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. (*)
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
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