Geology: Types of Rocks

Rock Types

• A rock is a solid mineral material found on or beneath the Earth's surface within soil or oceans.
• Rocks are categorized into three main types: igneous, sedimentary, and metamorphic.

Igneous Rocks

• Formed from the cooling and solidification of magma or lava.
• Magma types include basaltic, andesitic, and rhyolitic.
• Four main types of igneous rock are basaltic, granitic, intermediate, and ultra-basic.
• Melting temperatures of rocks vary based on minerals present. Rocks rich in iron and magnesium melt at higher temperatures than those with more silicon.
• Cooling rate influences crystal size. Intrusive rocks cool slowly beneath the surface resulting in large crystals, while extrusive rocks cool quickly on the surface forming small crystals.
• Volcanic glass forms when lava cools too quickly for crystals to develop.

Sedimentary Rocks

• Formed from the accumulation and cementation of sediments.
• Sediments include small pieces of rock, minerals, and organic matter moved and deposited by water, wind, glaciers, or gravity.
• Weathering breaks down materials into smaller parts.
• Erosion is the movement of these sediments.
• Deposition is the process where sediments settle out of a transporting medium
• Compaction is the process where the weight of overlying sediments forces the sediment together.
• Lithification involves the physical and chemical processes that turn sediments into rocks.

Metamorphic Rocks

• Formed from existing rocks (igneous, sedimentary, or even other metamorphic rocks) that have been changed by heat and pressure without melting.
• High pressure causes minerals to rearrange into layers and bands called foliation.
• Non-foliated metamorphic rocks have minerals with blocky crystal shapes. (Examples include Quartzite and Marble.)