Forces inside Earth and on Earth’s surface build, change, and destroy the rocks in the crust. A series of processes that operate inside Earth as well as on the surface of Earth slowly change rocks from one form to another. This series of processes is called the rock cycle.

There are three types of rocks in and on Earth’s surface. These are igneous rocks, sedimentary rocks, and metamorphic rocks. Each rock type can change into a different type of rock through the processes of the rock cycle. Igneous rocks form when lava or magma cools and hardens.

A diagram of the rock cycle. Legend: 1 = magma; 2 = crystallization (freezing of rock); 3 = igneous rocks; 4 = erosion; 5 = sedimentation; 6 = sediments & sedimentary rocks; 7 = tectonic burial and metamorphism; 8 = metamorphic rocks; 9 = melting.

Magma is molten rock that is found deep inside Earth’s crust. This magma will cool slowly as it rises toward Earth’s surface. As the magma cools, large crystals grow. These eventually become an igneous rock, much like the granite shown below.

Igneous rocks from cooling magma. Granite is an igneous rock formed from magma that cooled slowly underground. As the magma slowly cools, large mineral crystals form.

Igneous rocks can form on Earth's surface from lava as well. When magma reaches Earth's surface and spills on to the ground, it is known as lava. Cooling lava will form igneous rocks with much smaller crystals. Volcanic eruptions typically involve lava as well. The igneous rocks formed as the result of a volcanic eruption are typically glassy or full of holes.

Learn more about the formation of igneous rocks.

Cooling lava forms igneous rocks. Basalt and pumice are two examples of igneous rocks formed from cooling lava. Basalt typically forms where lava seeps out onto the ocean floor at spreading centers. Much of the ocean floors are made of basalt. Pumice is formed when lava erupts violently from a volcano. The holes in the rock are the result of gases escaping rapidly from the cooling lava. Pumice sometimes floats in water.
Sources: Pumice and Basalt

Sedimentary rocks form from sediment. Small bits of rocks or living things are called sediment. Sediment may be carried from place to place by the wind, by water, or by ice. When this sediment is deposited, or dropped, it usually forms into layers.

The layers of sediment pile up. Slowly, over time the bottom layers are compacted together. This compaction presses the sediment together. As more and more layers are added on top, the weight continues to compress the bottom layers. The sedimentary rocks that form this way often still show evidence of these layers.

Sediment is compacted into sedimentary rock. Over time, as sediment is deposited in horizontal layers, the lower most layer will become compacted and turn into sedimentary rock.

Sediment is often cemented together as well to form a sedimentary rock.

Often, as compaction is occurring, minerals in the sediment begin to dissolve. These dissolved minerals may move into the pore space between sediment particles and harden. There the dissolved minerals harden, forming a glue-like bond between the sediments. As the sediment hardens, the glue and the compaction help form the rock.

Calcite, quartz, and hematite are common cements in sedimentary rocks.

There are three types of sedimentary rocks: clastic, organic, and chemical.

Clastic sedimentary rocks are formed when broken pieces of other rocks are compacted and cemented.

The individual sediments in a clastic sedimentary rock can be clay-sized or boulder-sized. It is typically not too difficult to identify a clastic sedimentary rock – they do look as if bits of rock pieces have been glued together.

Shale, sandstone, breccia, and conglomerate are all common clastic sedimentary rocks.

Shale is a sedimentary rock that forms only from compaction. No cementation is needed. It is a brittle rock which often flakes apart. Sandstone will have a gritty feel as sand-sized particles of sediment have been compacted and cemented together. Conglomerate and breccia may look similar but closer examination shows a difference. The sediment grains in conglomerate are rounded and the grains in breccia are angular.

Organic sedimentary rocks form from the remains of plants and animals.
The remains of these organisms are deposited in thick layers. Organic sedimentary rocks often contain fossils.

Coal and limestone are common organic sedimentary rocks. Coal forms in swamps where layers and layers of organisms were deposited over time. It may take millions of years for coal to form.

Limestone is often formed in the ocean. Sea shells and other animal debris pile up over thousands of years, slowly compacting. Some of the shells may dissolve, releasing calcite which acts as a glue. Limestone very often does have fossils.

Coal and limestone are organic sedimentary rocks which contain fossils.

Chemical sedimentary rocks form when dissolved minerals come out of solution. For example, chemical sedimentary rocks may form when a shallow sea or saltwater lake evaporates. Deposits of rock salt and rock gypsum are examples of chemical sedimentary rocks.

Learn more the formation of sedimentary rocks.

Deep within Earth’s crust, heat and pressure can change any rock into a metamorphic rock.

When heat and pressure change an existing rock into a metamorphic rock, the mineral composition, appearance, texture, and crystal structure change as well. Metamorphic rocks can look much different from their original rock.

Gneiss is the metamorphic rock formed when the igneous rock granite is subjected to heat and pressure deep within the crust. Note how the minerals in the gneiss sample have been rearranged into bands.

Metamorphic rocks can be classified as foliated or nonfoliated.

Foliated rocks have grains arranged into bands or parallel layers. Metamorphic rocks such as slate, schist, and gneiss are foliated. These rocks tend to crumble or break along the parallel layers.

Slate is a common foliated rock. Slate forms as shale, a sedimentary rock, and is then compacted and heated. This makes slate essentially a harder, denser version of shale. Slate is used in many different ways, including roofing and tiling applications.

Slate is the metamorphic rock formed by the addition of heat and pressure to the sedimentary rock shale.

Nonfoliated metamorphic rocks do not show banding. The mineral grains within these rocks are arranged randomly. These rocks are not easily broken into layers.

The metamorphic rocks marble and quartzite are both nonfoliated rocks.

Marble forms when the sedimentary rock limestone is exposed to great heat and pressures. Marble has many applications in the building trades.

Quartzite forms when the sedimentary rock sandstone is metamorphosed.

Marble and quartzite, as shown here, are clearly nonfoliated. There is no layering or distinct pattern in these metamorphic rocks.

Can form from other metamorphic rocks.
The examples we have looked at so far involve changing igneous or sedimentary rocks into metamorphic rocks.

But if a metamorphic rock is subjected to more heat and pressure, it can change into a different metamorphic rock.

Consider this:
Shale is a sedimentary rock. If it is subjected to heat and pressure it changes into the metamorphic rock slate.

Slate, a metamorphic rock, can change into phyllite with the addition of heat and pressure.

And phyllite, another metamorphic rock, will change into schist when subjected to heat and pressure.

Fun fact: Rocks form at different rates – Some sedimentary rocks may form in less than 20,000 years. Others may take also long as 30 – 40 million years to form. It all does depend on how much water and minerals that could act as cement are available.

Notice how the foliation becomes more pronounced with each round of metamorphism as slate becomes phyllite and phyllite becomes schist.