unit 14

It depends on how the rock formed.

Igneous rock forms when magma cools and hardens. Examples include granite, basalt, gabbro, obsidian, and pumice.

Sedimentary rock forms when sediment deposits are compressed, cemented together, and hardened. Examples include sandstone, shale, limestone, and conglomerate.

Metamorphic rock forms when existing rock is changed by heat and pressure. Examples include gneiss, schist, marble, and slate.

The rock cycle is the series of processes in which rock can be changed from one type to another.

Sedimentary rock becomes metamorphic rock by heat and pressure. Metamorphic rock becomes sedimentary rock by weathering, erosion, deposition, and cementation.

Igneous rock becomes metamorphic rock by heat and pressure. Metamorphic rock becomes igneous rock by melting and cooling again.

Sedimentary rock can become igneous rock by weathering, erosion, deposition, melting, and cooling. Igneous rock can become sedimentary rock by weathering, erosion, deposition, and cementation.

The type of igneous rock formed depends on when it formed during the cooling process, which determines composition, and how the magma cooled, which determines structure.

Bowen's reaction series explains the order in which minerals form as magma solidifies.

Mafic igneous rocks are richer in ferromagnesian minerals and are typically darker, like basalt. Felsic igneous rocks are lighter in colour and rich in potassium feldspar and quartz, like granite.

Cooling rate determines grain size. Slow cooling makes coarse-grained rocks like granite, while fast cooling makes fine-grained rocks like basalt.

Both cool very rapidly and do not have well-formed crystals. Obsidian is smooth with a glassy lustre, while pumice is vesicular because it trapped gases as it cooled.

Tuff is volcanic ash that has been compacted and cemented to form solid rock. It can be classified as igneous but is also often referred to as sedimentary.

Intrusive rocks cool within Earth and are usually coarse-grained. Extrusive rocks cool on Earth’s surface and are usually fine-grained.

Batholiths are large intrusions covering hundreds of square kilometres. Stocks are similar but smaller. Laccoliths form dome-like hills within the crust. Sills are parallel to rock layers. Dikes cut through rock layers.

Compaction squeezes sediment grains together and reduces space between them. Cementation glues the grains together with minerals like calcite and quartz deposited by water.

Chemical sedimentary rocks form when minerals precipitate from a solution or settle from a suspension. Organic sedimentary rocks form from the remains of plants or animals. Clastic sedimentary rocks form when fragments of preexisting rock are compacted or cemented together.

Evaporites are chemical sedimentary rocks formed when water evaporates and leaves dissolved minerals behind. Examples include gypsum and halite.

A geode is a hollow rock containing large crystals. Groundwater deposits dissolved minerals such as quartz or calcite inside a cavity, and the minerals then crystallize there.

Limestone can form by deposition of minerals from water, or from shells and other marine organisms. It can be chemical or organic sedimentary rock.

Conglomerates and breccias are clastic sedimentary rocks. Conglomerates have rounded particles, while breccias have more angular particles.

Shale and sandstone are clastic sedimentary rocks. Shale is made of very small clay particles, while sandstone is made mostly of sand-sized quartz grains.

Stratification is the layering of sedimentary rock. It occurs when the conditions of sediment deposition change.

Fossils are most commonly found in sedimentary rock. They form when plant or animal remains are buried before they decompose, and hard parts or impressions are preserved.

Foliation is banding in metamorphic rock. It is caused by extreme pressure that realigns mineral crystals or by minerals separating into alternating dark and light bands.

Gneiss forms under extremely high heat and pressure, often from granite. It is identified by distinctive alternating light and dark bands.

Slate and schist are foliated metamorphic rocks. Slate is fine-grained and splits into thin sheets, while schist is coarser and has visible sheets.

Quartzite and marble are non-foliated metamorphic rocks. Quartzite forms from sandstone, and marble forms from limestone.

Contact metamorphism occurs when magma comes into contact with existing rock and heat from the magma changes that rock.

Regional metamorphism occurs over large areas during periods of high tectonic activity, such as mountain building.

Gemstones like rubies are usually found in metamorphic rock because the heat and pressure conditions needed to form them often occur during metamorphism.

Garnets can record the pressures and temperatures of metamorphism, so geologists use them as geothermometers and geobarometers.