Earth as a system is illustrated most vividly when we examine the rock cycle. The rock cycle allows us to see many of the interactions among the components and processes of the Earth system (Figure 1). It helps us understand the origins of igneous, sedimentary, and metamorphic rocks and how these rocks are connected. In addition, the rock cycle demonstrates that any rock type, given the right sequence of events, can be transformed into any other type.

The Basic Cycle

We begin our discussion of the rock cycle with molten rock, called magma, which forms by melting that occurs primarily within Earth’s crust and uppermost mantle (refer to Figure 1). Once formed, a magma body rises toward the surface because it is less dense than the surrounding rock. If magma reaches Earth’s surface and erupts, we call it lava. Eventually, molten rock cools and solidifies, a process called crystallization or solidification. Molten rock may solidify either beneath the surface or, following a volcanic eruption, at the surface. In either situation, the resulting rocks are called igneous rocks (from the Latin igneus, “of fire”).

If igneous rocks are exposed at the surface, they undergo weathering, the slow disintegration and decomposition of rocks by the daily influences of the atmosphere. The loose materials that result are often moved downslope by gravity and then picked up and transported by one or more erosional agents—running water, glaciers, wind, or waves. These rock particles and dissolved substances, called sediment, are eventually deposited. Although most sediment ultimately comes to rest in the ocean, other sites of deposition include river floodplains, desert basins, lakes, inland seas, and sand dunes.

Next, the sediments undergo lithification, a term meaning “conversion into rock.” Sediment is usually lithified into sedimentary rock when compacted by the weight of overlying materials or when cemented by percolating groundwater that fills the pores, spaces between sediments, with mineral matter.

If the resulting sedimentary rock becomes deeply buried or is involved in the dynamics of mountain building, it will be subjected to great pressures and intense heat. The sedimentary rock may react to the changing environment by turning into the third rock type, metamorphic rock. If metamorphic rock is subjected to still higher temperatures, it may melt, creating magma, and the rock cycle begins again.

Although rocks may appear to be stable, unchanging masses, the rock cycle shows that they are not. The changes, however, take time—sometimes millions or even billions of years. In addition, different locations on Earth are at different stages of the rock cycle. Today, new magma is forming under the island of Hawaii, whereas the rocks that comprise the Colorado Rockies are slowly being worn down by weathering and erosion. Some of this weathered debris will eventually be carried to the Gulf of America (Gulf of Mexico), where it will add to the already substantial mass of sediment that has accumulated there.

Alternative Paths

Rocks do not necessarily go through the cycle in the order just described. Other paths are also possible. For example, rather than being exposed to weathering and erosion at Earth’s surface, igneous rocks may remain deeply buried (refer to Figure 1). Eventually, these masses may be subjected to the strong compressional forces and high temperatures associated with mountain building. When this occurs, they are transformed directly into metamorphic rocks.

Uplift and erosion may bring deeply buried rocks of any type to the surface. When this happens, the material is attacked by weathering processes and turned into new raw materials for sedimentary rocks. Conversely, igneous or metamorphic rocks that form at depth (deep underground) may remain there, where the high temperatures and forces associated with mountain building may change (metamorphose) or even melt them. Over time, rocks may be transformed into any other rock type, or even into a different form of the original type. Then, rocks may take many paths through the rock cycle.

What drives the rock cycle? Earth’s internal heat is responsible for the processes that form igneous and metamorphic rocks. Weathering and the transport of weathered material are external processes, powered by energy from the Sun. External processes produce sedimentary rocks.