igneous rock formation molten rock comes from depth (less dense so works its way to surface) - full...
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Igneous Rock Formation
• Molten rock comes from depth (less dense so works its way to surface) - full of gases (H20, C02, S02) plus elements in silicates
• Eruption - gases escape as pressure lessons• Magma cools and hardens beneath the surface
(intrusive; plutonic) • Crystallization - process of cooling and solidifying • Lava cools and hardens on surface (extrusive) • In volcanic eruptions on or under the earth’s surface
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Intrusive Igneous Rocks
Form when magma hardens beneath Earth’s surface
Coarse grained due to slow cooling
Examples- Granite, Diorite, Gabbro, Periodotite
(image on left is diorite)
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Extrusive Igneous Rocks
Form when lava hardens above the Earth’s surface, when most of the gases have escaped
Fine grained due to rapid cooling
Examples- Basalt, Rhyolite, Scoria
(image on right is basalt)
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Classification of Igneous Rocks
• Texture- size, shape, arrangement of crystals
• Composition- proportions of light and dark minerals in the rock
(images from top to bottom: obsidian, diorite, basalt)
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Coarse Grained = Large Crystals
• Intrusive rocks• Magma cools slowly• Ions have time to
move large distances within magma
• Few centers of crystal growth develop
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Fine Grained: Fast Cooling
• Extrusive rocks• Magma or lava cools
rapidly resulting in small, interconnected mineral grains
• The ions in the melted material lose their motion and quickly combine
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Glassy Texture
• Fast Cooling, Extrusive
• No time for the ions in the lava to arrange themselves into a network of crystals
• Obsidian (top); pumice (bottom)
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Porphyritic Texture: Different Sized Crystals
• Minerals that do not crystallize at the same rate or time in magma --different sized crystals
• Inside volcano some magma never reaches the surface--two waves of crystallization
• Large crystals called phenocrysts (visible-crystals) in a matrix of fine grained crystals
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Granitic Composition
• 0%-25% dark• Felsic• Light colored• Quartz, feldspar• 10% dark silicate
materials-- with magnesium, iron
• 70% silica (light silicates)
• Example- Rhyolite(also granite)
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Basaltic Composition
• 45%-85% dark minerals• Plagioclase feldspar• Rich in magnesium and
iron• Darker and denser
(because of iron) • Mafic• Ocean floor = basaltExample- Basalt(also gabbro)
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Andesitic Composition
• Between granitic and basaltic
• 25%-45% ‘dark’• 25% dark silicate
minerals-- amphibole, pyroxene, biotite mica
• Other dominant mineral: Plagioclase feldsparExample- Andesite(also diorite)
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Ultramafic Composition
• 85%-100% dark• Olivine and pyroxene• Almost entirely dark
silicate minerals
• Peridotite rock. Rare at E’s surface but composition of much of mantle
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