In general, a rock is a group of minerals bound together

SECTION OUTLINE Igneous, sedimentary, and metamorphic rocks are formed, broken down, and reformed in a recurring process called the rock cycle.. click image to enlarge rock igneous magma

6.1 How Rocks Form

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In general, a rock is a group of minerals bound together

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Igneous, sedimentary, and metamorphic rocks are formed, broken down, and reformed in a recurring process called the rock cycle

click image to enlarge

rock

igneous

magma

sedimentary

sediment

metamorphic

rock cycle

Igneous rocks form as molten rock solidifies, as magma deep in the crust or lava at Earth’s surface cool

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Felsic magmas form light-colored, silica-rich rocks Mafic magmas form dark-colored rocks rich in iron and magnesiums

Basalt is igneous rock formed from mafic magma.

Granite is igneous rock formed from felsic magma.

Igneous rock texture depends mainly on the rate at which magma or lava cools

felsic

mafic

pluton

batholith

Texture coarse-grained

Chemical Composition

felsic

ultramafic

felsic-intermediate intermediate mafic

Igneous rocks are grouped into families by mineral composition and texture

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diabase

obsidian

basalt glass

most pumice

scoria

granite granodiorite diorite

gabbro

peridotite, dunite, pyroxenite

rhyolite

andesite basalt

felsic

mafic

pluton

batholith

Magma that solidifies underground forms various types of igneous intrusions

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Sill

Volcano

Batholith

Stock

Dike

felsic

mafic

pluton

batholith

Sedimentary rocks form from sediments that result from weathering and erosion of rock at Earth’s

surface They often occur in layers, formed over time as different sediments are deposited on top of each other

cementation

stratification

fossil

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Sedimentary rocks are grouped by the type of sediment from which they form: clastic, chemical, or organic

Clastic: Sandstone

Organic:

Limestone Cliffs

Chemical: Rock Salt Flat

cementation

stratification

fossil

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Clastic sediments are often sorted by water action before pressure and mineral cements turn them into rock

1 A river moves sediment into a lake.

2 Particles are sorted by size The largest gravels are the first to be deposited, followed by sands, and then silt and clay.

3 Over time, the sediments are buried, compacted, and may be

cemented

Sands and Gravels

Sands Silt and

Clay

Conglomerate Sandstone Shale

cementation

stratification

fossil

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Clastic sediments are often sorted by water action before pressure and mineral cements turn them into rock

Fossils, ripple marks, mud cracks, nodules, concretions, and geodes features associated with sedimentary rocks

cementation

stratification

fossil

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Metamorphic rocks form when heat or pressure or both alter parent, or preexisting, rocks

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becomes

Slate

becomes

becomes Shale

parent rock

metamorphism

deform

Metamorphic rocks form when heat or pressure or both alter parent, or preexisting, rocks

Metamorphism can occur across a region, as in mountain building events, or it can occur in smaller local areas

A metamorphic rock may be described and identified according to its parent rock, mineral composition, and texture

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parent rock

metamorphism

deform

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