5.3 igneous rocks
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Formation of Igneous Rock
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Formation of Igneous Rock
• When lava cools and hardens it forms igneous rock.
• The rock has two categories and is based on how the lava cools.
–Intrusive igneous rock–Extrusive igneous rock
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Intrusive Igneous
• The cooling of magma deep below the earth’s surface.
• Called intrusive because the lava that forms them intrudes other rocks.
• The magma then slowly cools to form rock.
• The slow cooling allows minerals to collect and form large crystals.
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Extrusive Igneous
• Formed when there is a rapid cooling of lava or melting of rocks on the earth’s surface.
• The rapid cooling does not give crystals time to form.
• The main difference between the two types of igneous rock is the size of the crystal when the rock is forming.
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Texture of the Rocks• The larger the crystals, the more rough the
rock will be.
• When rapid cooling occurs and crystal are not allowed the opportunity to form, the rock will appear extremely smooth.
• Crystal formation will be so small that they are not seen by the unaided eye.
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Texture of the Rocks
• An igneous rock with both large and small crystals is called porphyritic.
• The porphyritic texture is created when lava cools slowly and is then sped up.
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Texture of Rock
• When thick lava cools rapidly no crystals form at all.
• Usually thicker lava has an increased amount of trapped gases and this will cause the igneous rock to become full of holes.
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Composition of Rock
• The mineral make of the lava determines the chemical make-up of the rock.
• There are 3 different types of families:–Felsic–Mafic –Intermediate