On this page		Subsections
Introduction	Coarse pink 1.1 Ga Pikes Peak granite with green lichen near the summit of Devils Head, Rampart Range	Petrology Available Elements Basic Rock Types Rock Stability and the Rock Cycle End-Stage Products of Weathering Map Units
Igneous Rocks	Middle Phase intrusives and volcanic remnants form the West Elk Mountains	Melts — Buoyant and Reactive Igneous Settings Raw Materials for the Rock Cycle Magmatic Differentiation
Volcanic (Extrusive) Rocks	Late Tertiary basalts capping Triassic redbed hills north of Aspen	Volcanic Textures Volcanic Compositions Volcanic Habits Volcaniclastic and Metavolcanic Rocks Volcanic Map Units
Intrusive (Plutonic) Rocks	Oligocene intrusive below Cascade Lake	Intrusive Textures Intrusive Compositions Common Intrusive Rocks Intrusive Forms Intrusive Map Units
Sedimentary Rocks	Cliffs of resistant Wingate sandstone in the Colorado National Monument	Clastic Sedimentary Rocks Clastic Grain Sizes Clastic Grain Compositions Sedimentary Structures Porosity and Permeability Clastic Cements Clastic Rock Colors Marine Shelf Deposits — Sedimentology 101 Pelagic Deposits Deposits Left By Transgressive and Regressive Seas Terrigenous Deposits Clastic Sedimentary Map Units
Chemical Sedimentary Rocks	[photo coming] Pelagic chert	Carbonate Rocks Cherts Evaporites Chemical Sedimentary Map Units
Metamorphic Rocks	Maroon Formation strata indurated by contact metamorphism make up the ever-beautiful Maroon Bells	Eclogite, Subduction's Whip Schist Happens—The Shale Pathway Range of Metamorphism Under the Hood Types of Metamorphism Once a Gneiss, Always a Gneiss Metamorphic Fabrics and Textures Metamorphic Minerals, Grades and Facies [] Common Metamorphic Rocks [] Metamorphic Map Units
Rock Formations and Sequences	Cretaceous Dakota sandstone caps the east wall of the lower Animas canyon just above Durango; below it are the Jurassic Morrison Formation and the Entrada, Navajo and Wingate sandstones; and the Permo-Triassic redbeds of the Chinle Formation and the Cutler shale	What Are Rock Formations and Sequences And What Good Are They? Formation Names Formation Boundaries Lateral Variations and Terminations Formations As Mental Tools Sequence Stratigraphy
Rock Mechanics	Bold red Fountain Formation flatirons jut skyward toward the Front Range below a ridge of white Lyons sandstone at the north end of Roxborough State Park	Butter Just Add Time Heat and Pressure Help Brittle-Ductile Transitions
Page References

See also Colorado Geology Overview, The Earth At Work

Under construction

Last modified 10/22/04

Top    Page Index

Introduction

Skip to Igneous Rocks

When I first thought to name this article "Colorado Rocks", my hope was to limit its scope. But in truth, it's hard to name a rock Colorado doesn't have. Some of the higher-grade metamorphic rocks, like the 1.7 Ga arc-related metavolcanics, may bear little resemblance to their original state, but in one form or another, they're all here.

Petrology

Granodiorite boulder

Petrology, the study of rocks, may sound absurd to some, but it's more practical than it sounds. Rocks hold the only available record of the history of our one and only planet. They also hold the key to two of the great pillars of human economy—mineral wealth and agricultural vigor. They tend to hold up the planet's most inspiring scenery, and they ultimately support everything we build—including houses, schools, skyscrapers, roads, bridges, tunnels and nuclear power plants.

Our biological history is also more entwined with rocks than you might think. Cell biologists studying the origins of life now have good evidence that the precursors of modern cells used rock surfaces as both cell membranes and as as catalysts for the organic reactions they required. The chimney-like mid-ocean ridge hydrothermal vents known as ^black smokers carry on such such rock-cell partnerships to this day. It's no accident that many important human enzymes and physiologically active proteins require metal ions as ^co-factors—e.g., iron in hemoglobin, magnesium in chlorophyll, chromium in insulin, copper in cytochrome c oxidase, and zinc in angiotensin converting enzyme, to name just a few.

Since mantle and lower crust rocks are only rarely exposed, the rocks of the upper crust are the main focus of petrology, even though they constitute well under 1% of the earth by volume. Of course, geologists are eager to study any rock they can get their hands on, so they particularly prize the occasional plums thrust up to the surface from lower levels. 

Available Elements

Since the ^chemical elements are the fundamental building blocks of all ordinary materials, including minerals, let's start there. Elemental abundances at and near the surface of the earth tightly constrain the range of minerals and rocks observed, not to mention the range of possible biologic processes. 

The table below lists the 14 most abundant elements in the earth's crust in decreasing order.

With oxygen and silicon alone accounting for ~74% of the crust and aluminum for another ~8%, it's little wonder minerals composed primarily of these three elements dominate the crust. They do so primarily in the form of silicate and aluminosilicate minerals built on strong chains and sheets of tetrahedral and octahedral arrays of Si-O and Al-OH bonds. In nearly all common silicate minerals, including feldspars, micas and clays, positive ions of calcium, sodium, potassium and magnesium serve both to balance out the negatively charged silicate backbones and to bind them together neatly without disturbing their crystalline structures.

It takes seven of the eight major crustal elements to fill these vital chemical roles in the sialic (Si- and Al-rich) rocks typical of the upper continental crust. Iron, the odd atom out in the major element group, figures more prominently in the higher-density minerals inhabiting the lower continental crust, the oceanic crust and the mantle. Olivine, pyroxene, hornblende and biotite are among the most common of these ferromagnesian or mafic (Ma- and Fe-rich) minerals.

Trace Elements

Elements beyond the top 14 listed above fall into the trace element category. They're scarce in the crust for a very simple reason: By virtue of their size or charge distribution (see below), they fit poorly into the crystalline structures of typical of most crustal (silicate) minerals. The trace elements include gold, silver, copper, nickel, zinc, lead, lithium, beryllium, niobium, tantalum, tin, uranium, thorium, tungsten, zirconium and the rare earths. Many are more abundant in the mantle than in the crust. Mantle-derived, water-rich magmas are their primary means of transport to extractable crustal depths.

New Insights From a New Periodic Table

In 2003, geochemist Bruce Railsback published his revolutionary and ingeniously reorganized ^Earth scientist's periodic table of the elements and their ions showing not only the neutral elements but also their naturally-occurring ions. Since ions are with rare exception the stuff of earth materials, much can be learned from their habits and proclivities. Indeed, since oxygen is by far the most abundant element in both the mantle and crust, the way various cations (positively charged ions) interact with ionic oxygen constrains a great deal of geochemistry and to some extent biochemistry as well. Silicon, the 2nd most abundant element in the crust, also plays a defining role in geochemistry.

Ionic Potentials of Cations

One of the most important innovations in Railsback's periodic table is the addition of contour lines of equal ionic potential—the ratio z/r of ionic charge to radius. The higher the ionic potential, the more compact or intense the ionic electric field, and the more strongly the ion interacts with nearby charge centers. Since trends in ionic abundance, mineral formation, oxide melting point, solubility, and even nutrient value all tend to follow contours of ionic potential, the new table shows at glance important chemical relationships that the standard table obscures.

Cations (positively charged ions) of low ionic potential (z/r < 4) like Na⁺, K⁺ and Ca²⁺ bond relatively weakly to O^-2, do not form stable oxide minerals, remain in fluid phases until late in melt evolution, are highly concentrated and soluble in natural waters and serve as essential nutrients to both plants and animals.

Cations of intermediate ionic potential (z/r = 3-10) like Al³⁺, Fe³⁺ and Ti⁴⁺ bond strongly to O^-2. Their compact and largely shielded charge distributions allow them to coordinate with a single negative charge center in large numbers with little mutual repulsion. Such cations tend to form stable oxide minerals, to bond in igneous minerals early in melt evolution, to concentrate in soil, to linger in the mantle, to have low concentrations and solubilities in natural waters, to collect in ferromanganese nodules on the ocean floor, and to serve inconsequential roles as nutrients. 

Cations of high ionic potential (z/r > 8) like P⁺⁵, N⁺⁵ and S⁺⁶ also bond tightly to O^-2 to form highly stable and soluble radicals like PO₄^-3, NO₃^- and SO₄^-2, but they can't form stable oxide minerals due to mutual repulsion. However, the small C⁺⁴ cation (z/r ~ 27) forms the stable oxide and greenhouse gas CO₂ as well as stable carbonate oxysalts of the soluble radical CO₃^-2. The C⁺⁴ cation thus plays a very special role in the planet's surface temperature-regulating carbonate cycle. High potential cations share many properties with the cations of low ionic potential. Because they both readily leach out of soils due to high solubility, K⁺ (low potential) and NO₃^- (high potential) are both key ingredients in fertilizers. 

The most common silicon ion, Si⁴⁺, occupies another special niche as a highly abundant cation at the cusp (z/r = 8) between high and intermediate ionic potentials. Thanks to similar ionic potentials, Si⁴⁺, V⁵⁺, Mo⁶⁺ and Se⁴⁺ all stand at the upper margin of cations forming stable soluble oxysalts—e.g., silicate, SiO4^-4 or Si(OH)₄—that also form stable insoluble oxide minerals—e.g., silica, SiO₂, as in quartz.  (Interestingly, these 4 cations all serve as essential vertebrate micronutrients.) However, the crustal abundance of  Si⁴⁺ far exceeds that of all the others in this group. Thus Si⁴⁺ appears in large quantity in both the insoluble products of weathering, most notably as sand, and in natural waters as dissolved silica. Si⁴⁺ binds to igneous minerals only at intermediate to low temperatures and remains abundant in fluid phases to the end of the crystallization sequence. Along with their low densities, these properties insure the crustal accumulation of quartz and silicates during the earth's chemical differentiation.

Hard vs. Soft Cations

Another important innovation in Railsback's revamped periodic table is the grouping of ions according to their electronic configurations as ions, a dimension separate from ionic potential. Having lost all their outer shell electrons, the hard cations are left with a relatively inert noble-gas-like electronic configuration, while the soft cations retain some outer shell electrons—the more, the softer. Hard and soft cations behave quite differently. Hard cations like Ca²⁺ coordinate strongly with O^-2 and F^-; soft cations do not. When they form oxide minerals, hard and soft cation oxides have high and low melting points, respectively. Soft cations like Cu⁺ bond with S^-2 and the larger halides I^- and Br^- rather than with O^-2 and F^-; they tend not to form oxide minerals. Thus hard Ca²⁺ forms both oxide and an oxygen-rich sulfate (gypsum, CaSO₄) but not a sulfide, while soft Cu⁺ forms a sulfide (chalcocite, Cu₂S) but not an oxide. 

The metal cations commonly found in silicate minerals (Na⁺, K⁺, Ca²⁺, Mg²⁺) are all hard. Their low ionic potentials and noble-gas-like electronic configurations allow them to fit cleanly between large polymeric silicate and aluminosilicate sheets and chains and bind them together without disturbing them. The trace elements, on the other hand, generally have low to intermediate ionic potentials and soft to very soft outer shell configurations. Due primarily to the latter, they fit poorly in silicate lattices and for the most part remain sequestered in the mantle, where more hospitable non-silicate minerals dominate.

Anions

The most commonly occurring anions (negatively charged ions) are O^-2, S^-2, Cl^-, F^-, and the soluble oxo complex radicals  CO₃^-2, SO₄^-2, SiO₄^-4, NO₃^- and PO₄^-3. Hard cations prefer to coordinate with O^-2, by far the most common anion in both crust and mantle, while soft cations like Pb²⁺, Cu²⁺ , Zn²⁺ and Ag⁺ instead prefer S^-2 (number 13 of the 14 most common elements. This preference alone accounts for some of their rarity. The extremely soft Au+ cation can't form an oxide and can only form a sulfide with the help of other soft cations—hence the long-admired rarity and "nobility" of gold and its predilection to go native in elemental form. Not surprisingly, metal oxides and sulfides are the most common ore minerals in the Colorado Mineral Belt and elsewhere. After O^-2 and S^-2, the properties of the anions appear to be less important than those of the cations.

Basic Rock Types

Now that we've seen how crustal elements combine to form minerals, let's look at the rocks the minerals make.

Every grade-schooler knows that rocks come in three basic flavors—igneous, sedimentary and metamorphic, as detailed in the table below. That's still an excellent starting point, but we'll need some subtypes to make real headway in understanding the rocks of Colorado. It's worth emphasizing at the outset, however, that rocks in the field form a continuum of origins, compositions and textures beyond the reach of any rigid classification scheme. No matter how fancy the classification, there will always be important transitional rocks that can and will be classified more than one way by reasonable geologists. 

Transitional Rock Types

Much to the dismay of architects, students and users of rock classifications, transitional rock types pop up everywhere. Important examples include the following:

• Mildly altered sedimentary rocks may still look just like sedimentary rocks, but some geologists will label them metamorphic where others would not.

• Pelagic cherts derived from planktonic debris could just as easily be considered biochemical rocks, but by convention, they're classed as chemical because of a necessary recrystallization step.

• Marls are mixtures of clays and carbonates with at most minor amounts of quartz. Are they clastic or chemical sedimentary rocks? 

• Tuff—volcanic ash deposited in layers as it falls out of the air onto land or water—could be considered sedimentary, but by convention, tuffs are classed as igneous.

• Volcaniclastics formed from debris weathered from solid volcanic rocks have some unique properties. They might be considered igneous, but they're usually classified and mapped as clastic sedimentary rocks.

• Migmatites contain some melted rock but are still considered metamorphic, not igneous. 

• Cumulates—masses of precipitated crystals settling to the bottom of an otherwise fluid magma—are igneous sediments in a very real sense, but rocks formed exclusively from cumulates are considered igneous, not chemical sedimentary.

Confused? Hang around with rocks long enough, and you'll get used to it.

Crystalline Rocks

Rocks composed entirely of interlocking crystals of one or more minerals are said to be crystalline. All unequivocally igneous rocks are crystalline, as are higher-grade, recrystallized metamorphic rocks like gneiss and schist. Strictly speaking, some chemical sediments like limestone and chert (microcrystalline quartz) are also composed of crystals, but in common usage, the term excludes sedimentary rocks.

Mechanically, crystalline rocks tend to be stronger and more resistant than other types. They form the crests of the Rockies' highest ranges and hold up most of Colorado's Fourteener summits.

Rock Stability and the Rock Cycle

For the most part, rocks are equilibrium products relatively stable at the conditions under which they formed but chemically or mechanically unstable in all other environments. Crystalline rocks formed at depth are unstable at the surface, while sedimentary rocks formed at the surface are unstable at depth.

Rocks thrust into new settings by tectonic, magmatic or erosional events tend to move around the rock cycle (below) from one basic rock type to another. For example, igneous rocks formed at depth under high temperature (T) and pressure (P) in the absence of free oxygen and water are bound to change when brought to the surface to face chemical and mechanical weathering, erosion, transport, deposition and diagenesis. Given enough time, their debris will become sedimentary rocks best suited to surface conditions. With deep burial during a mountain-building event, the elements in the sedimentary rocks will reorganize  into new metamorphic minerals better suited to the extreme pressure-temperature (PT) conditions they now face. Under the right PT conditions, they might even come full circle to melt back into igneous rocks. Alternatively, uplift and erosion of the metamorphic rocks might ultimately produce a new batch of sedimentary rocks. All paths through the rock cycle are possible.

Rock Cycle

Rock Cycle

The diagram at right nicely summarizes the rock adjustments outlined above. It's useful to think of igneous rock as the starting point for the cycle, but material can jump in anywhere and end up anywhere. The transformations that occur with any frequency are already shown in the diagram, but since deeply buried sedimentary rocks can melt directly in certain tectonic environments, it would be reasonable to add another thin black curved arrow pointing from the sedimentary to the igneous node.

Acknowledgment: Rock cycle diagram courtesy ^Lynn Fichter.

Resistance to Weathering

Rocks that are slow to weather and erode are said to be resistant. All other things being equal, erosion will leave resistant rocks standing higher than the less resistant rocks around them. Crystalline (igneous and metamorphic) rocks then to be more resistant than unaltered sedimentary rocks, but chert is a notable exception.

Fusibles and Refractories

Fusibles are rocks or minerals that melt easily. Refractories are just the opposite. Sedimentary rocks tend to be fusible, while crystalline rocks tend to be refractory, some more than others. Not surprisingly, the most refractory rocks, like gabbro and peridotite, reside in the lower crust and mantle. Sedimentary rocks groomed for surface stability wouldn't stand a chance at those depths. 

Reworking

Rock materials don't necessarily move around the rock cycle as the rocks they compose evolve. In a tectonic disturbance, uplifted sedimentary rocks can be reworked into new sediments, igneous rocks can remelt, and metamorphic rocks can prograde. Reworking adds yet another layer of complexity to rock genealogy.

End-Stage Products of Weathering

Many earth processes play out at depth beyond the reach of the atmosphere and hydrosphere, but for many others (including weathering, erosion, transport, deposition, isostatic rebound and basin subsidence), the rubber meets the road at the surface, where the atmosphere, the hydrosphere and the land all interact strongly to shape both land and climate in a never-ending dance.

Once weathering gets a foothold on a rock exposure, erosion, transport and deposition are likely to follow, but weathering continues to break down the sediments, both en route and at the site of deposition. Given sufficient time and transport distance, the ultimate end-products of weathering are always pretty much the same, regardless of the initial rock type:

• Quartz (SiO₂) sand, typically derived from igneous, metamorphic and reworked quartz-bearing sedimentary rocks like sandstones and quartzites

• Clay mud, from the chemical breakdown of feldspars and ferromagnesian minerals in igneous, metamorphic and immature sedimentary rocks, and from reworked clay-bearing sedimentary rocks

• Dissolved calcite (calcium carbonate, CaCO₃, AKA lime), derived from calcium (Ca) weathered from common feldspars, and from reworked lime-bearing sedimentary rocks

Why end up with just these three mineral groups? Because all the rock-forming minerals commonly exposed on this silicate planet eventually break down into quartz, clay or calcite unless some other process (like melting) intervenes. These end-products are chemically stable under most surface and near-surface conditions, but their precursors are not. 

Quartz Grains

Quartz (SiO₂) grains are exceptionally stable at the surface. They may be ground down to silt-size during transport, but like glass (also SiO₂), they're chemically inert. (That's why chemists use glass containers.) Once silt-sized, they go back into suspension in moving water, where they escape further mechanical weathering. 

Clay Minerals

Clay minerals tend to form microscopic flat platy crystals with charged surfaces that slide easily against each other and have a hard time interlocking, especially when wet. Claystones tend to be weak as a result, and clay particles remain in transport the longest because of their size and shape. The most common clay minerals produced by weathering—montmorillonite, illite, kaolinite, in order of current abundance—reflect the stability of sheeted Si-O and Al-OH crystal structures. Montmorillonite is the expansile clay dreaded by homeowners and civil engineers everywhere. Kaolinite formation is restricted to low latitudes because it requires a hot wet climate.

Map Units

Some sections in this article close with a "Map Units" subsection describing how to find pertinent bedrock (surface rock) units on the Geologic Highway Map of Colorado.

Top    Page Index

Igneous Rocks

Skip to Extrusive (Volcanic) Igneous Rocks

Silver Plume granite

Rocks that solidify from a molten or partially molten state are said to be igneous. Rocks that freeze on or above the surface, whether in the air or underwater, are extrusive or volcanic. But if they freeze below the surface for any reason, as did the 1.4 Ga Silver Plume granites exposed so handsomely at ^Rocky Mountain National Park (right), they're called intrusive or plutonic instead. As we'll see, extrusive and intrusive igneous rocks differ chemically, texturally, and in other important ways. In either case, magma is the molten rock involved. Magmas reaching the surface in liquid state are called lavas.

For more on igneous rocks, read on, but also consider a visit to the extensive and well-illustrated ^Igneous Rocks site by educator and geologist ^Lynn Fichter.

Melts — Buoyant and Reactive

The geothermal gradient guarantees that all melts develop at depth. Since melts are almost always lighter than the solid rocks from which they derive, gravity impels them to rise toward the surface as best they can, just as a bubble eventually rises, however slowly, through semi-solid molasses in the refrig. (A rising rock body, whether solid or molten, is a diapir.) In fact, most rock melts are buoyant enough to approach the surface if they don't freeze first. 

On the way up, melts interact both physically and chemically with the rocks through which they pass. In the process, they give off heat and fluids and take in easily melted or dissolved wall rock components. The final igneous product, whether extrusive or intrusive, is always highly evolved relative to the initial melt, but some magmas reaching shallow levels are more primitive than others. On average, the basalts erupted at seafloor spreading centers are the least evolved relative to their asthenospheric source rocks. Continental granites and rhyolites are among the most differentiated of all magmas.

Igneous Settings

Since the onset of plate tectonics ~2.0 Ga, most of the planet's igneous activity has concentrated along plate boundaries. As the modern map below clearly shows, the situation is no different today. Unusually eruptive boundary segments like Iceland are called hot spots. The igneous centers found far from plate boundaries are also hot spots (Hawaii is the only one shown here, but others exist.) Hot spots arise for a variety of reasons, most of which ultimately relate to extensional failure of the plate(s) involved; the once-popular deep mantle plume explanation for hot spots is not supported by the observations. Volcanic outputs at hot spots can be truly prodigious, but the associated intrusive activity can be just as important.

Plate boundaries with major volcanic centers marked in red

Raw Materials For the Rock Cycle

In many ways, igneous rocks are the starting point for the rock cycle. Whether the parent melt derives from the mantle or from sedimentary rocks buried deep in the upper crust, it eventually cools and freezes into a solid mass of interlocking crystals derived from a handful of mineral families, including those listed in the table below. The igneous raw materials can then go on to become sedimentary or metamorphic rocks as events and conditions unfold.

* Table Note: Since quartz and feldspathoids (AKA foids) are chemically incompatible, they seldom occur in the same rock. Most of the remaining possible mineral family combinations can and do occur in common rocks.

Felsic and Mafic Rocks

The term felsic means feldspar- and silica-rich. Sialic rocks (those rich in silica and aluminum) are particularly felsic. Felsic rocks tend to be of continental origin. Felsic magmas like rhyolite have typically reacted strongly with continental (or at least felsic) crust on their way to the surface, regardless of  the source of melt.

Mafic means Mg- and Fe-rich. Rocks of the upper continental crust are felsic on average, but mafics are fairly common there. Rocks of the lower continental crust and the oceanic crust are almost always mafic. Mafic rocks are on average denser than felsics and tend to be found at greater depths as a result. Ultramafic rocks are exceptionally rich in Mg and Fe and poor in Si. They almost always come from the mantle and accordingly tend to be very dense.

Magmatic Differentiation

A single melt can produce a wide variety of different igneous rocks through a complex process known as magmatic differentiation. Exposed magma bodies, whether intrusive or extrusive, typically display some degree of differentiation over both space and time. Most melts develop in the lower crust or in the upper mantle's asthenosphere. As a result, many melts start out with a fairly primitive mafic or basaltic composition. Melts developing in the upper crust tend to have higher initial silica contents

Regardless of where they form, all melts evolve considerably during ascent. The rocks they ultimately produce depend on the composition of the original melt and on the properties of the wall rocks encountered en route. The main processes involved are fractional crystallization, assimiliation, exchange of volatiles, and magmatic mixing.

Fractional Crystallization

Fractional crystallization (or fractionation for short) occurs when circumstances prevent early-forming crystals from reacting with the remaining melt. This process accounts for most of the differentiation observed in igneous rocks.

As a rising melt cools and reacts with surrounding rock, the melt minerals with the highest melting points or the lowest solubilities (the refractories, like olivine and pyroxene) crystallize out first, while those with the lowest melting points or solubilities (the fusibles, like silica) freeze out last. Heat released by the crystallization of refractories replaces heat lost to the surrounding country rocks by simple conduction, by country rock melting and by the assimilation of country rock fusibles. Fusibles and refractories enter and leave the melt at specific temperatures and pressures, which tend to occur at specific depths along the ascent. As it continues to rise, the surviving melt loses volume, and its fusibles become more and more concentrated. It leaves behind a trail of solid refractories and country rock alterations. 

Gravitative differentiation, the most common form of fractionation, stems from the fact that most solid minerals are more dense than their parent melts. When their crystals settle to the bottom of the magma body, they are effectively segregated from the residual melt. Rocks formed from crystals amassed in this manner are called cumulates, and they're often zoned, with first crystals to leave the melt at the very bottom of the magma chamber. Cumulates formed from lighter crystals that occasionally precipitate out of the melt float to the top instead, with the lightest at the very top. Cumulate crystals are typically cemented by residual magmatic fluids. 

Assimilation

Ascending magmas also evolve chemically by recruiting easily melted or dissolved components (fusibles) from the walls of their conduits. Heat and magmatic fluids mediate the process. In so doing, they may pick up volatiles, extra silica, trace elements and even chunks of wall rock. The thermodynamics and geochemistry involved are exceedingly complex, but the heat the melt gains from leaving behind refractories (an exothermic process) is usually sufficient to cover the heat lost to the endothermic reactions involved in the assimilation of country rock components. Assimilation can thus proceed without tapping the heat required to keep the melt from freezing.

Partially-assimilated xenolith in granite boulder, Glenwood Canyon

Wall rock chunks that survive more or less intact, without completely melting or dissolving into the magma, are called xenoliths. Surviving wall rock crystals are called xenocrysts. Together, xenoliths and xenocrysts provide invaluable information about the rocks residing at rarely exposed lower crust and mantle levels.

Volatiles, Aplite and Pegmatites

Figuring prominently in the process of assimiliation are the volatiles found in varying amounts in nearly all wall rocks and magmas—CO₂,  SO₂, O_2, Cl₂ and most notably, H₂O. Water is particularly available in wall rocks of the mid-crust, both in free form and within the hydrated minerals commonly found at such depths. Some of the assimilated water goes into hydration reactions with predominantly anhydrous melt components, but most of it just builds up in the ever-shrinking surviving silicate melt. If it takes on enough water, the melt will eventually develop a water-saturated silicate fraction and a separate water-based fluid phase.

Pegmatite vein in granite boulder, Glenwood Canyon

Under certain conditions, the water-saturated silicate fraction can give off a whitish fine-grained vein-filling slurry of quartz and feldspar known as aplite. The water-based phase easily assimilates trace elements that don't fit well in most silicate crystals, including lithium, beryllium, niobium, tantalum, tin, uranium, thorium, tungsten, zirconium and the rare earths. Many ore deposits form when this hot, pressurized, mineral-laden hydrous fluid finally permeates fractured country rock and cools into veins of pegmatite—an igneous rock containing unusually large crystals of quartz, feldspar and, now and then, highly prized minerals as well. Pegmatite and aplite dikes and veins are common around intrusions. Pegmatite is the prospector's friend.

Magmatic Mixing

The mixing of two separate magmas just before eruption or final subsurface emplacement is uncommon, but in areas of active magmatism, adjacent magma bodies are bound to develop transient subsurface communications now and then. At right is a rare banded tuff from the Valley of 10,000 Smokes, Katmai, Aleutian Archipelago, Alaska. The banding reflects the last-minute mixing of lavas from two separately differentiated magma chambers underlying the valley during the cataclysmic 1912 eruption of Novarupta Volcano, which released a whopping 30 km³ of pyroclastic material at the time. [banded tuff photo]

A much more common form of magmatic mixing involves the secondary melting (anatexis) of mid- to lower crustal rocks on contact with much hotter rising mafic melts of mantle origin to produce felsic (feldspar- and quartz-rich) magmas in arc and continental rift settings. On reaching high crustal levels, such melts may arrive with more mantle heat than mantle material in tow.

Eruption

Mauna Loa, 1984

When magma reaches the surface, the excess volatiles escape in vapor form. Gases usually boil out of low-viscosity basaltic lavas relatively peacefully, as they usually do in Hawaiian eruptions. A good example shown at right is the ^March 26, 1984 fissure eruption on Mauna Loa's Northeast Rift Zone. But volatiles are more likely to explode than boil out of viscous lavas like rhyolite and andesite, as they did at Mt. St. Helens on May 18, 1980 (shown at the top of the next section). Volcanic habits are discussed in greater detail below.

Top    Page Index

Volcanic (Extrusive) Igneous Rocks

Skip to Intrusive (Plutonic) Igneous Rocks

Mount Pinatubo, 1991

Igneous rocks that solidify from melt on or above the surface of the solid earth are called volcanic after Vulcan, god of fire. The term extrusive is synonymous with volcanic. At right is the explosive June 12, 1991 eruption of Mount Pinatubo, Luzon, Philippines.

Volcanic rocks occur in many tectonic settings, including magmatic arcs at subduction zones (as in the Banda Sea at right), seafloor spreading centers, ocean islands, and along continental rifts and other leaky faults. Like their intrusive counterparts, extrusive rocks are categorized primarily on the basis of texture and composition.

Volcanic Textures

Pahoehoe lava, Mauna Ulu, Hawaii

Because volcanic rocks tend to cool quickly after eruption, individual mineral crystals have little time to grow and usually end up too small to see with the unaided eye. The resulting rock texture is said to be aphanitic. Occasionally, one mineral, often a feldspar, manages to grow phenocrysts (large crystals much bigger than all the rest) before venting. An otherwise aphanitic volcanic rock containing phenocrysts is called a porphyry; the fine-grained component is called the groundmass. Rock textures in which two very different grain sizes predominate are termed porphyritic. Volcanic glasses like obsidian tell of ultra-fast cooling rates.

Lavas

Sunrise, Mauna Loa, Hawaii

Lava flows are perhaps the simplest of volcanic deposits, but they show their share of complexities. Between the frozen gas bubbles (vesicles), if any, most lavas are predominantly aphanitic in texture, but porphyries also occur. Over time, flows tend to vary in texture and composition, in part because they tap different portions of the magma chambers that feed them. Flows often cross eroded surfaces and interact with their soil covers and groundwater along the way. Lavas quickly chilled in air or water develop glassy textures. ^Obsidian (usually of rhyolitic composition) and the glassy rinds on basaltic pillow lavas are examples. 

At right, Mauna Loa looms over Kilauea Caldera at ^Hawaii Volcanoes National Park as fume rolls off Steaming Bluff in the morning light. Mauna Loa is the world's largest mountain and largest volcano. Kilauea is the world's most active volcano. Basaltic lavas built both just in the last 1 Ma. Olivine basalt featuring macroscopic green olivine porphyrocrysts in a black groundmass is a common lava around Mauna Loa. Waves and currents have concentrated olivine dense crystals weathered out of sea cliffs at the southern tip of the Big Island into a unique green sand beach. 

Tephra

Solids thrown from a volcanic vent are called ejecta, and accumulations of ejecta are called tephra or pyroclastic deposits. Pyroclastics come in many sizes: Blocks and bombs are over 32 mm in diameter, with bombs showing some degree of aerodynamic rounding; lapilli are 4-32 mm across; and ash particles are under 4 mm. Wind can carry fine ash hundreds of kilometers, but, not surprisingly, larger and larger ejecta fall progressively closer to the vent. Tuff, rock made from consolidated ash layers, comes in water-laid and air-fall varieties.  When ash is hot enough and falls at high enough rates, individual particles can fuse on burial by subsequent ash falls. An extremely resistant welded tuff or ignimbrite results. 

Around 30 Ma, welded tuffs blanketed the entire Basin and Range to great thicknesses in a prolonged and undoubtedly unpleasant event known as the Ignimbrite Flare-up. During the Early Phase of Tertiary magmatism in Colorado (~37 Ma), massive ash flows from the Mount Princeton area rolled 90 km across the Eocene erosion surface to blanket the western Denver Basin with incandescent ash which compacted and fused to become the welded Wall Mountain tuff, apparently in a single day. Given the 10:1 compaction ratios typical of welded tuffs, modern Wall Mountain remnants up to 40' thick imply initial ash deposits up to 400' thick. 

I keep the clast of Wall Mountain tuff pictured at right on my desk to help me remember what a really bad day looks like.

Volcanic Compositions

Volcanic rocks vary widely in their elemental and mineral content. Silica content is perhaps the important single compositional property because it strongly controls viscosity. Viscous high-silica lavas like rhyolite and andesite tend to erupt explosively, because stickier lavas retain more of their volatiles until they near the vent and release them more violently on eruption. As with crustal rocks in general, silica (SiO₂) and alumina (Al₂O₃) dominate all volcanic rocks, even the most mafic basalts, but the silica variations shown below are more than adequate to create big differences in lava viscosity.

Compositions of common lavas, J. Johnston, USGS, Types of Igneous Rocks

In order of increasing silica content and explosive tendency, basalt, andesite, dacite and rhyolite are the most common lavas. As the lava of choice of at mid-ocean ridges, on seamounts and submarine plateaus, along continental rifts and other leaky faults, basalts outnumber and outmass all other lava types by a wide margin. Basalts are mafic (rich in ferromagnesian minerals), whereas rhyolites are felsic (rich in high-silica feldspars, feldspathoids and quartz). Andesites and dacites are in between. Compositionally, rhyolite is the extrusive form of granite, while basalt resembles the intrusive rock gabbro.

Table note: The external links above lead to excellent rock photographs and descriptions provided by the US Geological Survey Photo Glossary of Volcano Terms.

For the visual learner, the diagram below shows much the same information. 

Source: J. Johnston, USGS, Types of Igneous Rocks

Volcanic Habits

The way a particular volcano behaves depends on many things, including 

• the composition, temperature and viscosity of the lava currently being erupted

• the vigor of the magma supply

• the load of dissolved volatiles (gases) to be released

• the availability of free water (surface or ground, liquid or solid) to enhance the potential for explosive eruption and the generation of landslides and mudslides.

Of these, the most important influences relate to the properties of the lava itself.

Basalitic Volcanism

Mauna Loa, 1984

Immature oceanic arcs usually start out with basaltic lavas. Seafloor spreading centers, ocean island volcanoes, volcanic seamounts and volcanic submarine plateaus overwhelmingly produce basalts. Basalt is also the magma of choice in continental rift settings, at least for starters, and along leaky faults of all kinds.

Subaerial basaltic volcanoes generally have limited potential for devastation at a distance. Their lavas tend to erupt at high temperatures with low viscosities, the latter due to their low silica contents, and their products tend to accumulate near the vent. That makes for fairly peaceful eruptions and for relatively stable volcanic edifices, including shield volcanoes like Hawaii's Mauna Loa, shown at right in a 1984 fissure eruption. Note the calm observer in the lower left corner of the photo. Standing that close to an erupting vent would be totally insane on a non-basaltic volcano. 

Still, even basaltic volcanoes have their moments. Groundwater meeting hot rock beneath the Kilauea Caldera caused a massive week-long phreatic (steam-driven) eruption that blasted Halemaumau crater out of the caldera floor in 1924. When the dust had settled, the new crater was 915 m across and 550 m deep, and 750,000 m³ of ejected debris littered the caldera floor, including many angular car-sized blocks.

Arc Volcanism

Mt. Rainier looms over Takoma, Washington

Andesites, dacites and rhyolites are the magmas most commonly erupted at magmatic arcs associated with subduction zones, particularly those built on continental margins. These viscous lavas tend to build unstable stratovolcanoes—like Mt. Rainier (right) of the Southern Cascade Range—prone to release massive lahars (volcanic mudflows) and ground-searing ashflow (glowing cloud) eruptions. As Mount St. Helens so vividly reminded us, they also tend to erupt explosively, potentially spreading large volumes of ash over wide areas. The apocalyptic "X" bentonite layer of the Denver Basin records the fall of 15 m (48') of ash from an Idaho vent hundreds of kilometers upwind.

Bimodalism

Most arcs and many rifts go through the full range of magmas—first basalt, then andesite, then dacite and finally rhyolite—as source melts interact with wall rocks on the way up to their reservoirs, and as the magma reservoirs mature over time. But volcanic centers in both rift and arc settings not uncommonly jump straight from basaltic to rhyolitic products. If intermediate magmas are largely absent, the eruptive sequence is called bimodal. Bimodal sequences used to be taken as markers for continental rifting, but many bimodal arc sequences have been recognized as well.

Volcaniclastic and Metavolcanic Rocks

Clastic sediments derived from solid volcanic source rocks are called volcaniclastic and are usually classified and mapped as sedimentary rather than igneous. Volcaniclastics and their metamorphic derivatives deserve special mention here as an important transitional rock type in Colorado's physical evolution.

Clastic volcanic sediments can accumulate in large volumes around active magmatic arcs and volcanic fields. Their fine-grained feldspar and ferromagnesian minerals quickly weather to clays, and their free quartz crystals are already silt-size or smaller. Arkose (feldspar-rich) and lithic sandstones are common in immature volcaniclastic sediments deposited near the source, while silty mudstones are common among mature volcaniclastic sediments. Immature volcaniclastics shed to the east from early Laramide volcanic highlands in the Front Range uplift accumulated to form the Denver, Arapahoe and Dawson Arkose formations at the top of the Denver Basin.

Metavolcanics

1.7 Ga metavolcanics

Under regional metamorphism, volcaniclastics tend to evolve along the shale pathway detailed below. In metamorphic form, arc-derived 1.7 Ga volcaniclastics make up a good share of Colorado's Precambrian basement, including the craggy hornblende gneiss exposed atop Royal Mountain (right).

Volcanic Map Units

On the Geologic Highway Map of Colorado, the surviving volcanic outcrops are all Tertiary in age. The Oligocene andesites labeled "Tov" in brown are by far the most extensive and occur primarily in the San Juan and Thirtynine Mile volcanic fields. Widespread Miocene-Pliocene basalts and bimodal products are mapped as "Tuv" in orange. The legend shows a Quarternary basalt unit "Qv" in stippled orange, but I have yet to spot a mapped occurrence—not even at the Dotsero Volcano, last active about 4 Ka. (Tweeto's 1979, 1:500,000 Geologic Map of Colorado correctly shows a Quaternary basalt unit there.)

Top    Page Index

Intrusive (Plutonic) Rocks

Skip to Sedimentary Rocks

West Elk intrusions

By definition, intrusive igneous rocks solidify from a melt before reaching the surface. The term plutonic (after Pluto, god of the underworld) is synonymous with intrusive. Coherent bodies of plutonic rock are called intrusions. The host rocks surrounding the intrusion are referred to as country rocks. Intrusives are often involved in the plumbing system of a volcanic edifice or field, as were the beautifully exposed Oligocene intrusions seen at right in the West Elk Mountains, but some never communicate with the surface while still molten. Like their extrusive counterparts, intrusive rocks are categorized on the basis of texture and composition.

Mount Evans batholith

The highest point on the horizon in the photo at right is Mount Evans (14,264') as seen from Denver. Its summit area exposes a large intrusion (batholith) of resistant granite emplaced during the 1.4 Ga Berthoud Orogeny, perhaps the most common intrusive rock type found in Colorado.

Intrusive Textures

Granodiorite boulder

Slower subsurface cooling times lead to larger average intrusive grain sizes—much larger than the microscopic grains typical of extrusive (volcanic) rocks. Magmas solidifying underground tend to cool very slowly, in part because the already warm solid country rocks surrounding the intruding magma are effective thermal insulators. Individual crystals commonly reach 2-4 mm diameters, but in pegmatites, the quartz and feldspar crystals can exceed 100 mm. Such igneous textures are called phaneritic.

The grains in the granodiorite bolder at right are easily visible with the unaided eye. This particular boulder came from an Oligocene intrusion exposed along the lower Cathedral Peak in the Elk Range.

Intrusive Compositions

Most intrusive rocks contain the minerals shown in the diagram below. From left to right in the mineral plot at the bottom of the diagram, the rocks go from mafic (Mg- and Fe-rich) to felsic (feldspar- and silica-rich) in composition. 

A mix of light and dark grains is typical in intrusive rocks. The light-colored grains include quartz (usually colorless to gray) and one or more feldspars (off-white to pink, green or gray). Micas, if present, tend to be either black (biotite) or white (muscovite). Olivine, pyroxene and hornblende range from green to black. In intrusive rocks, all these minerals have characteristic crystal shapes readily observable with a hand lens when present, but the individual crystals aren't always well developed.

Source: J. Johnston, USGS, Types of Igneous Rocks

Common Intrusive Rocks

Intrusive rocks are easily recognized as such but difficult to classify, particularly in the field. The classification schemes used by geologists are far too complex to discuss there, but the most common intrusive rocks are worth exploring.

Most of the intrusives found in Colorado are light-colored felsic (feldspar- and silica-rich) to intermediate rocks along granite-granodiorite lines. Many of the Colorado rocks called and even mapped as "granites" turn out to be granodiorites or other felsic types on closer inspection. A few mafic (Mg- and Fe-rich) Laramide intrusives have been found in Colorado, but mafic intrusions are not uncommon worldwide. 

Table Note: The external links above lead to Lynn Fichter's well-illustration igneous rock descriptions.

Common Intrusive Forms

Depending on the composition of the magma, the final depth and the nature of the country rocks, intrusions can take many forms.

Intrusive Map Units

On the Geologic Highway Map of Colorado, 1.7 Ga granites are marked "Xg" and mapped in light gray. The 1.4 Ga Berthoud and 1.1 Ga Grenville granites are lumped under "Yg" in a darker gray. The Laramide intrusions are tagged "TKi" and mapped in a dark purple, while the mid-Tertiary intrusions are marked "Tmi" in hot pink. The TKi units are pretty much confined to the Colorado Mineral Belt; the Tmi units are also heavily concentrated there but have a much wider distribution, including an occurrence at Spanish Peaks. Younger intrusives under 20 Ma labeled "Tui" in stippled pink mainly occur north of Steamboat Springs.

Top    Page Index

Sedimentary Rocks

Skip to Chemical Sedimentary Rocks

Sedimentary rocks form from

• the accumulation and consolidation of loose surface materials, like sandstones and shales

• the chemical precipitation of minerals from over-saturated waters, like evaporites

• the remains or secretions of living organisms, like limestones and cherts. 

Because they're a lot more stable under surface conditions than igneous or metamorphic rocks, sedimentary rocks now cover two-thirds of the total area of the continents to an average depth of ~0.5 km (1,800').  By volume, sedimentary rocks are about two-thirds mudstones world-wide. In Colorado, the sedimentary cover ranges from absent over the Precambrian cores of Laramide uplifts to 4.0 km (13,000') thick at the west end of the Denver Basin.

It will once again prove useful to categorize sedimentary rocks, but bear in mind that there will always be transitional rock types. Nature, in her devotion to entropy, just loves to mix things up. This section will continue with the dominant class of sedimentary rocks known as clastics. The following section will describe the chemical sedimentary class—a very different kettle of rocks indeed.

For more on sedimentary rocks, visit the extensive and well-illustrated ^sedimentary rock site by educator and geologist ^Lynn Fichter. They're not as dull as you might think. For an introduction to stratigraphy, the science of reading stacks of sedimentary rocks, see Formations and Sequences below.

Clastic Sedimentary Rocks

Clastic sedimentary rocks form from clasts (fragments) weathered or otherwise disaggregated from other rocks. A synonym for clastic is detrital. Clastic rocks can be categorized in any number of ways, but most classification schemes key on both grain size and composition, which together tell a lot about the source rock, the depositional environment, and all the steps in between. Clastic rocks are generally held together by the interlocking of grains and by chemical cements, which also add color. Sandstones, shales and carbonates like limestone account for over 95% of all sedimentary rocks because they're the most stable rocks at surface conditions. When the source rocks are volcanic, the sediments are said to be volcaniclastic.

Clastic Sedimentary Classification Schemes

I can't improve on the sedimentary classification web sites developed by geologist and educator ^Lynn Fichter, so I'll just link his explanation of the ^QFL (quartz, feldspar, lithic) naming system here. The QFL system takes into account both texture and composition, but its use is beyond the scope of this site.

Clastic Grain Sizes

Clastics can be subdivided and are often named according to their average or dominant particle size according to the logarithmic Wentworth Grain Size Scale summarized in the table below.

Technical Note:  The ø (phi) scale facilitates grain size statistics; ø = -log₂ mm. It's worth repeating that the Wentworth scale is about grain size, not composition. Any particle in the 0.0625-4.0 mm range is sand, whether it's made of pure quartz or feldspar or undisaggregated rock fragments. In the last case, we might speak of a lithic sandstone, but it's a sandstone nonetheless. The same applies to the clay category, but here, size implies composition: Few clastic sedimentary rocks composed of clay-sized grains will contain anything other than clay.

Sedimentary Rock Names Based on Grain Size 

Common sedimentary rock names like sandstone, mudstone, siltstone, and claystone come directly from their average grain sizes, as defined in the Wentworth grain-size scale above. Thus, a rock with grains averaging 0.375 mm across is a medium sandstone. 

Shale, a term synonymous with mudstone, includes any fine-grained clastic sedimentary rock originally made of mud. Thus, siltstones and claystones are both shales. Pelite is yet another name for mudstones. Wind-deposited silts bears the special name loess. 

Of course, all size combinations are possible, including sandy siltstones and silty claystones. The term wacke lumps together silty and shaley sandstones, which can be hard to tell apart in the field. If a gently bitten shale feels gritty between the teeth, it's one-third to two-thirds quartz silt and may qualify as a wacke. If it feels smooth or creamy, it's at least two-thirds clay. 

Fountain conglomerate

A conglomerate is dominated by rounded gravel-sized clasts (> 2 mm), while a breccia contains angular gravel. At right is the basal layer of the Fountain Formation at Red Rocks Park, a moderately coarse conglomerate containing rounded pebbles and granules up to ~50 mm across in a sandy matrix—a typical terrigenous deposit. Since such deposits develop in alluvial fans, they're sometimes called fanglomerates.

Grain Sorting and Shapes

The standard deviation of grain sizes found in a clastic rock is a measure of its degree of sorting, which in turn says something about the rock's porosity, permeability and depositional history. In the field, the standard deviation is half the size range that includes two-thirds of all the grains. If two-thirds of all the grains in our medium sandstone above fall between 0.5 and 0.25 mm, the standard deviation is (0.50-0.25)/2 = 0.125 mm. Clastic rocks deposited near their source tend to have poorly sorted grain sizes. Well-sorted rocks are both more porous and more permeable than poorly sorted rocks with the same average grain size. The degree of sorting is a measure of the textural maturity of a clastic rock.

Grain shapes also tell a story. Grain shapes are usually described in terms of their sphericity (the degree to which all dimensions are equal) and roundness (the lack of sharp corners). Sphericity is often controlled by composition. Grains can be rod-shaped, disc-shaped, or spherical. The degree of rounding depends on grain size and hardness and on details of transport, deposition and diagenesis.

Clastic Grain Compositions

Sedimentary rocks typically include grains of the following components: