Rocks and their Formation in Geology

Rocks remember. This is a favorite maxim of geologists and planetary scientists. Essentially this means that the composition of a rock yields important clues about its origin. In geologic terms, a rock is an aggregate of one or more minerals. Minerals, in turn, are composed of one or more chemical elements. All rocks fall into one of three categories: igneous, sedimentary or metamorphic.

Igneous rocks are formed by volcanic activity. The term igneous literally means “fire,” an apt description of the high temperatures responsible for the formation of these rocks. Because the Earth began in a molten state over four billion years ago, all terrestrial rocks originated as igneous rocks. Geologists subdivide igneous rocks into two groups: intrusive and extrusive.

Intrusive igneous rocks solidify below the Earth’s surface and include granite, diorite, pegmatite and peridotite. Extrusive igneous rocks solidify at or above the Earth’s surface. They include rocks found in lava flows such as basalt, obsidian and pumice. Most of the rock found in oceanic crust consists of basalt. Obsidian forms when lava cools extremely fast. It contains no crystals, and, like glass, its pieces have exquisitely sharp edges. Pumice is also formed during volcanic eruptions. It contains numerous air pockets and is commonly used as an abrasive.

Today, new igneous rocks are formed by the upwelling of magma from the Earth’s mantle during volcanic eruptions, as well as in ocean basins where the seafloor is spreading. Another source of igneous rocks comes from the melting of crustal rocks as they are pushed into the mantle during tectonic plate subduction.

Sedimentary rocks are formed when layers of sediment become cemented together over long periods of time. Common examples of sedimentary rocks include shale, flint, sandstone and limestone. Sedimentary rocks often contain the fossils of marine organisms, which incorporated calcium carbonate into their shells during their lifetimes. When these organisms died, their shells sank to the ocean floor, where they were literally cemented into the surrounding sediments. Many deposits of sandstone and limestone contain the fossilized remains of trilobites, ammonites and other long-extinct sea creatures.

Metamorphic rocks are produced when igneous or sedimentary rocks are transformed by high temperatures and pressures or undergo chemical reactions. Approximately 27 percent of the Earth’s crust is thought to consist of metamorphic rock. Since the Earth’s crust comprises a mere one percent of the planet’s total mass, however, the vast majority of Earth’s rocks are either igneous or sedimentary.

An easy-to-remember example of a metamorphic rock is marble, found in caves (and monuments) throughout the world. Marble forms when porous limestone (composed largely of calcium carbonate) is heated and squeezed until it recrystallizes into a denser configuration. Under a microscope, limestone is packed together loosely, whereas marble fragments are locked together like pieces of a jigsaw puzzle. Other metamorphic rocks include amphibolite, gneiss, quartzite, schist, slate and soapstone.

Geologists broadly categorize metamorphic rocks as foliated or non-foliated. Foliated metamorphic rocks exhibit distinct bands or layers because their crystals formed from multi-mineral sedimentary rock exposed to unidirectional pressure. A good example of a foliated rock is slate. In contrast, non-foliated rocks show little or no layering because their crystals were exposed to pressure from multiple directions. Examples include marble and quartzite.