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Igneous Rocks

Igneous Rocks. Formed from the cooling and consolidation of magma. Plutonic (intrusive) = cooled below the surface. Volcanic (extrusive) = cooled on the surface. Igneous rocks. Magma: A mass of molten rock below the surface of the Earth.

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Igneous Rocks

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  1. Igneous Rocks

  2. Formed from the cooling and consolidation of magma. Plutonic (intrusive) = cooled below the surface. Volcanic (extrusive) = cooled on the surface. Igneous rocks

  3. Magma: A mass of molten rock below the surface of the Earth. Lava: Molten rock above the surface of the Earth. Magma vs Lava

  4. Basalt Basalt Granite Granite Macroscopic vs. microscopic Magnifying glass 1 cm Microscope 1 mm Figure 5.1

  5. Classifying based on crystal size …. Glassy no crystals Aphanitic rocks made of microscopically small crystals Phaneritic grains large enough to be seen with the naked eye Porphyritic mixture of coarse and fine Vesicular with bubble holes Pyroclastic formed of ash, glass, and mineral fragments blown into the air by volcanic explosion Igneous textures

  6. Glassy: Obsidian Glassy = no crystals.

  7. Aphanitic: Rhyolite Aphanitic = crystals too small to be seen with the naked eye.

  8. Phaneritic: Gabbro Phaneritic = grains large enough to be seen with the naked eye.

  9. Porphyritic Porphyritic = mixture of coarse and fine grains.

  10. Vesicular: Pumice Vesicular = with bubble holes.

  11. Pyroclastic Pyroclastic = formed from ash, glass shards, and mineral fragments thrown into the air by a volcanic eruption.

  12. slower cooling coarser crystals deep in ground What controls texture? How fast an igneous rock cools. What controls how fast an igneous rock cools? The setting where the rock is formed. In general,

  13. Formation and texture 1. Pyroclasts form from airborne lava in violent eruption 2. Extrusive igneous rocks. Cool rapidly on the Earth’s surface 3. Intrusive igneous rocks. Cool slowly in the Earth’s interior allowing large crystals to form 4. Porphyry starts to grow below the surface but before solidification is brought to the surface Figure 5.3

  14. ExtrusiveIntrusive Volcanic Plutonic Fast-cooling Slow-cooling Fine-grained Coarse-grained basalt gabbro andesite diorite rhyolite granite “Mafic” vs “felsic”? It turns out there are two major ways of classifying igneous rocks. We’ve already seen one, classification by texture: There’s also classification by composition: • magnesium (Mg) + iron (Fe) = mafic • feldspar + quartz (Si) = felsic

  15. Table 5-1

  16. When we talk about the chemical composition of a rock we usually speak in terms of the weight percent oxides, e.g., Typical Typical basalt granite SiO2 50% 70% Al2O3 15% 12% FeO+MgO 15% 3% CaO 8% 2% K2O+Na2O 5% 8% Composition of Igneous Rocks

  17. Classification of Igneous Rocks Mafic minerals crystallize at higher tempera-tures (earlier in the cooling of a magma) than felsic minerals. Figure 5.4

  18. Melt a whole rock, and the composition of the melt will be the same as the composition of the whole rock. However, whole melting is the exception! Most rocks undergo partial melting. In partial melting, different minerals melt at different times (have different melting points). Composition of the liquid is different from the composition of the residual solid. Like a melting a chocolate chip cookie (melt to the point that the chips melt but the rest of the cookie is intact). How do rocks melt?

  19. Melting takes place when material rises to the surface (effect of decreasing pressure). Melt is more buoyant than the residual rock, so it rises to the surface. How do rocks melt?

  20. Factors affecting melting temperatures (Table 5.3)

  21. More on pressure …. • Squeeze water hard, and it will freeze! • Rocks work the same way. • Pressure increases with depth in the Earth’s interior. • Rocks which will melt at 1000°C at the surface will melt at 1300°C in the interior.

  22. Even more on pressure …. • Under a mid-ocean ridge, temperature doesn’t change much with depth, but pressure does. • Decompression melting. Figure 1.11

  23. Tectonic Settings of Igneous Activity Figure 5.11

  24. Volcanic Island Arc, Indonesia

  25. Hawaii Oceanic Hot Spot

  26. Lava flow at Volcanoes National Park, Hawaii

  27. Continental Volcanic ArcN. Cascades

  28. Types of Igneous Structures Figure 5.7

  29. Some definitions • Pluton: Large igneous body formed deep within the Earth’s crust. • Batholiths: The largest plutons (at least 100 km2). • Stocks: Smaller plutons. • Sill: Tabular, sheetlike body formed by the injection of magma between layers of rock. • Dike: Tabular igneous bodies that cut across layers of rock. • Country rock: Rock surrounding an igneous intrusion.

  30. Figure 5.9 Sill

  31. Dike Figure 5.9

  32. Mid-ocean ridge magmatism Figure 5.13

  33. Subduction zone magmatism Figure 5.14

  34. Subduction zone magmatism Figure 5.15

  35. Mt. Rainier

  36. Fractional crystallization • The process of crystallizing and removal of crystals from the magma. • Composition of crystals is different than magma. • Fractional crystallization results in chemical change in the magma. • High temperature minerals crystallize first (melt last).

  37. Bowen’s Reaction Series • Kind of a mirror image of partial melting. • As magma temperature decreases, minerals crystallize out of the melt in an ordered sequence. • Two series: Mafic and felsic. Figure 5.5

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