nine Planets site.
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Compositional and mechanical layerings of the earth |
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In addition to the familiar compositional layering outlined above, it's essential to recognize an overlapping mechanical subdivision of the planet at depths above 660 km — into lithosphere, asthenosphere and remaining upper mantle. These overlapping boundaries make it hard to understand what "upper mantle" means in some contexts, but I'll try to be as clear about that as I can without adding too many extra words. Geoscience could really use a short recognized term for "remaining upper mantle", or what the table at right calls "lower upper mantle".
Since ~2.0 Ga, the crust and the cool, stiff layer of mantle immediately underlying it have moved together over the surface of the earth as a segmented but fairly coherent planetary shell called the lithosphere. By definition, the layer of mantle that tags along with the crust is the lithospheric mantle.
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Lithospheric plates with color-coded boundary types |
The plates of plate tectonics are nothing more than mobile lithospheric fragments — some large, some small, some mostly oceanic, some mostly continental, but all at least oceanic in part. The plates slide around on a slippery, barely-solid, ready-to-melt layer of upper mantle known as the asthenosphere that leaks out to the surface whenever it can. Like it or not, we're along for the ride
About 85% of the earth's surface can be understood in terms of physical interactions among plates and between plates and the asthenosphere. That is the fundamental lesson of plate tectonics. Colorado falls squarely in the remaining 15%, where other processes, perhaps of mantle origin, also seem to come into play.
The earth is a mighty heat engine constrained by its own gravity. To understand its workings, one must first sort out how the temperature, pressure and the density of its materials vary with depth.
Temperature and pressure increase steadily with depth throughout the earth but along different curves and more steeply at some levels and locations than at others.
The geothermal gradient, the curve describing the change of temperature with depth, reflects the cooling of the earth. It's fairly well known at most depths from well logs and geophysical observations, including the behavior of planet-crossing seismic waves. The global average is ~25°C/km, but the observed range is 5-90°C/km. At shallower depths, the geothermal gradient is flatter and varies considerably from place to place. For instance, at 10 km down, the temperature is 300°C under Los Angeles but only 150°C under Pittsburgh.
Pressure gradients in the earth are also fairly well worked out, at least at some depths. At all depths, the pressure is largely lithostatic—i.e., simply due to the combined weight of all overlying materials. In the crust and upper mantle, plate tectonic processes can vary pressure a bit around lithostatic values—not by much, but enough to make a difference, particularly vis-à-vis the asthenosphere's very tenuous hold on solidity. The unthinkable lithostatic pressures found at lower mantle depths severely suppress convection but foster chemical stratification.
For the most part, density also increases steadily with depth, particularly at the core. There is, however, one all-important exception:
Unlike continental lithosphere, oceanic lithosphere is nearly everywhere denser than the weak upper mantle supporting it.
This gravitational instability, not mantle convection, turns out to be the prime mover of plate tectonics, as we'll see.
The earth has been losing heat to space since it first formed but remains very hot internally, even after 4.5 Ga of cooling. Major impacts during the Hadean Eon (4.5-3.8 Ga, 00:00-03:40 hours) provided significant setbacks early on, but the planet's been cooling steadily ever since. The heat the earth now sheds into space comes from several non-renewable sources:
To put the importance of radiogenic heating into perspective, note that the heat flux at the surface is ~10 times that across the core-mantle boundary. Note further that radiogenic heat generation now is probably 2-3 times less than it was in the Archean Era (3.8-2.0 Ga) due to intervening radioactive decay.
Why bother with the cooling of the earth? Because processes in the mantle and crust have evolved both quantitatively and qualitatively as temperatures have fallen. Many high-temperature Archean processes have never occurred since, including the formation of continental cratons and their buoyant keels, the formation of domal granite-and-greenstone complexes, the intrusion of large anorthosite bodies, and the eruption of ultramafic komatiite lavas. These events required crust and upper mantle temperatures much higher than today's.
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Northern Pacific floor with Hawaii-Emperor and many other volcanic chains |
Since ~2.0 Ga, most of the heat escaping the earth does so through the oceans, primarily via seafloor spreading (60%) and subduction. These processes became possible only when the crust and uppermost mantle had become cool and stiff enough to support continents standing high above ocean basins, and the upper mantle had cooled enough to allow the gabbro-to-eclogite phase transition to occur at a shallow enough depth (now ~60 km) to foster subduction. Without this critical transition, which renders falling slabs of oceanic crust denser even than the upper mantle below the asthenosphere, gravity could not effectively drive subduction, and subduction could not effectively drive plate tectonics.
The lower mantle has probably changed little since its differentiation at ~4.4 Ga. Any residual temperature or density contrasts left in the lower mantle at that time were effectively frozen into place. It's circulation is very slow on time scales comparable to the current age of the earth, never mind the time frames within which surface processes evolve.
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Anti-Atlas Mtn folds |
Let's start our detailed dissection of the earth with its most tangible layer, the crust. The crust is the surface layer we inhabit and admire. It sits between the atmosphere or hydrosphere on one hand and the rarely exposed mantle on the other.
The accepted boundary between the crust and mantle is the Mohorovicic discontinuity, or Moho for short — a seismographic feature defined primarily by an abrupt increase in the seismic wave velocity (~7 km/sec above and ~8 km/sec below). Away from adornments like seamounts and submarine basaltic plateaus, the Moho on oceanic plates lies a fairly constant 5-6 km below the surface of the crust. On continental plates, the depth to the Moho is all over the lot, with a global mean of ~30 km and a 5-100 km range. On the continents, the Moho also coincides with related contrasts in consistency (stiffer below despite higher temperatures) and in the abundance of high-pressure minerals (more olivine, pyroxenes and garnet below).
On this planet, crust comes in four basic flavors— continental, oceanic, arc-generated, and collisional. Each type forms in a very different way, with resulting differences in composition, structure and mechanical properties.
Geologically, it's useful to divide the crust, whether continental or oceanic, into basement and cover layers. In most places, a stack of relatively thin, weak and easily-eroded sedimentary or volcanic layers (the cover) blankets a much thicker substrate of hard, resistant crystalline (igneous or metamorphic) rock (the basement) extending down to the brittle-ductile transition if not to the Moho. The basement is the foundation of the upper crust and the source of its mechanical strength. It's generally fairly homogeneous and isotropic, meaning that applied stresses find no particular directions or planes of weakness within it away from identifiable faults and shear zones.
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Red Canyon cover and basement |
The shape of Red Canyon (right) in the Colorado National Monument shows how very differently cover and basement tend to erode. Red Creek cut a wide and deep swath through the sedimentary cover (Wingate Sandstone) at Red Canyon. The meager notch in the distance is the best it could do in the hard 1.7 Ga Precambrian basement flooring the canyon.
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Painted Wall, Black Canyon |
Why even mention oceanic crust in a web site about Colorado geology? For starters, many of the exposed 1.7 Ga metamorphic gneisses and schists of the Colorado basement derive from oceanic crustal elements caught up in the collisional crust formed by the many ocean and backarc basin closures involved in the Early Proterozoic Colorado Orogeny. Some have been altered beyond recognition by heat and pressure attending deep burial during collisions and subsequent intrusions, but many retain signatures of their oceanic ancestry—as we see, for example, in the 1.7 Ga metasediments and metavolcanics exposed around the Mt. Evans batholith, in the east wall of Tenmile Canyon, and in Painted Wall of the Black Canyon of the Gunnison (right).
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Oceanic crust off Peru and Chile |
The plain crust flooring the abyssal plains of the oceans is the simplest case. This predictable, mostly igneous 4-layer package is usually ~6 km thick.
Structure of Simple Oceanic Crust |
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Relatively thin cover of marine sediments on top |
| 1 |
Submarine basalt erupted onto the surface before the sediments accumulated |
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Densely-packed vertical gabbro dikes with in deeper basalt |
| 3 | |
Layers 1-3 make up oceanic crust's stable basement, which rests in turn on lithospheric mantle. By mass, the gabbro predominates. Volcanic appendages like seamounts, submarine basaltic plateaus and aseismic ridges can thicken Layer 1 by several kilometers.
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Spreading center and subduction zone, USGS |
On average, oceanic crust is 3-10 times thinner than continental crust. A more mantle-like average mineralogy also makes it much denser (~3,000 vs. ~2,700 kg/m3). High magnesium and iron and low silicon and aluminum contents earn oceanic crust the compositional designation mafic, after the chemical symbols for magnesium (Ma) and iron (Fe). On the whole, oceanic crust much more mafic than the continental crust but less mafic than the ultramafic mantle.
The stable oceanic basement forms simultaneously with its underlying lithospheric mantle at seafloor spreading centers like the Juan de Fuca ridge at right. Together, these two layer make an oceanic plate or slab. The slab's mantle layer thickens over time as chilled asthenosphere freezes to its undersurface in a process called underplating, but the basement changes little until the plate subducts.
Sooner or later, all oceanic crust subducts. The oldest dated continental crust on earth contains ~4.2 Ga zircons, whereas the oldest crust on the ocean floor today is only ~180 Ma old. The average surface dwell time for oceanic crust is a mere ~100 Ma. Satisfactorily explaining these vast age differences was an early plate tectonic triumph.
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Marine limestones atop the high Andes in the Pachapaqui mining district |
Varying amounts of Layer 0-1 sediments and basalts get scraped into accretionary wedges during subduction, but much of the sediment goes down with the ship. Occasionally, more complete scraps of oceanic crust called ophiolites manage to escape subduction via subduction zone collisions, which can also elevate thick Layer 0 sedimentary packages like those found on outer continental shelves and slopes to great heights, as in the Andes at left. For the most part, however, subducted oceanic crust just falls back into the upper mantle along with its underlying lithospheric mantle. At a depth of ~60 km, the basement's basalt and gabbro undergo a phase change to eclogite, a metamorphic rock of even greater density. The density kick then accelerates the plate's fall like a sinker on a fishing line.
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Magnetic stripe generation, USGS |
The easily magnetized oceanic crust functions much like the magnetic coating on a recording tape. (In this analogy, the lithospheric mantle is the tape's plastic base.) Magnetic orientations frozen into cooling mafic basement minerals at the spreading center record the plate's time and place of birth relative to the prevailing geomagnetic field, which reverses at irregular intervals of ~0.5-10 Ma for poorly understood reasons. The field reversals create striking magnetic stripes in the oceanic crust, as diagrammed at right.
Geoscientists have made great use of the remanent magnetization of the ocean floor by cross-referencing its patterns against a hard-won timetable of geomagnetic field reversals. In fact, magnetic mapping of the sea floor first cracked open the plate tectonics revolution in earth science back in the early 1970s.
Freshly formed basalt-covered oceanic basement is bare. It acquires a sedimentary cover as it rafts away from its spreading center of origin. The sediments provide a variably detailed record of the travels and encounters of the plate on which they amass. In well-developed, sediment-starved deep ocean basins like the Atlantic, the sedimentary veneer rarely exceeds a few hundred meters in thickness. Uninterrupted accumulations of pelagic (deep ocean) sediments like radiolarian chert record time spent on the abyssal plain, far from land and well below the carbonate compensation depth, where sinking calcareous marine animal skeletons (those made of calcium carbonate) dissolve back into sea water, usually ~4 km down.
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Sumatra and its necklace of forearc ridge islands stand high above the Sunda trench to the southwest. |
Terrigenous (land-derived) sediments on oceanic crust tell of formation or passage near an eroding landmass. Continental shelves are thick sedimentary sequences piled for the most part onto faulted continental crust at the passive margin of a successful continental rift, but they sometimes advance over oceanic crust, as they do along the margins of the Red Sea. Volcaniclastic sediments resting on oceanic crust identify the nearby landmass as a magmatic arc. Large, mature oceanic arcs like Sumatra (right) can shed substantial shelf-like sedimentary aprons onto oceanic crust in both forearc and backarc settings.
Turbidity currents are the prime movers of terrigenous sediments onto oceanic crust. They're also the prime carvers of the submarine canyons found in river deltas, continental shelves and other sedimentary aprons. Turbidity currents form when coastal waters heavy with silt periodically roil down submarine canyons or along trenches to spread their loads onto the deep ocean floor in large volumes, sometimes over distances of hundreds of kilometers. For example, turbidity currents coming off the Ganges-Brahmaputra delta at the north end of the Bay of Bengal carry sediments along the Sunda trench as far south as Bali. The resulting distinctive sedimentary rocks, called turbidites, contribute heavily to accretionary wedges at subduction zones and also to continental slopes, particularly around the mouths of major rivers.
Hydrated low-density sialic (silicon- and aluminum-rich) crustal residues floating on a global ocean of dense hydrated mafic (magnesium- and iron-rich) magma probably developed shortly after the time of earth's formation at 4.5 Ga and may have persisted well into the Archean Eon. Buoyant patches of sialic crust eventually coalesced into a number of large, durable, unsinkable rafts that became the cratons (stable cores) of our modern continents — e.g., the Wyoming Province and the Canadian shield. Through fractionation and underplating of subjacent mantle, these early Archean cratons developed thick (>100 km) hulls and very deep (>250 km) keels of cool, strong, buoyant and refractory lithospheric mantle that to this day protect them from disruption by heat and currents in the underlying upper mantle.
Most of the planet's supply of sialic continental crust formed in the hot early Archean via high-temperature processes never repeated since. Much of that crust was recycled into the upper mantle prior to the advent of plate tectonics at ~2.0 Ga. The remaining Archean cratons have been tied up in continental cores ever since. Continental crust formed since 2.0 Ga, primarily at post-Archean collisional and convergent plate boundaries, has been much more mantle-like (mafic) than the surviving Archean cratons.
BTW, Colorado contains no cratonic crust to speak of, but adjacent Wyoming is a different story.
Relative to oceanic crust, the crust found in cratons is usually much lighter (~2,700 vs. 3,000+ kg/m3), much thicker (for the most part, 20-60 km vs. ~ 6 km) and much more complex with regard to both structure and composition. Continental crust is typically composed of crystalline igneous and metamorphic rock at depth and all manner of sedimentary, igneous and metamorphic rocks near the surface. On average, sialic continental crust contains much more silicon and aluminum and much less magnesium and iron than the mafic oceanic crust and the ultramafic mantle. Rocks of this composition are said to be felsic, because they contain abundant feldspars (K, Na, Ca, Ba, Rb, Sr and Fe aluminum-silicon oxides) and silica (Si02).
Mechanically, continental crust divides into a cooler, stronger, brittle upper crust and a hotter, weaker, ductile (putty-like) lower crust. An important thermodynamic and mechanical boundary known as the brittle-ductile transition (BDT) separates the upper and lower crust, usually 10-15 km below the surface. The terms middle crust and mid-crust usually refer to a horizon around the brittle-ductile transition.
The hot, plastic ductile lower crust flows, slowly of course, under the influence of gravity and regional stress fields, just as an incandescent but quite solid steel ingot flows to take on the shape of a I-beam under the force of a hydraulic roller press. The less compliant brittle upper crust folds, buckles and snaps when stressed beyond its limits but is nevertheless the main locus of mechanical strength in continental lithosphere. The upper crust generates most of the world's earthquakes, but the lower crust can get into the act now and then, too. Below the Moho, the ductile mantle is itself seismically silent. All subcrustal earthquakes originate in still-brittle slabs of oceanic crust falling through the upper mantle below subduction zones but above the "660", the effectively impenetrable physicochemical boundary between the upper and lower mantle. Claims of earthquakes (and therefore brittle behavior) below the "660" are based on highly suspect data.
Over most of the globe, the upper crust further divides into cover and basement layers, each with markedly different rock types and mechanical properties. The strength of the upper crust and to a large extent, that of the entire continental lithosphere, resides in the basement. The cover may have its own secondary BDT, but it's the primary basement BDT that separates the upper from the lower crust.
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Costa Rican arc |
We'll examine this important intermediate form of crust in detail when we discuss the magmatic arcs that accompany subduction below. Suffice it to say here that arc-generated crust is complex and highly variable but on average falls somewhere between mafic and felsic in composition. The Colorado basement includes a substantial portion of metamorphic arc-generated crust of Early Proterozoic age. At right is the Costa Rican magmatic arc built on the southwest margin of the Caribbean plate, where the Cocos and Nazca plates subduct along the dotted cyan line.
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Much of the western 3rd of the US has been accreted in plate-margin collisions just in the last 500 Ma |
Continents add mixed crustal materials at or near their margins through subduction zone collisions of all kinds. Over deep time, long-lived continental arcs can accrete substantial volumes of arc-generated crust of intermediate mafic-to-felsic composition, fragments of partially subducted mafic aseismic ridges, submarine basalt plateaus and ocean islands, slivers of mafic oceanic crust and occasional felsic continental fragments (e.g., ribbon continents) as they scrape or break these buoyant incoming structures off falling slabs. Between mid-Paleozoic and Late Jurassic time, North America grew by a third or more along its western margin through the accretion of many such far-flung exotic terranes, some of which traveled north 3,000 km or more to become California, Nevada and parts of Alaska and Utah.
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The Sea of Japan is a classic backarc basin separating the Japanese and Islands from the Asian mainland. The Sea of Okhotsk (between the Kuril Islands and the mainland), however, is more like a large continental graben. |
Lesser collisions at continental arcs many not result in terrane accretion, but they can still generate complex crust by crushing backarc basins and arcs together. Submarine basaltic plateaus and aseismic ridges can subduct successfully if not gracefully. Their buoyancy can generate enough compressive stress in the overriding plate to close and invert a backarc basin like the Sea of Japan (right) and can even mix the arc itself back into the resulting orogen. A new arc may then form outboard of or sometimes over the mess created by the collision. In fact, backarc basins can form and close repeatedly, accordion-like, along the same continental margin in a process known as tectonic switching. Backarc basins are important nurseries for collisional continental crust.
Continent-continent collisions may also entrap exotic terranes other oceanic elements as they close both ocean and backarc basins in complex time-progressive patterns along strike.
However they form, these mixed-bag continental add-ons, called mobile or orogenic belts, can produce extraordinarily thick, complex and puzzling crustal structures and juxtapositions overprinted by metamorphism varying widely in both degree and P/T regime. Things get even crazier when erosion and later deformations and magmatic events begin to tamper with the evidence. Between 1.78 and 1.65 Ga, the Colorado Province quickly grew into a large and very complex mobile belt through backarc mashing and the accretion of many primarily oceanic terranes. Nearly 1.7 Ga later, after long bouts of deep erosion and several major superimposed deformations and intrusions, most of its pieces are hopelessly scrambled into the basement, and some are missing or metamorphosed beyond recognition. Nevertheless, basement exposures like those in sections of the Black Canyon of the Gunnison occasionally preserve enough original structure and composition to give an idea of what went on.
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Modern Banda Sea |
To get an even better feel for what students of Colorado's Precambrian past are up against, take a look at the modern Banda Sea (right), which contains many oceanic elements and a few continental fragments as well. As Australia advances on Southeast Asia, the sea will close, probably in the next 10 Ma or so. Geologists studying the resulting suture 50 Ma from now will have a heck of a time unraveling all the pieces, even without further tectonism. I wish them luck.
Mobile (orogenic) belts are less stable than the cratons they encrust for a number of reasons. Structurally, they're usually on thinner lithosphere, even when they include thickened crust, because they lack the deep refractory keels that insulate cratons from potentially disruptive upper mantle heat and currents. As a natural consequence of their creation in collision zones, they tend to be hotter and more densely and thoroughly faulted, with more built-in zones of weakness of lithospheric scale. Their greater compositional diversity can include weaker materials, and on average, they're more mafic and therefore less buoyant relative to the asthenosphere. As a result, they're more likely than cratonic crust to pass from continent to continent in the supercontinent cycle, or to disappear down a continent-continent subduction zone, temporarily or permanently. That's why they're called mobile.
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Ocean floor ages; red ~ 0-10 Ma, purple ~ 160-180 Ma |
Nearly all of the planet's oceanic crust gets recycled through the upper mantle via the processes of subduction and seafloor spreading. Oceans as we know them have been around since at least ~2.0 Ga and probably in much shallower form since at least 3.5 Ga, but nowhere is today's oceanic crust older than ~180 Ma. With the exception of a few slivers (ophiolites) squeezed into mobile belts here and there, all older oceanic crust has been returned to the upper mantle by subduction. The average surface dwell time for oceanic crust is a mere ~100 Ma.
Continental, collisional and arc-generated crusts are a much different story. They're too buoyant to subduct in any significant quantity (try pushing a basketball underwater), so they're usually left behind at the surface at subduction zones, where they generate more collisional crust. They get passed back and forth among the continents du jour as the plates do their dance in the supercontinent cycle. Continental mobile belts are particularly susceptible to exchange among the seldom-rifted continental cores of ancient cratonic crust, but the cores change partners among themselves. The oldest exposed continental crust, part of such a core in southwest Australia, dates back to 4.2 Ga.
The continents slowly lose crustal mass "off the top" to minor subduction of the marine sediments they shed, but most of their eroded materials are simply redistributed and recycled into different continental crust via processes ranging from sediment transport and deposition to the formation of collisional crust to the supercontinent cycle. Total continental area has grown slowly since the inception of plate tectonics at ~2.0 Ga through the formation of mobile belts, primarily by way of arc magmatism and backarc spreading.
The earth's outermost shell, the lithosphere, is segmented into semi-rigid mobile plates that give the planet its facial expression. The plates are fairly rigid internally, particularly around the ancient continental cores known as cratons, but are variably deformable at their margins. At times, plates also deform far inboard, as the Rockies and the Colorado Plateau loudly testify.
In aggregate, the plates form a self-organizing, far-from-equilibrium dissipative system driven only by gravity and the top-down cooling of the earth, subject to spontaneous reorganization without notice on long and short time scales and on large and small distance scales. In other words, the plates behave chaotically in the mathematical sense of the term. The upper mantle moves primarily in response to the motions of the plates, not the other way around, as originally thought. Interestingly, there always seem to be about 12 roughly pentagonal major plates, with many minor plates filling in the gaps.
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Modern tectonic plates, USGS |
Plates (like the Pacific) that include subducting oceanic lithosphere are the freest to roam, and they contribute most to the changing face of the earth over time. Most plates lack subducting boundaries, however. Their wanderings are heavily restricted by the motions of adjacent plates. Together, the plates form a global circuit of mutually related motions that can get pretty complicated at times.
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Plates with color-coded boundary types |
You can watch the plates dance to the beat of gravity through time in the animations at the ^PALEOMAP Project. You can examine the plates and their current relationships in many fascinating ways at the Jules Verne Voyager, Jr. interactive mapping site, where I produced the map at right.
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Structure of the Earth, USGS |
Plates carry their crust on a relatively cool, stiff basal layer of outermost solid mantle called the lithospheric mantle, simply defined as the portion of the upper mantle that moves in lockstep with the crust. Together, the crust and lithospheric mantle make up the lithosphere. The table below may help you sort out these overlapping divisions of the earth.
The lithospheric mantle slides around on a very hot, barely solid, highly deformable upper mantle layer known as the asthenosphere, from the Greek for "weak shell". Extending down to a depth of ~200 km, the unstable asthenosphere is ready to melt at the slightest provocation. More often than not, the excuse is relief of lithostatic pressure (depressurization) secondary to extension within or between plates, as at spreading centers and subduction zones. The strong global concentration of volcanoes along plate boundaries is no accident.
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Compositional and mechanical layerings of the earth |
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Compared to the stiff, groaning crust, the lithospheric mantle is weak and seismically silent, and the asthenosphere is like so much grease under the skids.
Mechanically speaking, a plate's strength (i.e., its ability to resist deformation and fracture under tensile, compressional and bending stresses) resides in its stiff upper crust—the only portion brittle enough to generate earthquakes. The weaker, ductile lower crust and lithospheric mantle (slightly stiffer and denser than the lower crust but still weak relative to the upper crust) are supported by the hard shell of upper crust in exactly the same way that a crab's exoskeleton supports its soft internals. The hot asthenosphere is too plastic to support much of anything except by buoyancy, but since all continental lithosphere literally floats on the asthenosphere, that's no small contribution.
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Himalayan orogen |
Plates support large mountain ranges in 2 main ways—through flexural strength and through buoyancy. The diving board's flexural strength supports the diver at its tip as she gathers herself, eyes closed and motionless. In the same manner, the flexural strength of the thick upper crust of the partially subducted northern margin of the Indian plate supports the soaring height and immense weight of the Himalayas and the Tibetan Plateau (top center at right), both of which stand on the shattered and weak overriding southern margin of the Eurasian plate. Without the flexural support of the Indian plate, the Himalayas would sink and drown in their own sediments.
The majority of mountain ranges not associated with continent-continent collisions receive little flexural support from surrounding crust. Instead, such ranges support their locally thickened crust through buoyancy relative to denser rock below. Once you take to heart the viscous behavior of rocks below the upper crust, this is nothing more than a straightforward application of Archimedes principle. Isostasy is a fancy name for the obvious requirement that
Down to at least the asthenosphere, every square kilometer column of rock must weigh the same in the absence of external (e.g., flexural) support.
Otherwise, lithostatic pressure differences would force denser, more ductile rock below the upper crust to flow into underweight columns until the pressures equalized.
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Andean orogen |
Since earth materials are too weak and earth processes too slow to generate dynamic (tectonic) overpressures large enough to override isostasy, it follows that equally large roots of light-weight crust should underlie and buoy up all mountain ranges lacking flexural support. Most ranges not associated with continent-continent collisions have geophysically detectable roots within the crust—among them the Andes (right) and the Sierra Nevada but not the Rockies.
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Four Corners from space, NASA |
The Rockies seem to be a very peculiar case here. They now stand at great average elevation over 1,000 km from any plate margin without any crustal roots or flexural support to speak of, and have done so in the face of erosion for perhaps some 70 Ma—quite a long time as mountain ranges go. In fact, they're as high as ever and still rising!
What keeps them up then? The best bet is isostasy operating at upper mantle rather than at the usual crustal levels. A large low-velocity upper mantle seismic anomaly (the Aspen anomaly) underlying the central Rockies is widely interpreted as a zone of higher than usual temperature. What that means is far from clear, but if lower seismic velocities also mean lower densities, as they should at the depths involved, then the upper mantle may be providing deeper than usual buoyant support for the Rockies. Dynamic support by a regional upwelling of the upper mantle (an inertial "jet" effect) is unlikely. Intraplate disturbances have repeatedly rumpled this portion of the continent for at least 1.4 Ga, since the time of the Berthoud Orogeny. That kind of time frame also raises the question of a long-standing lower mantle influence. More at 11...
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Northern Pacific floor with Hawaii-Emperor and many other volcanic chains |
Whether continental or oceanic, plates have little tensile strength. In other words, they resist compression fairly well but pull apart easily. Most if not all large volcanic outbursts (hot spots) result, not from mantle outbursts and thermal anomalies, but from the failures of plates in tension. Indeed, the plate need not crack overtly; all that's required is a horizontal axis of least compressional stress. (This is known as the diking condition.) Once it gets a foot in the door, the buoyant force behind a depressurization melt does most of the work involved in breaching the plate. A single meter-wide dike can spew out tremendous volumes of lava in geologically short timeframes. In all likelihood, the intense and widespread pulse of magmatism that swept Colorado in the mid-Tertiary resulted, at least in part, from regional extension associated with the passing of the Rio Grande Rift.
As your read on, keep this firmly in mind:
The near-molten asthenosphere is barely contained by the lithosphere. It will leak out as melt wherever and whenever it can, with little provocation.
Spreading centers like the Mid-Atlantic Ridge and the East Pacific Rise; ocean islands like ^Iceland and the ^Emperor-Hawaiian chain; flood basalts like the Snake River Plain and the ^Deccan Traps of India; leaky faults like the San Andreas transform and the many normal faults of the Basin and Range; massive intraplate volcanic disturbances like ^Yellowstone; and vast oceanic plateaus like the Ontong Java all owe their existence to this fundamental fact of life on planet Earth. What comes out depends critically on how thoroughly the asthenosphere melts and on what the melt passes through to reach the surface, but basalt is the quintessential leakage lava, especially in oceanic settings.
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Structure of the Earth, USGS |
The lithospheric mantle and the asthenosphere are the uppermost layers of the upper mantle, which ends at depth at the "660"—the fundamental upper-lower mantle boundary found globally at 660±30 km. An obvious discontinuity in seismic probes of the mantle, the 660 appears to represent a solid-solid phase transition localized at 660 km by relatively fixed temperature and pressure gradients.
In the diagram at right, the 660 runs along the top of the yellow lower mantle. The more complex upper mantle consists of a basal layer (salmon), the mobile asthenosphere (orange) and the plate-bound lithospheric mantle (gray).
Due to a number of mutually-reinforcing physicochemical influences, material has a very hard time crossing the 660 in either direction, if it crosses at all. This barrier has profound implications for earth dynamics. For starters, it means that
All plate tectonic processes play out in the crust and the upper mantle above the 660.
Subducted oceanic lithosphere drops through the upper mantle and piles up along the 660 to await recycling. If the the lower mantle convects at all, it does so very slowly in isolation below the 660, passing heat to the upper mantle only through conduction. Observed and inferred currents in the upper mantle derive primarily from plate motions and from small-scale convection cells related to plate boundaries, not from currents in the lower mantle.
Thanks to the self-organizing plates, the upper mantle is heterogeneous in both temperature and composition. It has absorbed a great deal of oceanic lithosphere since the onset of plate tectonics at ~2.0 Ga, and it absorbed a good deal of felsic Archean crust before that. As a result, it remains rich in volatile elements (C, H and O) and also in fertile minerals (those with low melting points). Among other things, these constituents make the asthenosphere—a key player in both plate tectonics and intraplate volcanism—all the more eager to melt and the deeper upper mantle less dense than sinking slabs of oceanic crust. We'll return to the upper mantle many times as this overview of earth dynamics unfolds.
By far, the lower mantle's greatest contribution to observable geologic processes is the gravity generated by its immense mass—well over half that of the planet. Since gravity is main drive in most surface processes, including plate tectonics, gravitational collapse, mass wasting, erosion and sediment transport, that's no small matter, but the lower mantle's influence pretty much ends there.
The lower mantle appears to be stratified chemically, with likely boundaries at 1,000 and 2,000 km. If it convects, it may do so very slowly in several layers isolated from each other and also from the upper mantle and crust. Having lost most of its heat-generating radioactivity to the crust and upper mantle in the planet's thorough chemical differentiation prior to ~4.4 Ga, for most of earth's history, it has passively conducted original heat amounting to only ~10% of the surface flux from the core to the upper mantle with little help from convection.
Can lower mantle features have surface expressions? Theoretically, density lows in the lower mantle should lead to broad topographic highs at the the surface, while temperature highs should heat the overlying upper mantle with potential effects on plate distribution. At lower mantle densities, viscosities and thermal expansion rates, any such contrasts should be very large and semi-permanent. No such associations have been documented, but I wonder about the curiously rootless Rockies, which lack a demonstrable means of support.
Narrow-based "plumes" of hot material rising directly from the core-mantle boundary to manifest as surface hot spots are physically impossible, despite many educational cartoons and professional publications to the contrary. These plume proponents ignore the convection-stifling effects of pressure on thermal expansivity, viscosity and thermal conductivity at planetary scale. Add the pressure back into the models, and plumes don't form. Nor have any such plumes been detected by geophysical means.
Since the lower mantle has nothing else to do with plate tectonics, and certainly nothing to do with mantle plumes or hotspots, we'll ignore it henceforth.
BTW, an excellent up-to-date online read on the role of convection in earth dynamics is ^Mantle Convection by renowned geophysicist Don Anderson. If you fancy plumes, be prepared to change your mind.
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Plates with color-coded boundary types |
Since 2.0 Ga, the plates have clumped and split and regrouped many times, but by most counts, there are currently 20 plates—12 major and 8 minor. The 7 largest plates now account for 94% of the earth's surface, oceans included, and the smallest of the 12 major plates is larger than all of the minor plates combined.
The observation that there always seem to be ~12 major plates in the geologic record suggests that a dodecahedral division of the lithosphere involving 12 roughly pentagonal plates may be energetically and statistically favored over time. Experimentally and mathematically, there are only so many good ways to crack up and move a spherical shell held together by gravitationally-induced lateral compression, which acts something like surface tension. In the case of the earth, the weight of the plates and their willingness to spread under gravity at free margins (AKA subduction zones) together provide the lateral compression that keeps plates in contact.
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Pangea, complete and during break-up |
To understand why plates move, consider these geometric and physical constraints on plate tectonics:
Since the earth's size and shape seem to have been stable since the planet's formation at 4.5 Ga, available surface area is constant. The plates have only so much room. If some plates grow, others must lose an equal area, at least at the surface.
For a variety reasons, gaps between plates are simply not sustainable. For starters, the asthenosphere on which the plates rest is too plastic and too close to its melting point to stay out of any gap that might develop. In fact, there's a name for asthenosphere that manages to "plate" itself out onto the surface: Oceanic lithosphere.
Gravity impels lithospheric plates to move toward any free edge unrestrained by other plates.
The only truly free edges available in the system are at subduction zones where an oceanic plate segment (which may or may not have attached continental segments) manages to slip beneath the edge of another plate.
Oceanic plate segments move toward their subduction zones because they are under constant compression, both by their own weight and by neighboring plate segments, and simply have no place else to go. Overriding plates at such subduction zones move or stretch toward the subducting plates as the latter fall out of their way. Through these simple local interactions, the plates enter a ^self-organized dance in the same way that shoals of fish manage to dart in unison without a leader.
You can watch the plates dance through time in the online animations at the ^PALEOMAP Project.
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Earth, NASA |
The earth's daily rotation appears to energize, or at least foster, plate motions to some extent. On average, relative plate velocities near the equator are double those near the poles. This stable pattern and the net westerly drift of lithosphere apparent in the Antarctic frame of reference together strongly imply an input from the planet's spin.
Slow convective circulation of hot, plastic mantle rock is often said to be the energy source powering the dance of the plates, but many lines of geophysical evidence rule that out. Upper mantle convection certainly occurs in small scattered cells, but it doesn't exert enough horizontal force on the lithosphere to push plates around. In fact, upper mantle currents result from plate motions, not the other way around. Lower convection, if it exists, is confined below the "660" and is thus effectively decoupled from the upper mantle and lithosphere. Heat rising from the lower mantle enters the upper by conduction, not by convection.
The bottom line:
Top-down cooling and gravity acting on oceanic lithosphere are the only documentable driving forces for plate tectonics, perhaps with a slight boost from planetary rotation.
Relative plate motions are the most important because they control plate interactions directly. Absolute plate motions are also of interest, but they're difficult to pin down for lack of a suitable reference frame. Of the many possible frames proposed, the most commonly used are the hot spot frame pegged to Hawaii or to a collection of selected "fixed" hot spots (on the assumption that the proposed underlying mantle plumes must be fixed relative to the mantle), and the "no net rotation" (NNR) frame designed to minimize all plate motions. But hot spots turn out to migrate at rates (3-6 mm/year) comparable to plate velocities, and the NNR frames assign plate velocities and relative motions that just don't jibe with observed plate boundary types. In other words, plate velocities don't necessarily point toward convergent boundaries, away from divergent boundaries or along transform boundaries, as one would expect.
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Antarctic frame |
A simpler and more realistic frame plots recent plate motions against a fixed Antarctica (right) since, more than any other plate, it has boundaries that are nearly all divergent. (The only exception involves the tiny Scotia plate east of Cape Horn.) In the Antarctic frame, all slab hinges roll back (as they must), all ridges migrate (to tap fresh asthenosphere) and plate velocities actually point toward convergent plate boundaries, away from divergent boundaries and along transform boundaries, as they should. You can give the Antarctic and other frames a try online at the Jules Verne Voyager, Jr. interactive mapping site.
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Tibetan Plateau |
For the most part, the plates move as rigid bodies over the sphere of the earth along fairly sharp boundaries and interact and deform primarily along those boundaries. But significant areas of intraplate deformation affect 15% of the earth's surface, including much of Asia within and around the Tibetan Plateau (right).
The Rockies and the Colorado Plateau fall squarely in that 15% minority,
thanks in part to a 1.6 Ga-long legacy of weakness in the Colorado basement. Mantle stirrings probably also play a role in the Rockies, but relevant data is hard to come by. Intraplate deformations remain an active and challenging area of research in contemporary geoscience.
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Plates with color-coded boundary types |
The plates are semi-rigid spherical caps with varying shapes and sizes at the earth's surface. On a planetary scale, most plate boundaries are surprisingly sharp, but some are diffuse—in some cases, up to hundreds of km across. The boundary around any one plate may vary in character from place to place, but the nature of one plate's boundary with any other single plate is in large measure dictated by the relative motion of the two plates involved. Plate boundaries are therefore predominantly one of three basic types—divergent, convergent or transform.
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Currently, 80% of the earth's plate boundaries are divergent mid-ocean ridges and convergent subduction zones. As the globe at right shows, most boundaries are on the ocean floors, and this is no accident: By weakening rock and promoting melting, water tends to facilitate plate interactions.
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Mid-Atlantic Ridge |
Plates move away from each other at divergent boundaries, most of which are seafloor spreading centers. Mid-ocean ridges like the Mid-Atlantic ridge shown at right are the purest and most developed examples of divergent plate boundaries, but less oceanic examples include the Red Sea and the Gulf of California. Divergent boundaries create all of the planet's oceanic lithosphere.
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Himalayan orogen |
Plates move toward each other at convergent boundaries. Subduction zones and collision belts mark convergent boundaries, depending on the lithospheric types involved. Most convergent boundaries involve oceanic lithosphere on at least one side, but continent-continent convergences make for some of the planet's most dramatic scenery, including the Himalayas (right) and the Alps.
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San Andreas Fault |
Plates slide past each other horizontally along transform boundaries, or transforms for short. Over time, transforms have been responsible for many large-scale redistributions of continental crust. California's San Andreas fault is the main locus of motion (5 mm/yr) between the Pacific and North American plates, but their rather diffuse transform boundary also includes faults far to the east beyond the Sierras. At right, the main trace of the San Andreas fault rumples up the Temblor Range across central California's Carrizo Plain.
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Complex Pacific-North American boundary, USGS |
Things can get pretty messy after 2.0 Ga of semi-rigid plates bumping and grinding helter-skelter on the surface of a sphere. Mixed plate boundaries tend to occur where the margins and relative motions of the two plates involved fail to align neatly.
In California, the Pacific-North American boundary is both diffuse and mixed. In its drive to the NW relative to the California margin of the North American plate, the Pacific plate must also pull away to the west as it slides along the San Andreas fault, which trends too much to the north to accommodate the relative plate motions cleanly. Since plates lack the cohesion to allow gaps to open up between them, ongoing Basin and Range extension carried by gravity-driven lower crustal flow accommodates the westward component of divergence along the predominantly transform Pacific-North American boundary.
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Mendocino Triple Junction |
So far, we've considered only boundaries involving two plates. The coming together of three plates is called a triple junction, the ridge-ridge-ridge (RRR) variety being the most common. Eventually, all plate boundaries end at triple junctions, as do all seams on a soccer ball. California's Cape Mendocino (right) marks a seismically dangerous trench-transform-transform triple junction terminating the Gorda Trench, the San Andreas Fault, the Mendocino Fracture Zone.
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Afar Triple Junction |
Most RRR triple junctions are oceanic, but the Afar triple junction of Ethiopia and Somalia is a notable exception. Particularly leaky oceanic RRR triple junctions are the most common source for the seamounts and large submarine basaltic plateaus that dot the ocean basins.
Triple junctions exhibit many other surprising behaviors, but they're not a significant part of Colorado's story and won't be discussed further here.
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Structure of the Earth, USGS |
The earth's mobile lithosphere consists of the crust and the chilled topmost layer of the upper mantle. These two layers are usually inseparable, but the lithospheric mantle can occasionally peel off (delaminate), as it is doing now beneath the Carpathian Mountains of the Southern Alps. The plates of plate tectonics are lithospheric rafts that glide across the surface of the planet on the barely solid, plastic asthenosphere.
Like crust, lithosphere comes in two basic end-member flavors, continental and oceanic, but intermediate forms occur, particularly at passive continental margins. Most plates include both continental and oceanic lithospheric components.
Relative to the upper mantle on which it floats, continental lithosphere is buoyant and unsinkable. Oceanic lithosphere, on the other hand, is both denser and stiffer than the underlying mantle and will sink at its first opportunity.
This is the fundamental fact of plate tectonics.
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Mid-Atlantic Ridge |
Geoscientists call oceanic segments of lithospheric plates slabs for short, and I'll do the same. Slabs are created at seafloor spreading centers and consumed at subduction zones. Their materials are continually recycled through the upper mantle in the process. Compared to continental lithosphere, slabs are thin—on average 80-90 km at subduction zones and closer to 6 km when they first form. They are also denser and more flexible. The oldest dated continental crust (and presumably lithosphere) contains ~4.2 Ga zircons, whereas the oldest oceanic lithosphere is only ~180 Ma old.
More importantly, slabs are both denser and stiffer than the hot, barely solid asthenosphere on which they temporarily rests. This density inversion drives subduction and all of plate tectonics in turn.
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Juan de Fuca ridge and trench with Cascade arc, USGS |
Many processes create continental lithosphere, but oceanic lithosphere is created only at the divergent plate boundaries known as seafloor spreading centers (or ridges for short). Most of the planet's oceanic lithosphere forms along the vast, continuous ~40,000 km-long network of mid-ocean ridges that divide the globe like so many seams on a sloppily-made soccer ball. The Mid-Atlantic Ridge (above) is a world-class example. The pint-sized Juan de Fuca ridge (right and below) off the coast of Washington is a remnant of the East Pacific Rise, a Pacific spreading center that rivaled the Mid-Atlantic Ridge before its mid-section subducted beneath California.
Significant amounts of oceanic lithosphere also develop at smaller but often more complex spreading centers located in backarc basins near subduction zones and in maturing continental rifts like the Red Sea. Since mid-ocean ridges are the cleanest examples of spreading centers, we'll describe one next, but underneath their sedimentary covers, if any, all spreading centers look and work pretty much alike.
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Major Pacific fracture zones, USGS |
Think of a ridge as a snaking slit-like window exposing ready-to-melt asthenosphere to the hydrosphere. Mafic asthenospheric melts well up along the axial valley of the ridge to freeze against the edges of twin slabs forming on each side. In the map at right, the Pacific slab on the west is paired with much smaller twins across the Explorer, Juan de Fuca and Gorda ridges. South of the Mendocino fracture zone, the Farallon plate, a much larger Pacific plate twin, has already subducted beneath California along with the intervening ridge, the East Pacific Rise.
Submarine fissure volcanoes venting pillow basalts dot the axial valley of the spreading center, but down deep in the ridge, gabbro intrusions take over. Together, the basalt and gabbro make up the igneous basement of the oceanic crust. Ultramafic lithospheric mantle forms simultaneously beneath the basement as chilled asthenosphere freezes to its underside in a process known as underplating. Underplating progressively thickens the slab's mantle layer, but the basement layer changes little during the slab's time on the surface.
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Magnetic stripe generation, USGS |
As they form, the twin slabs record the earth's current magnetic field as they move away from the ridge and from each other in more of less symmetrical fashion. They develop tell-tale magnetic stripes in the process. What drives the divergent plate motion across the ridge is still not entirely clear, but gravity appears to be the prime mover via a combination of hinge rollback and ridge slide. Superheated brines circulating through the fault-ridden ridge chemically alter the fresh slabs as they pass off the ridge and onto the adjacent ocean floor. Basted basalt, anyone?
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Ocean floor ages; red ~ 0-10 Ma, purple ~ 160-180 Ma |
Once formed, the twin slabs go their separate ways, eventually to meet similar if not simultaneous fates in the most fundamental cycle in plate tectonics. As they cool, they'll soon become denser than the asthenosphere on which they rest, but they'll manage to stay topside for a while, mostly via lateral compression by adjacent lithosphere. Their residence times at the surface will be brief by continental standards—only ~100 Ma on average. Ultimately, they'll sink into the upper mantle from whence they came at separate and oppositely polarized subduction zones that may or may not have existed at the time of their birth. After piling up and dissolving back into the upper mantle along the "660", their materials will reappear at the surface as new oceanic lithosphere generated at some other ridge, usually one or more continents away from the ridge of origin.
The word "center" notwithstanding, spreading centers are linear, not radial, seafloor features. They are segmented and offset at varying intervals by perpendicular linear seafloor features known as fracture zones or transforms (which are not quite the same as transform boundaries). Ultimately, fracture zones serve both to fit linear spreading center segments to the earth's spherical surface, and to allow them to tap the ripest asthenospheric magma sources available along strike.
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Mendocino fracture zone |
Although they do show some seismicity, fracture zones are neither fractures nor continuously moving faults. Rather, they are welded sutures between slab segments generated at adjacent offset ridge segments. Because fracture zones may offset ridge segments by hundreds of kilometers, the slabs across them may differ substantially in age and therefore also in depth. The Men
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