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Igneous Rocks

Igneous Rocks. Igneous Rocks are rocks that were once partially or entirely liquid Magma is molten rock that is below the surface of the earth Lava is molten rock that has reached the surface. Magma and lava can be found in a variety of different settings.

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Igneous Rocks

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  1. Igneous Rocks • Igneous Rocks are rocks that were once partially or entirely liquid • Magma is molten rock that is below the surface of the earth • Lava is molten rock that has reached the surface

  2. Magma and lava can be found in a variety of different settings

  3. The rate at which magma cools determines the texture of the igneous rock • Texture is a term used to describe the mineral sizes, shapes, and arrangements within a rock. There are 7 different textures for igneous rocks.

  4. Glassy texture – Very rapid cooling, forms volcanic glass, no crystals present • Aphanitic texture – Rapid cooling, small to invisible • crystals present (< 1mm), generally cools above ground

  5. Phaneritic texture – Large, visible crystals (1-10 mm), generally cools under ground • Pegmatitic texture – Very large crystals (  1 cm), • indicates that the rate of cooling was very slow

  6. Porphyritic texture – Rock contains two distinct crystal sizes, large crystals are called phenocrysts, small crystals are called the matrix. Generally due to a change in the rate of cooling • Vesicular texture – Rock contains vesicles (gas • bubbles trapped in lava), which form from • depressurization of the magma

  7. Pyroclastic texture – Igneous rocks that contain pyroclasts (ejected material from explosive volcanic eruptions)

  8. There are several common minerals seen in igneous rock, each of which can be grouped as either Felsic (light-colored) or Mafic (dark-colored)

  9. Classification of igneous rocks is based on the type and amount of minerals present and the rocks texture.

  10. There is a relationship between the temperature of a magma and its composition. This is called Bowen’s Reaction Series

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