We are studying stuff like sedimentary rocks, metamorphic rocks, weathering, lithification, etc. The teacher (maybe) and librarian don't like wikipedia, but I can't find any other good sites for the info. Please send links, as well.
2006-10-04 10:46:38 · 3 answers · asked by George 3 in Education & Reference ➔ Homework Help
If your teacher (maybe) and librarian don't like wikipoedia but you can't find any other good sites for the info, you haven't looked very hard. There is a box on this page entitled 'search the web'. All you have to do is type Black Canyon of Gunnison into that slot, hit the 'enter' button and you'll be knee deep in information.
The reason your teacher (maybe) and librarian don't like wikipedia is because it is an unreliable source of information. It is made up of 'facts' that people put on the internet, which may or may not be genuine.
2006-10-04 10:51:06 · answer #1 · answered by old lady 7 · 0⤊ 1⤋
INTRODUCTION: WRITTEN IN THE ROCKS
Like pages in a book, the rock layers of Black Canyon and Curecanti tell a story of past environments, ancient animals and dynamic processes of change. But unlike a book that we can read in a short time, this geologic book has to be read from a different point of view. Time is thrown out of balance here, and we need to see the land from a very different perspective.
GEOLOGIC TIME
Time is an everyday part of our lives. We keep track of time with a marvelous invention, the calendar, which is based on the movements of the Earth in space. One spin of the Earth on its axis is a day, and one trip around the sun is a year.
While this concept seems rather straightforward, the calendar we use today is very different from earlier versions. It is a great achievement, developed over many thousands of years as theory and technology improved. For centuries, scholars have sought to understand time and its relationship to the age of the Earth. Today, geologists estimate that the Earth is 4.6 billion years old! Who can fathom such an expanse of time?
Geologists have designed a very special type of calendar in order to grasp Earth's long history. This geologic time scale is very different from the familiar calendar we use to keep track of our busy lives. In some ways, the geologic time scale is more like a book, with the rocks as pages. Some of the pages are tattered and torn, and some are missing -- especially the early parts. To make matters worse, the pages aren't numbered. Luckily, geology gives us the tools to help decipher and read this incredible book!
Just like a calendar is divided into months, weeks, days, and so forth, the geologic time scale has its own unique set of time divisions. The largest division is called an eon. Eons, which can span billions of years, are subdivided into eras, which are subdivided into periods, which are subdivided into epochs, which are subdivided into ages, and... well, you get the picture!
The names used to designate the divisions of geologic time may seem bewildering at first glance, but nearly every name represents an historic breakthrough in geologic thought.
Most of the rocks exposed along the length of the Black Canyon are Precambrian in age (older than 500 million years) and are either metamorphic, or igneous, with some sedimentary layers evident along the North Rim. The rocks in the Black Canyon have a wide variety of minerals. Here is a brief look at some of them and where they may be found.
METAMORPHIC ROCK
The word "metamorphic" has its origins in the Greek language and means to transform or change. Metamorphic rocks usually start out as sedimentary, or igneous rock, but when buried deep in the earth, intense heat and pressure "cook" or "bake" them into a completely new rock. Heat and pressure are the two most important parts of this process, but time also plays a role; the longer the rock has been baked and squeezed, the greater the changes.
Rock is usually buried deep within the Earth's crust (six to eight miles, for instance) before temperatures and pressures are high enough to melt and change their physical and chemical composition. Black Canyon's metamorphic rocks have been altered to the point that little trace of the original rock remains. However, geologists suspect that the original rocks, or protoliths were sands, mud and volcanic debris that accumulated on the floor of an ancient sea. The time of metamorphism is estimated at 1.7 to 1.9 billion years ago. Gneiss and schist are examples of metamorphic rocks found in the Black Canyon. These rocks blend from one to another because of variations in the heat and pressure which occurred when some rocks were buried deeper than others.
Gneiss
Gneiss represents some of the most advanced stages of metamorphism, with the most intense temperatures and pressures exerted upon the rock. That means the original rocks were buried deeper and were hotter, almost to the point of melting. In places the rock has been partially melted and the melt was injected, or squeezed into the layers of the remaining solid portions of the gneiss, creating a type of gneiss known as migmatite. Migmatite gneiss is a rock that almost melted and is an intermediate between igneous and metamorphic. More information »
Schist
Schists are the other metamorphic rocks found in the Black Canyon and are at the other end of the heat and pressure scale. The original rocks (protoliths) were not buried as deeply so there was less heat and pressure. Although still considered metamorphic, these schists have been altered less because of the lower pressures and temperatures. More information »
IGNEOUS ROCKS
Igneous rocks are those that cooled from a molten rock, or magma, deep beneath the surface of the earth. If magma cools before it reaches the surface, it is called intrusive. Magma that reaches the surface, as in a volcanic eruption, is referred to as extrusive. Examples of igneous rocks in the Black Canyon are intrusive rocks. Here the magma was pushed into the existing metamorphic rock and never reached the Earth's surface. The striking, pinkish banding evident throughout the canyon walls is intrusive-igneous rock.
Quartz Monzonite
Quartz Monzonite may sound intimidating, but it's only a type of granite. Granite is a crystalline, igneous rock, composed mainly of quartz, orthoclase and microcline. The name monzonite means that the magma that created the rock had approximately equal amounts of sodium and calcium-rich feldspars. When "quartz" is added to the title, it means that a large amount of silica was present in the magma. Silica, when cooled, becomes quartz.
One of the most famous examples of the quartz monzonite is the Curecanti Needle (along Morrow Point Reservoir). The monzonite is harder than the metamorphic rocks and weathers more slowly. The needle was created when the waters of the Gunnison River and Blue Creek carved away the surrounding metamorphic rock, leaving the Needle in their wake. The third side is separated by weathering of a fault system.
Pegmatite
Pegmatite is another type of granite with the three main minerals of quartz, feldspar, and mica added to it. It has unusually large, intergrown crystals and is the last and most water-rich portion of a magma to cool. As magma cools and solidifies, water becomes concentrated. This concentration makes the magma more fluid and easier to squeeze, like toothpaste out of the tube, into the surrounding rock. The crystals can be huge - up to 6 feet in length. There were three episodes of pegmatite intrusion in the Black Canyon rocks. The most spectacular example of these intrusions is the Painted Wall, at 2,250 feet, it's the tallest cliff in Colorado.
ADDITIONAL RESOURCES
To learn more about the rock origins at Black Canyon consider the following sources:
Black Canyon/Curecanti Online Bookstore
A variety of titles dealing specifically with the geology of the Black Canyon and Colorado in general, are available for online purchase.
The Interior of the Earth
Online Edition by Eugene C. Robertson
This publication, available through the U.S. Geological Service, gives an excellent introduction to what lies below the surface of the Earth.
Natural Gemstones
Also from the USGS, this page provides an indepth look at gemstones from around the country.
http://www.nps.gov/blca/naturescience/minerals.htm
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.
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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.
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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.
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.
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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.
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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
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.
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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.
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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
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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.
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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.
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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
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.
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.
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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, O2, 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
[photo coming]
Banded tuff, Valley of 10,000 Smokes
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.
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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
[photo coming]
Banded tuff, Valley of 10,000 Smokes
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.
[photo coming]
Wall Mountain ignimbrite clast from Castlewood Canyon near Franktown, CO
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.
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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.
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"Black Canyon of the Gunnison National Park." Britannica Concise Encyclopedia. 2006. Encyclopædia Britannica Online. 3 Nov. 2006
Black Canyon of the Gunnison National Park. (2006). In Britannica Concise Encyclopedia. Retrieved November 3, 2006, from Encyclopædia Britannica Online: http://www.britannica.com/ebc/article-9357385
To cite this page:
MLA style:
"Black Canyon of the Gunnison National Park." Britannica Concise Encyclopedia. 2006. Encyclopædia Britannica Online. 3 Nov. 2006
Black Canyon of the Gunnison National Park. (2006). In Britannica Concise Encyclopedia. Retrieved November 3, 2006, from Encyclopædia Britannica Online: http://www.britannica.com/ebc/article-9357385
natural area in western Colorado, U.S., encompassing a deep, narrow gorge 15 miles (24 km) east of Montrose. It was established as a national monument in 1933 and was elevated to national park status in 1999; the park occupies an area of 47 square miles (122 square km). Curecanti National Recreation Area borders it to the southeast. The canyon was cut by the Gunnison River (named for the army engineer John W. Gunnison) and its tributaries. At the section where its walls are steepest, it is 10 miles (16 km) long with depths ranging from 1,730 to 2,425 feet (525 to 740 metres), while its rim width narrows to 1,300 feet (400 metres) and its floor width to 40 feet (12 metres).
The Black Canyon derives its name from its black-stained, lichen-covered walls, which accentuate the gloom of the chasm. Roads on the North and South rims, with overlooks and foot trails, reach about 8,000 feet (2,400 metres) above sea level. The South Rim, which is somewhat more accessible than the North, receives most of the park's visitors. Block “islands” and pinnacles form the canyon's most striking features. The Black Canyon is the habitat of mule deer, coyotes, bobcats, foxes, rock squirrels, and a wide variety of birds, including the golden and bald eagles. Most of the monument has a vegetation cover of Gambel oak and serviceberry.
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To cite this page:
MLA style:
"Black Canyon of the Gunnison National Park." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 3 Nov. 2006
Black Canyon of the Gunnison National Park. (2006). In Encyclopædia Britannica. Retrieved November 3, 2006, from Encyclopædia Britannica Online: http://www.britannica.com/eb/article-9015470
Hope this helps.
PS. None of this is from Wikipedia
2006-10-04 11:14:24 · answer #2 · answered by Shalamar Rue 4 · 0⤊ 0⤋
i dunno know
2006-10-04 10:48:22 · answer #3 · answered by Chelsie W 2 · 0⤊ 1⤋