Earth's Crust and its Composition

The elements of the Earth's crust occur in chemical compounds that we recognize as minerals. Minerals are combined into rock, which we can broadly define as an assemblage of minerals in the solid state.

Earth's Crust and Its Composition

The eight most abundant elements in the Earth's crust, measured by percentage of weight. Oxygen and silicon dominate, with aluminum and iron following.

The Cycle of Rock Change

There are three classes of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. These classes of rocks are constantly forming from one another in a continuous circuit—the cycle of rock change.

Igneous rock is formed when molten pre-existing rock deep within the Earth moves toward the surface and cools.

Sedimentary rock is formed when pre-existing rock is broken down and accumulates in thick layers; over long spans of time the sediments become hardened into sedimentary rock.

Metamorphic rock is formed when high heat and tremendous pressure alter igneous and sedimentary rocks.

* See the Animation on the Rock Cycle found on WileyPLUS or your text’s website.

The Cycle of Rock Change

Igneous Rock

Igneous rocks are formed when molten material moves from deep within the Earth to a position within or atop the crust. There the molten material cools, forming rocks composed of mineral crystals.

* See the Animation on Igneous Rocks found on WileyPLUS or your text’s website.

Igneous Rock

Only the most important silicate mineral groups are listed, along with four common igneous rock types. The patterns shown for mineral grains indicate their general appearance through a microscope.

Features of Igneous Rocks

Joints form in intrusive rocks as the overlying material is eroded away, reducing the pressure. Once exposed, the rock expands leaving fractures or joints. Joints may be differentiated from faults as there is no relative movement along a joint.

Extrusive rocks that cool quickly do not form crystals that can be seen by the naked eye. Under very rapid cooling conditions, the rock will form a volcanic glass or obsidian.

Features of Igneous Rocks

Joint blocks in granite exposed at Acadia National Park.

A large block of obsidian or volcanic glass.

Sedimentary Rocks

The mineral particles in sedimentary rocks can be derived from pre-existing rock of any of the three rock classes as well as from newly formed organic matter. However, igneous rock is the most important original source of the inorganic mineral matter that makes up sedimentary rock.

In the process of mineral alteration, solid rock is weakened, softened, and fragmented, yielding particles of many sizes and mineral compositions. When transported by a fluid medium, these particles are known collectively as sediment.

Over long time spans, the sediments can undergo physical or chemical changes, becoming compacted and hardened to form sedimentary rock.

Sedimentary Rocks

Natural arches in massive sandstone, Lake Powell, Utah.

Red shales with thin, interbedded layers of sandstone.

Clastic Rocks

Clastic sedimentary rocks are made up of individual particles or clasts. The term comes from the Greek word klastos, meaning broken. They differ in size and shape and are derived from eroded rock debris.

*See the Animation on Clastic Rocks found on WileyPLUS or your text’s website.

Hydrocarbons in Sedimentary Rocks

Hydrocarbons are an important type of organic sediment. Natural gas and petroleum occupy open interconnected pores in thick sedimentary rock layers. Uparching of the layers of rock is most effective in trapping the fossil fuels and so producing deposits that are easily accessed by drilling.

*See the Animation on Hydrocarbons in Sedimentary Rocks found on WileyPLUS or your text’s website.

Metamorphic Rock

Any type of igneous or sedimentary rock may be altered by the tremendous pressures and high temperatures that accompany the mountain-building processes of the Earth's crust. The result is a rock so changed in texture and structure as to be reclassified as metamorphic rock. For example, metamorphosed limestone produces marble, and high-grade metamorphism of shale produces a schist.

Metamorphic Rock

Marble. Freshly exposed schist shows shiny surfaces where shearing has occurred. It represents the end product of deep deformation of black mud once deposited on the floor of an ancient oceanic trench.