Rock Classification
by Joe C.
What's a Rock?
Rocks are solid substances mainly composed of crystals. They are classified into one of three groups by their texture, physical appearance, and origin.
The Three Rock Classes
Igneous Rock
- Formed by Melting & Solidification
- Form inside or outside a volcano
- Main types: Intrusive & Extrusive
- Intrusive forms slowly inside a volcano and has large crystals
- Extrusive forms quickly outside a volcano, but has small crystals
Sedimentary Rock
- Formed by Compaction & Cementation
- Form
- Main types: Clastic, Bioclastic, & Crystalline
- Clastic forms when many sediments are compacted together
- Bioclastic forms when living things or remains of living things (like plants and shells) are compacted together
- Crystalline forms from chemical evaporates and/or precipitates
Metamorphic Rock
- Formed by Heat & Pressure
- Form deep within the earth
- Main types: Foliated & Nonfoliated
- Form from either Regional (intense pressure) or Contact (extreme heat) Metamorphism
Igneous Rock (Left)
- Formed by Melting & Solidification
- Form inside or outside of volcanoes
- Main types: Intrusive & Extrusive
- Intrusive formed inside a volcano, cools slowly, has large crystals
- Extrusive formed outside a volcano, cools quickly, has small crystals
Sedimentary Rock (Middle)
- Formed by Compaction & Cementation
- Form on the earth's surface
- Main types: Clastic, Bioclastic, Chemical
- Clastic formed when many sediments are compacted together
- Bioclastic formed when living things or remains of living things (e.g. shells or plants) are compacted together
- Chemical formed from chemical evaporates and/or precipitates
Metamorphic Rock (Right)
- Formed by Heat & Pressure
- Form deep within the earth
- Main types: Foliated & Nonfoliated
- Formed by either Regional (extreme pressure) or Contact (intense heat) metamorphism
Rock Examples
Rhyolite
- Formed outside a volcano--Crystals formed quickly
- Texture is fine--Crystals are small
- Non-Vesicular
- Felsic--Light in color, low density
- 15% Amphibole, 20% Biotite, 50% Plagioclase Feldspar, 15% Quartz, 5% Potassium Feldspar
Bituminous Coal
- Organic
- Texture is very coarse
- Composed of carbon
- Contains compacted plant remains
Phyllite
- Texture is foliated
- Grain size is fine to medium
- Composition is mica, quartz, feldspar, amphibole, and garnet
- Type of Metamorphism: Regional
- Foliation surfaces shiny from microscopic mica crystals
The Rock Cycle
The Rock Cycle is a flowchart that describes how one type of rock becomes another type of rock.