Rocks

Geologists define rocks as aggregates of minerals. Minerals are naturally occurring, inorganic substances with specific chemical compositions and structures. A rock can consist of many crystals of one mineral, or combinations of many minerals. Several exceptions, such as coal and obsidian, are not composed of minerals but are considered to be rocks. Common uses for rocks include building materials, roofs, sculpture, jewelry, tombstones, chalk, and coal for heat. Many metals are derived from rocks known as ores. Oil and natural gas are also found in rocks.

Prehistoric humans used rocks as early as 2,000,000 B.C. Flint and other hard rocks were important raw materials for crafting arrowheads and other tools. By 500,000 B.C., rock caves and structures made from stones had become important forms of shelter for early man. During that time, early man had learned to use fire, a development that allowed humans to cook food and greatly expand their geographical range. Eventually, probably no later than 5000 B.C., humans realized that metals such as gold and copper could be derived from rocks. Many ancient monuments were crafted from stone, including the pyramids of Egypt, built from limestone around 2500 B.C., and the buildings of Chichen Itza in Mexico, also of limestone, built around A.D. 450.

Since at least the 1500s, scientists have studied minerals and mining, fundamental aspects of the study of rocks. Georgius Agricola (the Latin name for Georg Bauer) published De Re Metallica (Concerning Metallic Things) in 1556. By 1785, the British geologist James Hutton published Theory of the Earth, in which he discussed his observations of rocks in Great Britain and his conclusion that Earth is much older than previous scientists had estimated.

Geologists, scientists who study the earth and rocks, distinguish three main groups of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. These distinctions are made on the basis of the types of minerals in the rock, the shapes of individual mineral grains, and the overall texture of the rock, all of which indicate the environment, pressure, and temperature in which the rock formed.

Igneous rocks

Igneous rocks form when molten rock, known as magma (if below the surface of the Earth) or lava (at the surface of the Earth), solidifies. The minerals in the rock crystallize or grow together so that the individual crystals lock together. Igneous rocks and magma make up much of the oceanic and continental crust, as well as most of the rock deeper in the Earth.

Igneous rocks can be identified by the interlocking appearance of the crystals in them. Typical igneous rocks do not have a layered texture, but exceptions exist. For example, in large bodies of igneous rock, relatively dense crystals that form early can sink to the bottom of the magma, and less dense layers of crystals that form later can accumulate on top. Igneous rocks can form deep within the Earth or at the surface of the Earth in volcanoes. In general, igneous rocks that form deep within the Earth have large crystals that indicate a longer period of time during which the magma cools. Igneous rocks that form at or near the surface of the Earth, such as volcanic igneous rocks, cool quickly and contain smaller crystals that are difficult to see without magnification. Obsidian, sometimes called volcanic glass, cools so quickly that no crystals form. Nevertheless, obsidian is considered to be an igneous rock.

Igneous rocks are classified on the basis of their mineral content and the size of the crystals in the rock. Extrusive igneous rocks have small crystals and crystallize at or near the Earth's surface. Intrusive igneous rocks cool slowly below the Earth's surface and have larger crystals. Rocks made up of dense, dark-colored minerals such as olivine, pyroxene, amphibole, and plagioclase are called mafic igneous rocks. Lighter-colored, less dense minerals, including quartz, mica, and feldspar, make up felsic igneous rocks.

Common igneous rocks include the felsic igneous rocks granite and rhyolite, and the mafic igneous rocks gabbro and basalt. Granite is an intrusive igneous rock that includes large crystals of the minerals quartz, feldspar, mica, and amphibole that form deep within the Earth. Rhyolite includes the same minerals, but forms as extrusive igneous rock near the surface of the Earth or in volcanoes and cools quickly from magma or lava, so its crystals are difficult to observe with the naked eye. Similarly, gabbro is more coarse-grained than basalt and forms deeper in the Earth, but both rocks include the minerals pyroxene, feldspar, and olivine.

Fabulous exposures of igneous rocks occur in the volcanoes of Hawaii, volcanic rocks of Yellowstone National Park (located in Wyoming, Idaho, and Montana), and in Lassen Volcanic National Park and Yosemite National Park (both in California).

Sedimentary rocks

Sedimentary rocks are those made of grains of preexisting rocks or organic material that, in most cases, have been eroded, deposited, compacted, and cemented together. They typically form at the surface of the Earth as sediment moves as a result of the action of wind, water, ice, gravity, or a combination of these. Sedimentary rocks also form as chemicals precipitate from seawater, or through accumulation of organic material such as plant debris or animal shells. Common sedimentary rocks include shale, sandstone, limestone, and conglomerate. Sedimentary rocks typically have a layered appearance because most sediments are deposited in horizontal layers and are buried beneath later deposits of sediments over long periods of time. Sediments deposited rapidly, however, tend to be poorly layered if layers are present at all.

Sedimentary rocks form in many different environments at the surface of the Earth. Eolian, or wind blown, sediments can accumulate in deserts. Rivers carry sediments and deposit them along their banks or into lakes or oceans. Glaciers form unusual deposits of a wide variety of sediments that they pick up as the glacier expands and moves; glacial deposits are well exposed in the northern United States. Sediments can travel in currents below sea level to the deepest parts of the ocean floor. Secretion of calcium carbonate shells by reef-building organisms produce large quantities of limestone. Evaporation of seawater has resulted in the formation of widespread layers of salt and gypsum. Swamps rich in plants can produce coal if organic material accumulates and is buried before aerobic bacteria can destroy the dead plants.

Sedimentary rocks are classified on the basis of the sizes of the particles in the rock and the composition of the rock. Clastic sedimentary rocks comprise fragments of preexisting rocks and minerals. Chemical precipitates are sedimentary rocks that form by precipitation of minerals from seawater, salt lakes, or mineral-rich springs. Organic sedimentary rocks formed from organic matter or organic activity, such as coal and limestone made by reef-building organisms like coral. Grain sizes in sedimentary rocks range from fine clay and silt to sand to boulders.

The sediment in a sedimentary rock reflects its environment of deposition. For example, wind-blown sand grains commonly display evidence of abrasion of their surfaces as a result of colliding with other grains. Sediments transported long distances tend to decrease in size and are more rounded than sediment deposited near their precursor rocks because of wearing against other sediments or rocks. Large or heavy sediments tend to settle out of water or wind if the energy of the water or wind is insufficient to carry the sediments. Sediments deposited rapidly as a result of slides or slumps tend to include a larger range of sediment sizes, from large boulders to pebbles to sand grains and flakes of clay. Such rocks are called conglomerate. Along beaches, the rhythmic activity of waves moving sediment back and forth produces sandstones in which the grains are well rounded and of similar size. Glaciers pick up and carry a wide variety of sediments and often scratch or scrape the rocks over which they travel.

Sedimentary rocks are the only rocks in which fossils can be preserved because at the elevated temperatures and pressures in which igneous and metamorphic rocks form, fossils and organic remnants are destroyed. The presence of fossils and the types of fossil organisms in a rock provide clues about the environment and age of sedimentary rocks. For example, fossils of human beings are not present in rocks older than approximately two million years because humans did not exist before then. Similarly, dinosaur fossils do not occur in rocks younger than about 65 million years because dinosaurs became extinct at that time. Fish fossils in sedimentary rock indicate that the sediments that make up the rock were deposited in a lake, river, or marine environment. By establishing the environment of the fossils in a rock, scientists learn more about the conditions under which the rock formed.

Spectacular exposures of sedimentary rocks include the Grand Canyon (Arizona), the eolian sandstones of Zion National Park (Utah), the limestones of Carlsbad National Park (New Mexico), and glacial features of Voyageurs National Park (Minnesota).

Metamorphic rocks

Metamorphic rocks are named for the process of metamorphism, or change, that affects rocks. The changes that form metamorphic rocks usually include increases in the temperature (generally to at least 392°F [200°C]) and the pressure of a precursor rock, which can be igneous, sedimentary, or metamorphic, to a degree that the minerals in the rock are no longer stable. The rock might change in mineral content or appearance, or both. Clues to identifying metamorphic rocks include the presence of minerals such as mica, amphibole, staurolite, and garnet, and layers in which minerals are aligned as a result of pressure applied to the rock. Common metamorphic rocks include slate, schist, and gneiss. Metamorphic rocks commonly occur in mountains, such as the Appalachian Mountains, parts of California, and the ancient, eroded metamorphic rocks in the Llano Uplift of central Texas.

Metamorphic rocks are classified according to their constituent minerals and texture. Foliated metamorphic rocks are those that have a layered texture. In foliated metamorphic rocks, elongate or platy minerals such as mica and amphibole become aligned as a result of pressure on the rock. Foliation can range from alternating layers of light and dark minerals typical of gneiss to the seemingly perfect alignment of platy minerals in slate. Some metamorphic rocks are unfoliated and have a massive texture devoid of layers. Mineralogy of metamorphic rocks reflects the mineral content of the precursor rock and the pressure and temperature at which metamorphism occurs.

As sediments undergo metamorphism, the layers of sediment can be folded or become more pronounced as pressure on the rock increases. Elongate or platy minerals in the rock tend to become aligned in the same direction. For example, when shale metamorphoses to slate, it becomes easier to split the well-aligned layers of the slate into thin, flat sheets. This property of slate makes it an attractive roofing material. Marble-metamorphosed limestone-typically does not have the pronounced layers of slate, but is used for flooring and sculptures.

Metamorphism of igneous rocks can cause the different minerals in the rocks to separate into layers. When granite metamorphoses into gneiss, layers of light-colored minerals and dark-colored minerals form. As with sedimentary rocks, elongate or platy minerals become well-aligned as pressure on the rock increases.

It is possible for metamorphic rocks to metamorphose into other metamorphic rocks. In some regions, especially areas where mountain building is taking place, it is not unusual for several episodes of metamorphism to affect rocks. It can be difficult to unravel the effects of each episode of metamorphism.