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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

Earth as a system: The rock cycle Full cycle does not always take place due to "shortcuts" or interruptions e.g., sedimentary rock melts e.g., igneous rock is metamorphosed e.g., sedimentary rock is weathered e.g., metamorphic rock weathers

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 magma Consists 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 magma Texture 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 texture Very 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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