There are three main types of rocks: sedimentary, igneous, and metamorphic. Each of these rocks are formed by physical changes—such as melting, cooling, eroding, compacting, or deforming—that are part of the rock cycle.

Sedimentary Rocks

Sedimentary rocks are formed from pieces of other existing rock or organic material. There are three different types of sedimentary rocks: clastic, organic (biological), and chemical. Clastic sedimentary rocks, like sandstone, form from clasts, or pieces of other rock. Organic sedimentary rocks, like coal, form from hard, biological materials like plants, shells, and bones that are compressed into rock.

The formation of clastic and organic rocks begins with the weathering, or breaking down, of the exposed rock into small fragments. Through the process of erosion, these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock.

Chemical sedimentary rocks, like limestone, halite, and flint, form from chemical precipitation. A chemical precipitate is a chemical compound—for instance, calcium carbonate, salt, and silica—that forms when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. This occurs as water travels through Earth’s crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates.

Metamorphic Rocks

Metamorphic rocks are rocks that have been changed from their original form by immense heat or pressure. Metamorphic rocks have two classes: foliated and nonfoliated. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation. Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the direction of pressure that is applied. An example of this transformation can be seen with granite, an igneous rock. Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added, those minerals shift to all point in the same direction while getting squeezed into flat sheets. When granite undergoes this process, like at a tectonic plate boundary, it turns into gneiss (pronounced “nice”).

Nonfoliated rocks are formed the same way, but they do not contain the minerals that tend to line up under pressure and thus do not have the layered appearance of foliated rocks. Sedimentary rocks like bituminous coal, limestone, and sandstone, given enough heat and pressure, can turn into nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks can also form by metamorphism, which happens when magma comes in contact with the surrounding rock.

Igneous Rocks

Igneous rocks (derived from the Greek word for fire) are formed when molten hot material cools and solidifies. Igneous rocks can also be made a couple of different ways. When they are formed inside of the earth, they are called intrusive, or plutonic, igneous rocks. If they are formed outside or on top of Earth’s crust, they are called extrusive, or volcanic, igneous rocks.

Granite and diorite are examples of common intrusive rocks. They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow.

Alternatively, rocks like basalt and obsidian have very small grains and a relatively fine texture. This happens because when magma erupts into lava, it cools more quickly than it would if it stayed inside the earth, giving crystals less time to form. Obsidian cools into volcanic glass so quickly when ejected that the grains are impossible to see with the naked eye.

Extrusive igneous rocks can also have a vesicular, or “holey” texture. This happens when the ejected magma still has gases inside of it so when it cools, the gas bubbles are trapped and end up giving the rock a bubbly texture. An example of this would be pumice.

  

The Rock Cycle

Active volcanoes like this one on Reunion Island—east of Madagascar, in the Indian Ocean—forms a type of igneous rock. Extrusive, or volcanic, igneous rocks are formed when molten hot material cools and solidifies.

chemical
Noun

molecular properties of a substance.

clast
Noun

fragment of a rock, often broken off through weathering.

compact
Verb

to pack tightly together.

cooling
Verb

to become cool lose heat or warmth

cycle
Noun

regularly occurring event that repeats over a period of time.

deform
Verb

to put out of shape or distort.

Noun

act in which earth is worn away, often by water, wind, or ice.

extrusive igneous rock
Noun

any rock derived from magma that was poured out or ejected at the Earth’s surface.

foliation
Noun

layering that occurs when pressure squeezes flat or long minerals so they become aligned.

Noun

rock formed by the cooling of magma or lava.

intrusive igneous rock
Noun

plutonic rock; formed from magma forced into older rocks at depths within the Earth’s crust, which then slowly solidifies below the Earth’s surface.

melting
Verb

to become altered from a solid to a liquid state usually by heat

metamorphic rock
Noun

rock that has transformed its chemical qualities from igneous or sedimentary.

organic
Adjective

produced according to standards using limited amounts of chemical additives.

Noun

processes that explain the relationship between the three rock types: igneous, sedimentary, and metamorphic. Any rock type can become any other.

Noun

rock formed from fragments of other rocks or the remains of plants or animals.

Noun

the breaking down or dissolving of the Earth's surface rocks and minerals.