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The Grand Canyon's walls display a cross section of the Earth's crust.
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Josemaria Toscano/Shutterstock
Earth's Crust
From Grolier's New Book of Popular Science
Convection currents in the Earth’s mantle, thought to result from the intense heat of the core, may be responsible for the movements of the tectonic plates that make up Earth’s crust.
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Grolier
Earth’s crust, the relatively thin outermost layer of the planet, is composed primarily of just eight elements.
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Grolier

A patch of bare rock that interrupts the more pleasing pattern of a lush meadow or green forest reveals the true nature of Earth's crust, the solid outer layer of the planet.

The soil of the land, upon which life flourishes, is an exceedingly thin film. So, too, is the layer of ooze that carpets the ocean floor. Beneath the blanket of these sediments is bedrock, the layer of solid rock that makes up Earth's crust. When compared to the planet's actual diameter, Earth's crust is itself relatively thin, extending downward for some 22 miles (35 kilometers) under the continents and about 4 miles (6 kilometers) beneath the ocean floor.

There are two main types of crust: the continental crust, which forms the landmasses and consists primarily of low-density rock such as granite; and the oceanic crust, which is a thin layer of volcanic rock called basalt that lies under the seas.

The ocean crust is younger than the crust that underlies the continents. This is because new crust is constantly formed along a ridge that runs through the middle of the world's oceans. The new crust material wells out of Earth's interior and cools as it emerges from the mid-oceanic ridge, causing the seafloor on either side of the ridge to spread outward. It is this creation of new oceanic crust that is believed to power the slow movement of the continents across the face of Earth.

The rocky crust of Earth—both the oceanic and the continental crusts—is made up of three kinds of rock:

igneous,  sedimentary, and  metamorphic.

Igneous rock forms when molten rock cools either within Earth's crust or on its surface. The crust of the primitive Earth is believed to have once consisted entirely of igneous rock. Sedimentary rock is made from small fragments of eroded rock and the remains of tiny sea animals and plants that have become compacted and cemented together over thousands of years. Most sedimentary rocks are formed on the ocean floor. Metamorphic rock is derived from preexisting rock that sinks deep into Earth and is subjected to intense heat or pressure or both, causing the characteristics of the rock to change.

IGNEOUS ROCKS

The molten mass from which igneous rocks form is called magma. It contains various gases in solution, but, for the most part, it is made up of rock-forming minerals dissolved in a more or less haphazard arrangement.

According to contemporary theory, Earth was never in a completely molten state during its formation. Instead, Earth was formed from a mixture of the minerals that are now found segregated into the core, mantle, and crust. Then, as heat accumulated through radioactivity, melting occurred deep within Earth. By about 3.8 billion years ago, this melting had resulted in the layering of materials according to their density: the densest elements formed the core of Earth, while the lightest materials floated to the planet's surface and cooled, forming a solid crust. In between the core and crust was the largely molten mantle.

Earth's crust is constantly being recycled. Large pieces of the crust called tectonic plates are in constant motion. Where the plates meet, the edge of one may dive beneath the other, becoming partially or entirely molten as it reaches the mantle. At the same time, fresh mantle materials push their way upward into magma chambers, from which they are expelled into the crust as new igneous rock.

In some cases, the advancing magma is blocked by layers of solid rock in its path, and only the gases in the magma escape. The magma gradually cools and solidifies under the rock layers that make up the upper part of Earth's crust. Igneous rocks that form in this way are called intrusive masses.

In other cases, the magma continues moving upward to the surface, unimpeded by surrounding rocks. When it reaches the surface, it discharges through a volcanic vent or some other opening in the crust. Igneous rocks that form from magma extruded, or thrust out, from Earth are known as extrusive masses.

Intrusive Masses. Intrusive masses of igneous rock form a considerable portion of Earth's crust. On the basis of their shape and their relation to the rocks that surround them, they may be divided into several types—dikes, sills, laccoliths, volcanic necks, batholiths, and stocks.

Dikes are formed by magma that has cut its way across existing rock layers and solidified in the fissures created by its movement. In certain instances, dikes are visible. This is either because the original fissures extended to the surface or because the rocks that once covered them have eroded away.

The mass of igneous rock called a sill is formed in much the same way as a dike, except that the magma of a sill thrusts itself into the natural spaces between layers of the surrounding rock. Typically the magma displaces the rock beds between which it flows. The extent to which the surrounding rock is shifted determines the thickness of the resulting sill. Some are several hundred feet thick. A sill may be horizontal, or it may lie tilted along with the rock layers in which it forms. The latter situation may result when magma flows between the layers of already tilting or dipping rock beds. Or the sill may have originally formed between horizontal rock beds, which were later shifted by tectonic or volcanic forces.

Sometimes, during the formation of a sill, the magma does not spread evenly, but instead causes the overlying layer of host rock to arch upward. The resulting igneous mass, having the shape of a lens in cross section, is called a laccolith. It has a flat floor, connected by a "stem" to the magma source below. As a whole, it resembles a toadstool in side view.

The vent of a dead volcano may be filled with a cylindrical mass of solid lava known as a volcanic plug, or neck, which may be connected to a large underground mass of igneous rock called a stock. The stock may have supplied lava to the erupting volcano when it was active. Dikes and sills often emanate from the plug. If the surrounding rocks have been eroded away, the plug stands like a tower.

Huge, intrusive masses of igneous rock enlarging and extending downward to unknown depths are called batholiths. They differ from laccoliths in that they do not show signs of a definite floor. The magma from which a batholith is formed made its way through rock layers and not between them. As in the case of other intrusive masses, the batholiths are exposed to view only when the overlying rocks erode away. One of the world's largest batholiths forms the Sierra Nevada Range in California. It has been uncovered over wide areas by erosion. Batholiths form the cores of many other mountain ranges as well.

Extrusive Masses. There are several kinds of extrusive masses. Once magma reaches the surface, it is properly called lava, which then solidifies in one of various forms of igneous rock such as obsidian, pumice, or basalt. Sometimes magma is extruded through a volcanic vent with explosive force. The result is a spray of ash and other fragments. When these fragments cement together, they form pyroclastic rocks. Pyroclastic rock made primarily of fused ash is called tuff; when composed of larger particles, it is called volcanic breccia.

Minerals in Igneous Rock. The minerals that have contributed to the formation of igneous rocks are rather limited in number and variety. The most abundant and widespread of all minerals is quartz, a very hard compound consisting of silicon and oxygen. Feldspar, which is almost as abundant as quartz, is light to dark in color, and contains potassium, sodium, or calcium, as well as aluminum, silicon, and oxygen. Pyroxene and hornblende make up a family of diverse and important rock-forming minerals that contain various metals plus silicon and oxygen. The micas are sheetlike minerals composed of aluminum, other metals, oxygen, and silicon. Magnetite, an iron-oxygen compound, is heavy and magnetic. Olivine consists of iron, magnesium, oxygen, and silicon.

Different Textures. Igneous rocks differ from one another in texture: that is, in the size, shape, and arrangement of their internal particles. The size of the individual grains depends upon the rate at which the magma cooled. If the process was slow, the mineral crystals had a longer time to form, and so grew comparatively large. The result is a coarse-grained rock. If, however, the magma cooled rapidly, the crystals did not have much time to grow. The resulting rock is fine-grained. Generally there is also a relationship between the depth at which the rock was formed and the coarseness of its grain: the greater the depth, the coarser the grain.

Varieties of Igneous Rocks. Several hundred varieties of igneous rocks are recognized by geologists; the crust of Earth is made up principally of only a few kinds. One of the most common is the plutonic, or deep-seated, rock called granite, which occurs in the form of huge, irregular masses. Granite consists chiefly of quartz and feldspar. It also contains some hornblende and mica (the black variety called biotite). Generally this type of rock has an even texture. The variety of granite called pegmatite, which is coarse and grainy, is often found in dikes. Pegmatite is the source of much of the white mica used by industries.

The basalts, the most common of all the extrusive igneous rocks, are black, brown, dark gray, or dark green. The basalt formation in the Columbia Plateau, in the northwestern part of the United States, covers an area of more than 200,000 square miles (520,000 square kilometers), and ranges to more than 4,000 feet (1,200 meters) in thickness. Other sizable basalt formations occur in western India and in the spectacular Giant's Causeway on the north-facing coast of Northern Ireland.

Rhyolite, an extrusive rock with a very fine grain, is similar to granite in composition. It may have a porphyritic texture if there are large crystals of feldspar, quartz, or mica in it. (Porphyritic rocks contain a mixture of coarse and fine grains.)

Among the most striking of the glassy rocks found among the extrusive masses is obsidian—a lustrous rock that is black, gray, yellow, or brown. It is found in many different parts of the world, including California, Mexico, Hungary, and New Zealand. One of the largest and best-known formations is Obsidian Cliff, in Yellowstone National Park in the western United States.

SEDIMENTARY ROCKS

Some sedimentary rocks are formed from particles of igneous rock. The igneous-rock layers exposed at the crust's surface are continually weathered by forces such as temperature fluctuation, water, and wind. This breaks the igneous rock into fragments that are then swept along by winds, glaciers, streams, and shore currents. Once deposited in lakes or in shallow parts of the sea, they are pressed together under the weight of later sediments. Gradually, they may cement together and transform into sedimentary rock. Sedimentary rock is also formed from the decayed remains of ancient plants and animals as well as from the minerals left behind by the evaporation of seawater.

Sedimentary rock makes up most of the surface of Earth's crust. Each layer of sedimentary rock ranges in thickness from a few inches to several feet. In some places, the crust consists of only a few layers; in others, there are vast accumulations of sedimentary beds, several miles thick.

Generally speaking, the older the sedimentary-rock bed, the more thoroughly cemented is the sediment within it. The fragments of young rocks are so loosely held together that they can often be broken apart with a spade. Older sedimentary rocks, by contrast, are generally so solid that they cannot be quarried unless they are drilled, blasted with dynamite, and then broken into smaller fragments with a pick or sledgehammer.

Fragmental, or Clastic, Rocks. The sedimentary rocks that are made up entirely of particles of other rocks are known as fragmental, or clastic, rocks. The fragments from which the clastic rock is derived are generally classified on the basis of size, in four groups:

gravel,  sand,  silt, and  mud.

Gravel is the coarsest sediment of all. It is a loose deposit of fragments that range from small pebbles at least 0.08 inch (2 millimeters) in diameter to big boulders. Sand sediments consist of rounded grains ranging from 0.08 inch in diameter to 0.002 inch (0.062 millimeter). Particles smaller than 0.002 inch (0.062 millimeter) but greater than 0.0002 inch (0.0039 millimeter) in diameter are called silt. The finest sediments—muds and clays—consist of fragments less than 0.0002 inch (0.0039 millimeter) in diameter.

When rounded gravel particles—pebbles, cobbles, and boulders—are cemented together, they form the type of rock called conglomerate. In the variety known as breccia, the gravel fragments are not rounded, but angular.

Cemented sand grains give rise to the porous formation known as sandstone. The pores of this rock may make up as much as 30 percent of its total volume. Liquids move quite freely through the pores. For this reason, sandstones are often reservoirs for oil deposits, as well as groundwater. Silt particles are converted into siltstone; mud particles, into mudstone and shale.

Rocks Derived from Plants and Animals. Coal, the most common rock derived from plants, was formed from the green or woody remains of plants that thrived ages ago in swampy areas. These plant remains were buried under later sediments. The various strata sank under ancient seas and rose periodically. Some were folded and broken up by crustal movements. As a result of tremendous pressure, intense heat, and the exclusion of air, the organic materials were transformed in the course of time into coal. Peat, lignite, bituminous coal, and anthracite are successive stages in this transformation.

Animal remains also have contributed to the formation of various familiar kinds of sedimentary rocks. Coral reefs and coral islands are built of the skeletons of countless coral polyps and other marine organisms. One generation of coral after another has contributed to these huge deposits.

The rock called limestone is composed of calcite, or calcium carbonate, a mineral compound of calcium, carbon, and oxygen. Calcite is commonly found in the shells of animals such as snails, clams, brachiopods, and other shellfish. Many limestone beds are derived from such shells. Other limestones are made up of calcium carbonate that precipitated directly out of water. Some limestone deposits arise on land, others beneath oceans and lakes. As waters retreat, marine deposits are exposed and become part of the land.

Evaporites. Some sedimentary deposits are formed from salts that have precipitated, or separated, out of water as it evaporated. These sediments are called evaporites. They may develop from seawater or from the freshwater of streams and lakes. Many of the salts in question are found in both oceanic and inland waters. The concentrations of the salts may differ, and the mineral deposits they form vary accordingly.

Evaporites generally develop under dry, hot conditions. An arm of the sea may be cut off and become a lake of concentrated brine as the water evaporates. Once evaporation starts, the least soluble compounds precipitate first. In deposits derived from seawater, calcium carbonate and iron oxide form the first, or lowest, layer. Then comes gypsum, a sulfur-oxygen compound of calcium.

Common salt, or rock salt, is the next to precipitate. Various other compounds, such as sulfates of magnesium and sodium, precipitate later. These four groups of dissolved minerals, when present together, form successive layers of evaporites from the bottom up. Such deposits persist as long as conditions remain dry. Otherwise, returning waters or rain redissolve the evaporites or cover them over with layers of mud and sand.

Fossils. Accumulating sediments can contain the remains of plants and animals. These sometimes turn into stone or leave their imprints in the surrounding rocks—and so produces fossils. Scientists study fossils to gain clues about the evolution of life through the ages. All known fossil-containing sedimentary rocks, arranged layer on layer, make up what is known as the stratigraphic column. It is the geologist's historical standard, used by scientists to date geologic events.

METAMORPHIC ROCKS

The third classification of rock—metamorphic—includes igneous or sedimentary rocks that have been transformed deep within Earth's crust. Preexisting metamorphic rock can also be altered to create new forms of metamorphic rock. Whatever the original material, some traces of its structure are usually preserved in the resulting metamorphic rock.

Formation. One of the most important factors in metamorphism—the formation of metamorphic rock—is pressure. It may be applied by the weight of overlying sedimentary beds; it may be caused by magma making its way into surrounding rock layers; or it may be due to the mountain-building forces that deform Earth's crust. These colossal pressures generate tremendous heat that quickens and heightens various chemical reactions within the rock. The presence of water is another factor in metamorphism. The water may be so scanty that it forms a mere film around the particles. Yet it provides a medium in which rock substances can pass into solution and from which they can condense on the surface of new and growing crystals.

Under extreme heat and pressure, the original rock particles are forced into new arrangements. In some cases, the rock constituents recombine with those in the immediate vicinity and form new minerals, many of which grow with nearly perfect crystal form. Garnet is such a mineral.

Layering. Metamorphic rocks may exhibit layers resembling those of sedimentary rocks. Metamorphic layering, or foliation, is due to mechanical and chemical changes in the original rock. Grains, crystals, and fossils are shifted or broken up and strung out in a linear series. Parallel rows of platelike minerals, not at all related to the original bedding, may form. Shale is changed by pressure into slate—a dark rock with an extremely fine grain that splits readily into thin, smooth slabs. Slate may later become a rock called phyllite and, with continued pressure, a crystalline foliated rock known as schist.

Phyllite represents a stage intermediate between slate and schist. Its grain is very fine, consisting primarily of mica, but here and there a few larger crystals appear. The foliation is rougher and wavier than that of slate. Schist is visibly crystalline, with wavy foliation. It consists of such flakelike or tabular minerals as mica, chlorite, hornblende, or talc, as well as distinct crystals of quartz and garnet.

Other kinds of rock, such as sandy shales and granites, for example, are transformed into a more coarsely foliated crystalline rock called gneiss. Gneiss looks irregular and streaky because it has alternating layers of different minerals. Gneisses originate from several different kinds of rock. Each type of gneiss has much the same composition as the mother rock. Granite gneiss, for example, is derived from granite.

Some types of metamorphic rock show no foliation at all. Pure sandstones are changed into a more compact mass called quartzite, where pore spaces have been compressed, making a very hard and durable rock. Limestone is converted by heat and pressure into marble, in which the carbonate grains become visibly crystalline. Pure marble is white. Most limestones, however, contain impurities that often react under metamorphism to produce new minerals. These often impart striking colors or mottling to the resulting marble, making it more attractive as a building stone.

The Rock Cycle. Metamorphism in reverse also occurs. Once the transforming forces cease, metamorphic rocks tend to return to their original unaltered condition. Rocks may be remelted into magmas. Or they may be thrust to the surface and worn away into particles that eventually form sedimentary rock. The rock cycle is ongoing. And while it is useful to think of this cycle as starting with igneous rock, progressing through sedimentary, and ending up as metamorphic, the actual order is variable. The net result is a continuing geologic cycle in which new rocks are constantly formed to replace those destroyed by the effects of air and water.

 

Terence T. Quirke