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The Rock Cycle

Igneous Rocks

THE ROCK CYCLE

Rocks are naturally occurring combinations of minerals, fossils or other hard materials. They are classified by the way in which they form. The three rock types are: igneous, sedimentary and metamorphic.

All rocks on Earth are locked into a system of cycling and re-cycling known as the ROCK CYCLE. (*)

Earth as a system: The rock cycle

Igneous Rocks

Form as magma cools and crystallizes

Rocks formed inside Earth (intrusive) rocks

Rocks formed on the

surface (extrusive) rocks

Magma

The nature of magmaConsists of three components:

• A liquid portion, called melt, that is composed of mobile ions

• Solids, if any, are silicate minerals that have already crystallized from the melt

• Volatiles, which are gases dissolved in the melt, including water vapor

Magma

Crystallization of magma Crystal size is determined by the rate of cooling

• Slow cooling = large crystals

• Fast cooling = microscopic crystals

• Very fast cooling = glass

Magma

Crystallization of magmaTexture in igneous rocks is determined by the size and arrangement of mineral grains

Igneous rocks are typically classified by• Texture

• Mineral composition

Igneous textures

Fine-Grained or (Aphanitic)

Rapid rate of cooling

Microscopic crystals

May contain holes from gas bubbles

Typically occurs in extrusive / volcanic rocks

Example is Rhyolite

Igneous textures

Course-Grained or (Phaneritic)

Slow cooling

Crystals can be identified without a microscope

Typically occurs in intrusive / plutonic rocks

Example is granite

Igneous textures

Large Crystals or (Porphyritic)

Minerals form at different temperatures as well as differing rates

Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass

Igneous textures

Glassy textureVery rapid cooling of molten rock

Resulting rock is called obsidian

Igneous textures

Vesicular Cellular; full of holes; often light weight.

Example is Pumice.

Igneous Textures

Fragmental Composed of fragments ejected during a volcanic eruption

Varieties• Tuff – ash-sized fragments

• Volcanic breccia – particles larger than ash

Igneous Compositions

Composed mainly of silicate minerals Dark silicates = rich in iron and/or magnesium, creates dark rocks

• Olivine

• Pyroxene

• Amphibole

• Biotite mica

Light silicates = greater amounts of potassium, sodium, and calcium, creates light colored rocks

• Quartz

• Muscovite mica

• Feldspars

Igneous Compositions

Igneous rocks are often classified by mineral (chemical) composition

Mafic (or basaltic))= Dark colored

Felsic (or granitic)= Light colored

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