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Understanding Igneous Rocks: Formation and Classification

Explore the formation process of igneous rocks from molten rock to solidification and learn about the classification based on where they are formed and their mineral composition. Discover examples of intrusive and extrusive rocks and volcanic glass.

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Understanding Igneous Rocks: Formation and Classification

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  1. Igneous Rocks Hot rocks/Fire Rocks

  2. Igneous Rock • Igneous rocks form when: molten rock cools and solidifies.

  3. Igneous Rocks • Molten rock is called magma when it is below the Earth’s surface and lava when it is above.

  4. Igneous Rocks • Remember: • Rate of cooling affects crystal size • Fast cooling – small crystals • Slow cooling – large crystals

  5. Crystal size Magma (below) when molten rock cools and solidifies Lava (above)

  6. Igneous Rock classification • Igneous rocks are classified two ways: • Where they were formed • What they are made from (mineral composition)

  7. Part 1 Classifying igneous rocks by where they are formed.

  8. Igneous Rocks - Classification • Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic).

  9. Igneous Rocks - classification • The word “plutonic” comes from Pluto, the name for the Roman god of the underworld.

  10. Igneous Rocks - Classification • They form when magma enters a pocket or chamber underground that is relatively cool and solidifies into crystals as it cools very slowly.

  11. Intrusive Igneous Rock • Most intrusive rocks have large, well formed crystals.The mineral crystals within them are large enough to see without a microscope.

  12. Igneous Rocks - Classification • The more slowly molten rock cools within the Earth, the larger the igneous rock crystals will be.

  13. Crystal size magma Intrusive slowly Magma (below) when molten rock cools and solidifies large Lava (above)

  14. Intrusive Igneous Rocks - examples • Gabbro

  15. Intrusive Igneous Rocks - examples • Diorite

  16. Intrusive Igneous Rocks - examples • Granite

  17. Crystal size magma Intrusive slowly Magma (below) when molten rock cools and solidifies large Lava (above) Granite, gabbro, diorite

  18. Extrusive Igneous Rocks • Extrusive igneous rocks, or volcanic rocks, form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools RAPIDLY forming rock.

  19. Extrusive Igneous Rocks • Most extrusive (volcanic) rocks have small crystals.

  20. Extrusive Igneous rocks - examples • Basalt

  21. Extrusive Igneous rocks - examples • Andesite

  22. Extrusive Igneous rocks - examples • Rhyolite

  23. Lava Crystal size Quickly Small or not visible Basalt, ryolite, andesite Extrusive magma Intrusive slowly Magma (below) when molten rock cools and solidifies large Lava (above) Granite, gabbro, diorite

  24. Volcanic Glass • Pumice, obsidian, and scoria are examples of volcanic glass. • These rocks cooled so quickly that few or no mineral grains formed. • Most of the atoms in these rocks are not arranged in orderly patterns, and few crystals are present.

  25. Glassy Igneous Rocks Pumice (left) Scoria (bottom left) Obsidian (bottom) Note gasses in the lava can cause fine holes called vesicles as seen in the pumice and scoria. Glassy Igneous Rocks cool so rapidly, that atoms don’t have enough time to get together, bond and form crystals. To cool this quickly the rocks MUST be extrusive.

  26. Part 2 Classifying by mineral composition

  27. Composition – Magma types • A way to further classify these rocks is by the magma from which they form. An igneous rock can form from, granitic, andesitic, or basaltic magma.

  28. Composition - Properties • Magma composition determines the physical & chemical properties of an igneous rock

  29. Lava Crystal size Slowly granitic Small or not visible Physical & chemical properties Extrusive Basalt, ryolite, andesite andesitic magma Intrusive slowly Magma (below) basaltic when molten rock cools and solidifies large Lava (above) Granite, gabbro, diorite *SiO2 = Silicon Fe = Iron Mg = Magnesium

  30. Basaltic Igneous Rocks • They form from magma that is rich in iron (Fe) and magnesium (Mg) and LOW in silica (SiO2.)

  31. Basaltic Igneous Rocks • The presence of iron(Fe) and magnesium (Mg) in basalt gives basalt its dark color. • Basalt rocks have a high density

  32. Basaltic Igneous Rocks • Basaltic lava is fluid and flows freely from volcanoes in Hawaii, such as Kilauea. • Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). • Makes up the Oceanic Crust

  33. Lava Crystal size Slowly granitic Small or not visible Physical & chemical properties Extrusive basalt, ryolite, andesite andesitic magma Intrusive Rich in Fe & Mg poor in SiO2 slowly Magma (below) basaltic Dense & dark colored when molten rock cools and solidifies large Lava (above) Ocean floor & Hawaii Granite, gabbro, diorite

  34. Granitic Rocks • Granitic igneous rocks are light-colored rocks of lower density than basaltic rocks.

  35. Granitic Rocks • Granitic rocks are coarse-grained • Granitic magma is thick and stiff, high silica content, lesser amounts of Fe and Mg.

  36. Granitic Rocks • It is the most common rock type on the continental land masses. • Yosemite Valley in the Sierra Nevada Mtn. range and Mt. Rushmore are two notable examples of granitic rocks • Makes up the continental crust

  37. Lava High SiO2 Low Fe & Mg Crystal size Slowly granitic Light colored, less dense Small or not visible On the continents Physical & chemical properties Extrusive basalt, ryolite, andesite andesitic magma Intrusive Rich in Fe & Mg poor in SiO2 slowly Magma (below) basaltic Dense & dark colored when molten rock cools and solidifies large Lava (above) Ocean floor & Hawaii Granite, gabbro, diorite

  38. Andesitic Rocks • Andesitic igneous rocks have mineral compositions between those of basaltic and granitic rocks. • Color tends to be between dark and light. • Medium density

  39. Andesitic Rocks • Many volcanoes around the rim of the Pacific Ocean formed from andesitic magmas.

  40. Andesitic Rocks • Like volcanoes that erupt granitic magma, these volcanoes also can erupt violently. • Rocks made from andesite tend to be fine-grained.

  41. Lava High SiO2 Low Fe & Mg Crystal size Slowly granitic Light colored, less dense Small or not visible On the continents Physical & chemical properties b/t basaltic & granitic Extrusive basalt, ryolite, andesite andesitic Grey, medium magma Pacific Rim Intrusive Rich in Fe & Mg poor in SiO2 slowly Magma (below) basaltic Dense & dark colored when molten rock cools and solidifies large Lava (above) Ocean floor & Hawaii Granite, gabbro, diorite

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