Igneous Rocks

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Dont you wish you were here?
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The Rock Cycle

• Igneous Rocks

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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.
  ()

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Earth as a system The rock cycle
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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

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Igneous Rocks

• Form as magma cools and crystallizes
• Rocks formed inside Earth
• (intrusive) rocks
• Rocks formed on the
• surface
• (extrusive) rocks

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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

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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

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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

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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

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Igneous textures

• Course-Grained or (Phaneritic)
• Slow cooling
• Crystals can be identified without a microscope
• Typically occurs in intrusive / plutonic rocks
• Example is granite

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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

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Igneous textures

• Glassy texture
• Very rapid cooling of molten rock
• Resulting rock is called obsidian

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Igneous textures

• Vesicular
• Cellular full of holes often light weight.
• Example is Pumice.

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Igneous Textures

• Fragmental
• Composed of fragments ejected during a volcanic
  eruption
• Varieties
• Tuff ash-sized fragments
• Volcanic breccia particles larger than ash

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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

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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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• End