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Chapter 10 Igneous Rocks

Chapter 10 Igneous Rocks. Igneous Rock Classification. Classified according to WHERE the magma cools and hardens Intrusive: slow cooling below the crust Extrusive: rapid cooling on earth’s surface Differ mostly in the size of their crystal grains– “texture”

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Chapter 10 Igneous Rocks

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  1. Chapter 10Igneous Rocks

  2. Igneous Rock Classification • Classified according to WHERE the magma cools and hardens • Intrusive: slow cooling below the crust • Extrusive: rapid cooling on earth’s surface • Differ mostly in the size of their crystal grains– “texture” • Texture is determined by the cooling rate of magma that formed the rock

  3. Texture: Intrusive • Forms when magma cools and hardens slowly, deep underground • Slow loss of heat allows for magma to form large, well-developed crystals/mineral grains • “Coarse-grained” rock • Core of continental crust

  4. Intrusive Rocks • Ex: granite

  5. Texture: Extrusive • Forms when lava cools rapidly on earth’s surface. • Rapid loss of heat to air or sea water does not allow time for large crystalline grains to form. • “Fine-grained” rocks. Grains cannot be seen by unaided eye • Oceanic crust composed mainly of basalt.

  6. Extrusive Rocks • Ex: basalt

  7. Texture: Porphyritic • Some igneous rock form when magma cools slowly at first, then rapidly as it nears the surface • Large crystals embedded within mass of smaller ones • Porphyritic: rock with mixture of large and small crystals

  8. Texture: Others • When highly viscous, silica-rich magma cools rapidly, result is no crystals at all • Obsidian, or volcanic glass • When magma containing large amount of dissolved gases cools rapidly- gases are trapped in rock • Pumice

  9. Composition • Mineral composition of igneous rocks is determined by the chemical composition of the magma from which it was developed • Different types of igneous rocks have similar compositions • 3 families: felsic, intermediate, mafic

  10. Composition: Felsic • Magma high in silica • Light coloring • Main minerals: orthoclase feldspar and quartz • Ex: • Coarse: granite • Fine: rhyolite • obsidian

  11. Composition: Mafic • Magmas low in silica, but rich in iron and magnesium • Dark in color • Main minerals: plagioclase feldspar, pyroxenes, olivine, hornblende • Ex: • Coarse: gabbro • Fine: basalt

  12. Composition: Intermediate • Medium-colored • Contain less quartz than felsic rocks • Main minerals: plagioclase feldspar, hornblende, pyroxene, biotite mica • Ex: • Coarse: diorite • Fine: andesite

  13. Hmm…. • If you know only that an igneous rock has coarse grains… can you identify the family it is in? Why?

  14. Igneous Rock Structures: Intrusions • Form underground • Batholith • Largest of all intrusions. At least 100 km2 • “Deep rock” • Form cores of mountain ranges (Sierra Nevada, Coast Range, British Columbia) • Wolf River Batholith • Stock • Similar to batholith, but covers less than 100 km2

  15. Wolf River Batholith, WI

  16. Igneous Structures: Intrusions • Laccolith • “Lake of Rock” • Magma flows between rock layers, pushing them into an arc. Stays parallel with underlying layers • Push small dome-shaped mountains onto surface • Sill • A sheet of magma that flows parallel to rock layers surrounding it, no matter what angle they are at. • Big Bend National Park, TX • Dike • Magma forces its way through rock layers, following existing fractures or forming new ones. • Cut across rock layers

  17. Igneous Rock Structures: Extrusions • Form on the surface • Volcanic neck • When a volcano stops erupting, it eventually wears away. Softer parts of cone carried away by wind and water. Solidified central vent, volcanic neck, remains • Shiprock, NM • Lava plateau • Lava flow: flat masses of rock • Lava plateau: develops from lava flowing out of cracks on earths’ surface, spreading over vast area. Fills in valleys and covers hills.

  18. Shiprock, NM

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