The Rock Cycle Digital Handout

The concept of a rock cycle was proposed by James Hutton in the late 1700’s. It shows how rocks of one rock class may be recycled by geologic processes to become the raw material for other rock classes.

Like water in the hydrologic cycle, earth materials are constantly recycled in the rock cycle. The driving forces for the recycling of earth materials are geothermal heat, solar energy, and gravity.

Rocks may be classified into three main groups according to their method of formation. Rocks that form from the cooling and crystallization of magma are igneous rocks. Rocks composed of fragments or sediments of other rocks form sedimentary rocks. Rocks that form from the alteration of existing rocks by heat, pressure, and chemical change are metamorphic rocks.

The earliest rocks must have been igneous rocks. Igneous rocks that form on or near the earth's surface are termed extrusive (or volcanic). These rocks have small crystals with a fine-grained texture (e.g., basalt, the most common rock of the seafloor). If the magma cooled so quickly that no crystals formed, the rock has a glassy texture and is obsidian. Volcanic rocks that have holes where the bubbles of gas escaped from the lava, are pumice and scoria.

Igneous rocks that form deep below the earth's surface are intrusive (or plutonic). They cool slowly developing large crystals. Granite with its coarse-grained texture is a plutonic rock. It is the most common rock underlying the continents.

Sedimentary rocks are usually classified according to the origin of the sediments. Cemented or compressed rock fragments are clastic sedimentary rocks. Minerals precipitated or evaporated from a solution are chemical sedimentary rocks (e.g., salt or crystalline limestone). Organic sedimentary rocks form from plant and animal remains (e.g., coal or fossil limestone).

Wind or running water deposit clastic sediments by size. As these sediments become cemented together, the gravel and pebbles form conglomerate, the sands form sandstone, and the clays form shale. The change from sediment to solid rock is called lithification. Lithification takes place with the help of cementation, compaction, and crystallization.

Metamorphic rocks are formed from other rocks that have been subjected to heat, pressure, and chemical alteration. Metamorphic rocks are usually harder and more crystalline than the original rock.

Metamorphism occurs when rocks are deeply buried during mountain building. Metamorphic rocks often show evidence of being squeezed. Squeezed rocks such as slate, schist, and gneiss have their minerals reoriented and realigned at right angles to the pressure. These rocks are foliated.

Rocks that have recrystallized without showing any parallel alignment are non-foliated. Marble is a metamorphic rock formed from recrystallized limestone.