ANTHOLOGY SAMPLE

Tributary 4

THE BASIC STUFF OF ROCKS

This part of the text restates some information about types of rocks presented in other Tributaries, as well as introducing more detailed information. Therefore, the kind of subject matter with a focus on geology, and more to the point, the substance of rocks, as well as a variety of rock formations that comprise the materials (building blocks) of the Grand Canyon (including all other environments). Because the canyon’s horizontally-stacked upper layers represent changing environments over hundreds of millions of years, each layer is different. Each layer is also uniform, varies in thickness, and each was laid on top of a timeworn mountain range (i.e., the Vishnu) that accounts for the primal base foundation. The distinctive, and mostly upright, basement rocks underlying the upper layers contrast nicely with the stratified formations above (i.e., arranged into strata). Everything seen on display given the Grand Canyon’s primeval geologic showcase is the result of three primary geologic events created by spent materials of ancient volcanoes long before the atmosphere was life-friendly, a soaring mountain range that was followed by something geologists refer to as fault-block mountains, and numerous shallow marine and freshwater seas that flowed into the interior of the continent. When the calm waters of the Paleozoic Era’s inaugural life-producing geologic chapter receded, these respective events (as defined by “periods”) deposited a record of mostly limestone and sandstone. It is also these sedimentary rocks that account for canyon’s upper tier. Indeed, there is an epic story of life viewed in these multicolored formations rising above the inner canyon gorge where the Colorado River flows.

To further illustrate this introductory and noteworthy information, sandstone and limestone, as well as shale and mudstone, can be thought of as ideal materials preserving the earliest life forms appearing on the planet. Hence, fossilized remains. Each formation can be thought of as a catacomb cataloging a benchmark of time when this or that specific life form appeared and died. Consequently, an untold number and variety of marine and terrestrial creatures that emerged on the planet relatively soon after the close of the much longer Precambrian Era (meaning, “before life”). Aquatic species, plants, trees, and insects were, therefore, the precursors, whose remains were later fossilized. (Bear in mind reptiles and mammals came after the close of the Paleozoic Era.) All of the earliest evolutionary life forms account for a replete roster of fossilized life on display inside the Grand Canyon’s formations. Swamps, rivers, lagoons, and, at least, one Sahara-like desert delineate the many changing environments that occurred here over hundreds of millions of years. 

To mention the obvious, everything about the Grand Canyon comes down to rocks—rocks from the ages made from different materials and fashioned over the eons by the physical and chemical agents of erosion.

The following particulars may be considered the official Geology 101 explanation of this text.

The major Building Blocks Of The Planet: The Grand Canyon is rock heaven and a geologist’s ideal haven. Rocks are divided into three basic families. Each is based on how their respective base materials form. Geologists classify all rocks in these three categories:

IGNEOUS

SEDIMENTARY

METAMORPHIC

Geomorphology entails the study of the physical features of the surface of the planet and their relation to its geological structures and rock has everything to do with the Earth’s landscape and features. The following breakdown will soon sort out what you know from what you don’t know about rocks. More importantly, what makes each rock’s ingredient what it is.

I) Igneous Rocks: This material originates from a melt (or magma) deep within the planet. Such rocks can be extrusive (i.e., volcanic) in origin or intrusive. Geologists place intrusive rocks in two categories: plutonic and hypabyssal. Plutonic rocks comprise great masses of material formed in mountain-building zones. Hence, deep below the planet’s surface. Some masses are formed by partial fusion of lower continental crust while others are formed from magma rising from the mantle. The cooling of magma yields large mineral crystals, denoting coarse-textured rocks (i.e., granite, diorite, gabbro, and peridotite). Granite, mainly that which is made of quartz, feldspar, and mica, is the main igneous rock of continental crust.

By contrast, hypabyssal rocks are relatively smaller masses, and often form strips or sheets. These materials cool at a lesser depth and much faster than plutonic rocks. Therefore, they are comprised of smaller crystals (i.e., microgranite, microdiorite, and diabase). All intrusive rocks produce the following features:

• Batholith (a huge deep-seated, dome-shaped intrusion, usually of acid igneous rock).
• Stock (similar to a batholith but smaller; also have an irregular surface area under 40 square miles/64.3 km or thereabouts).
• Sill (a sheet of usually basic igneous rock intruded horizontally between rock layers).
• Laccolith (a lens-shaped usually acidic igneous intrusion that domes overlying strata).
• Lopolith (a saucer-shaped intrusion between rock strata; up to hundreds of miles across).

Regarding extrusive igneous rocks, these occur mainly at volcanic vents along the active margins of lithospheric plates. Magma erupts as lava, which cools and hardens quickly on the surface as fine-grained or glassy rock (i.e., obsidian). When a magma flow comes into contact with cold water or land, it cools and crystallizes very quickly. Consequently, such rocks leave little or no time for the mineral crystals in the rock to grow to a very large size. Sometimes, the rocks cool so quickly that gasses from the magma are trapped in the rock, thereby forming gas bubbles that are also known as vesicles. These small holes are also plainly visible when the final product is complete (as a process). Examples of the most common volcanic rock types are basalt, andesite. and rhyolite. 

All basic lavas are rich in metallic elements though poor in silica. These lavas flow easily and erupt relatively gently. Basalt is the most famous kind of magma, which accounts for more than ninety percent of all volcanic rock. Basalt also forms by partial melting of peridotite, which is the chief rock of the upper mantle. Welling up from oceanic spreading ridges, the basalt builds new ocean floor material. There are also acid (i.e., silica-rich) extrusive lava flows. These appear at plate margins. Likely, these flows may comprise selected substances from the basic lava of the upper mantle or reprocessed crust. Acid lavas are, therefore, very explosive and slow-flowing. The lava produces such rocks as dacite, rhyolite, and obsidian.

The igneous contribution to the planet’s geophysical transformation is, of course, magma that has to go somewhere. Hence, occurring where heat melts parts of the planet’s upper mantle and lower crust. Most magma that has cooled and solidified oozes through the crust from oceanic spreading ridges. Smaller quantities come from destructive plate boundaries, colliding continents, and so-called “hot spots.”

In addition to the main magma release, there are intermediate lavas in the group that contain minerals of plagioclase, feldspar, and amphibole. Alkali feldspar and quartz are also found in this group. The intermediate lavas stem from partial melting of certain minerals in subducted oceanic crust.

Amazingly, some 860 known volcanoes have erupted in the last two thousand years. Those that emit continuously or periodically are considered active. Volcanoes that don’t erupt in recent times are labeled dormant while inactive volcanoes for a long period are said to be extinct. The cooling process of both extrusive and intrusive rocks is important for this reason: as the magma cools the chemical elements in the material begins to join together in crystalline forms or minerals. Accordingly, as the magma cools, it crystallizes and turns into solid rock. The time it takes for some of these rocks to cool may take minutes, years, or even hundreds of thousands of years.

II) Metamorphic Rocks: Think of this material being new rocks made from old rock material. In short, metamorphic rocks are morphed by physical or chemical alteration. By definition, the alteration occurs by extreme heat and pressure of an existing igneous or sedimentary material and results in a much denser form. Metamorphic rocks can be warped and deformed, thereby compacting into a smaller volume of space. Reasons for this have to do with plate tectonic collisions, stress, and shearing forces, as well as compression over long periods of time. Consequently, metamorphic rocks are denser than the original material these rocks came from. They also endure longer in the erosional process. Ideal examples of metamorphic rocks are the following:

• Schist (coarse grained)

• Gneiss (medium to coarse)

• Slate (fine grained)

• Marble (metamorphosed limestone or dolomite)

• Quartzite (coast grained derived from sandstone

At the bottom of the Grand Canyon, what can be called deep time, this basement metamorphic rock foundation mainly consists of schist and gneiss (i.e., a former granite), with sporadic intrusions of quartzite-like material (i.e., pegmatite). Metamorphic rocks make up a large part of the planet’s crust, and may be formed deep beneath the surface. Therefore, subjected to extremely high temperature and pressure from rock layers above. The material can also form from plate tectonic processes (i.e., continental collisions), causing horizontal pressure, distortion, and friction. As a geologic term, metamorphism occurs when rock material is heated by the intrusion of magma. 

Another facet of metamorphism entails metamorphic minerals that form only at extreme temperature ranges and coincide with exacting pressure. The so-called index minerals associated with the process include andalusite, garnet, kyanite, sillimanite, and staurolite. Other popular index minerals are amphiboles, feldspars, micas, pyroxenes, and olivines. Any of these minerals may be found in metamorphic rocks. 

Another feature within metamorphic rocks is called foliation. So-called foliated rock is a product of differential stress factors that deforms the rock in one plane and sometimes creates a plane of cleavage. Slate, which originates from shale, is an example of a foliated metamorphic rock. Marble, however, is usually not foliated. Hence, it is often used in architecture (i.e., countertops or flooring), and sculpturing. The essence of metamorphic rocks comes down to these distinguishing factors:

• Grains that make up the rock grow together tightly during metamorphism, which creates a stronger rock.
• Metamorphic rocks are made of different minerals compared to other kinds of rocks and typify a wide range of color, even luster.
• Because they are metamorphosed, these type of rocks tends to show signs of stretching or squeezing, denoting a striped appearance.

As for the physics of metamorphic rocks, extreme heat, force, and pressure are latent in the process. Given such extremes, minerals in most rocks break down and are altered into an entirely different set of minerals. It follows how metamorphic rocks have undergone a solid-state recrystallization process in direct response to temperature and pressure, and sometimes abetted by chemically reactive water (in the original material). During metamorphosis, the original grains may grow larger, then break down and form new material, and sometimes nothing happens. What ultimately happens, therefore, depends on the starting minerals and the pressure, temperature, as well as fluid conditions directly associated with metamorphism.

One more thing to mention about this subject matter and that is to consider how metamorphic rocks tend to be the best-looking variety in the rock showcase of Mother Nature. For example, part of the Grand Canyon’s basement rock complex is comprised of gneiss (pronounced “nice”). In these rocks are swirling or stripes of bands, with the befitting name gneissic banding. The banding is also a direct indication the rock was developed under high temperature and pressure. A common cause of the banding forms when the protolith (i.e., the original rock material under metamorphism) is subject to extreme shearing forces. These forces also stretch out the rock, as it were, almost like plastic material. The original material is, therefore, smeared out into sheets. One would have to see firsthand the lovely pink swirls mixed in with grayish colored material in varying shades, and then realize the winsome look of gneiss. Lighter bands indicate relatively more felsic minerals (i.e., igneous rocks relatively rich in elements that form quartz and feldspar while darker bands have relatively more mafic minerals (i.e., a silicate mineral or rock that is rich in magnesium and iron). 

III) Sedimentary Rocks: Sedimentary rocks are the easiest to understand their nature because they are the least complicated given how they form. This type of rock also forms entirely at the planet’s surface. Unlike igneous or some metamorphic rocks, the geologic clock in sedimentary rocks is not reset. This odd statement means the materials are not changed or altered from their original constituent properties as are igneous and metamorphic rock types. Primary sedimentary rock examples are limestone, sandstone, shale, and conglomerate. (i.e., similar to mudstone). These relatively softer rocks, as compared to the harder metamorphic rocks mostly account for the formations of the Grand Canyon’s upper formations, and are horizontally-placed strata. Of course, each sedimentary rock has different properties.

For instance, limestones are rich in calcium and magnesium carbonates. They make up about eight percent of all sedimentary rock; only shale and sandstone are more plentiful. Organic limestones also contain calcium carbonate extracted from seawater, by plants and animals that used this compound for their protective shells. Thus, limestone rocks include the following:

• reef limestones made from the stony skeletons of billions of coral polyps and algae inhabiting the beds of shallow seas; 
• coquina is a cemented mass of shelly debris; 
• chalk, which is a white, powdery, porous limestone comprising tiny shells of fossil microorganisms. 

All these microorganisms once drifted in the surface waters before they perished, then rained down on the bottom of the sea. By contrast, sandstone is a common sedimentary rock composed primarily of particles of sand, with minor amounts of silt and clay. These particles are cemented together, mainly by trace amounts of calcite or silica. Sandstone accounts for eight to element percent of the sedimentary rocks on the planet.

Add to this list the smaller fragments found in sedimentary rocks. For example, mudstone, like siltstone, as well as shale, which denote the very soft materials of sedimentary rocks. These classifications of rocks are made of clay minerals of less than 0.004 mm diameter while siltstone is formed of particles 0.004 to 0.06 mm in diameter. Shale accounts for more than eighty percent 80 of all sedimentary rock. Shale also indicates a low energy environment where silt and mud settles. Thus, the accumulation of silt and clay equals siltstone and mudstone that ends up as shale in one form or another. Rivers, floodplains, coastal tidal flats, lagoons, even deeper water offshore environments, typify places where sedimentary materials are found. 

All three of these rock types, including similar fine-grained rocks of silt and clay, are easily split along their bedding planes. This is why these much softer sedimentary rocks are fragile and easy to break. All sedimentary rocks form in three ways:

• CLASTIC—formed from pieces of preexisting rocks (i.e., Coconino Sandstone).

• ORGANIC—formed from the accumulated shells or body parts of once-living creatures (i.e., Kaibab Limestone).

• CHEMICAL—formed when minerals precipitate directly out of the water and later form rocks (i.e., travertine, which comes from calcium carbonate).

The Substance Of Rocks: All rocks are varied mixtures of minerals. Most consist of interlocking grains or crystals held together by a natural cement. There are a few dozen minerals that account for the main ingredients for the most common rocks. For example, silicates (i.e., quartz), carbonates (i.e., dolomite), sulfides (i.e., pyrite), oxides (i.e., hematite), halides (i.e., rock salt), hydroxides (i.e., limonite), sulfates (i.e., gypsum), phosphates (i.e., apatite), and tungstates (i.e., tungsten ore). For those who are interested in geology and its academic discipline, much can be gained by examining any rock, particularly, noticing the features in sedimentary rocks. Like fossils, the clues to the environment of deposition can be deduced by analyzing certain qualities within the rock. Consider this next example how to go about it: 

• Examining the texture of a rock by its size of grains––is it small like sand or large like a boulder? 
• Composition––does the rock contain clasts such as quartz, mica or feldspar?
• Roundness––is the rock well-rounded or perhaps partially angular? 

These aspects, including trace fossils within the rock, provide helpful clues to the origins of any rock. There is even a way to tell how far the rock sediment traveled from its source (i.e., if it's well-sorted, whereby all the particles are about the same size). Such scrutiny indicates the specimen is further from its original source. A good example is the Coconino Sandstone Formation, which is the third layer below the rim of the canyon. 

Other clues to understanding sedimentary rocks are the fingerprints found within the sediment. Here are some excellent examples:

• Raindrop impressions in sand or silt indicate “sub-aerial” conditions (i.e., continental).
• Worm burrows denote irregular tubes or imprints left in the rock caused by organisms moving around and were later in time often filled with another type of sediment.
• Horizontal stratification (i.e., bedding) indicates undisturbed sediments as laid down by the action of wind or water.
• Cross-bedding is another residual sign produced by wind or water current. 
• Mudcracks found in sedimentary rocks tell us these rocks were formed in environments that were alternately wet and dry. 

Combined, these telltale signs point the way to a floodplain environment. Indeed, and like a hardened fingerprint, nearly every rock tells its unique story. If the geologic clock has not been disturbed, as often happens with igneous and metamorphic rock types, it’s fairly easy to understand how and where the rocks were formed, not to mention how they came into existence. 

As a reminder, the information presented in all the chapters (Tributaries) can be read sequentially or disregarded, depending on the reader’s interest and what he or she chooses to peruse. Therefore, this anthology is written in the guise of an encyclopedia. Feel free to return to any Tributary’s subject matter when the need or urge arises. 

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