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Igneous Rocks

Igneous Rocks. An igneous rock is formed when magma or lava cools and solidifies as a result of the process of crystallization. Remember: Definition of a Rock. Is a solid 2) Contains a mixture of one or more minerals 3) Occurs naturally as part of our planet. Molten Rock.

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Igneous Rocks

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  1. Igneous Rocks Anigneous rock is formed when magma or lava cools and solidifies as a result of the process of crystallization.

  2. Remember: Definition of a Rock Is a solid 2) Contains a mixture of one or more minerals 3) Occurs naturally as part of our planet

  3. Molten Rock In the Earth is magma. Magma is buoyant and therefore, rises to the surface. Sometimes it breaks through. When magma reaches Earth’s surface, it is called lava.

  4. Why Should We Care? • Igneous rocks make up the bulk of Earth’s crust. • Earth’s mantle is basically one huge igneous rock. • Igneous rocks are important economically. • Creates and provides striking landscape features.

  5. Igneous rocks that form at the surface are volcanic or extrusive. Igneous rocks that form deep down are plutonic or intrusive.

  6. Magma body Plutonic Rocks: To see them, they must be uplifted to the surface and softer surrounding rock must be weathered and eroded away. El Capitan, Yosemite National Park, California

  7. As magma cools, atoms of elements of minerals arrange themselves in an orderly crystal structure. (Crystallization) Note the cooling and growth of crystals.

  8. Coarse-grained Igneous Rock Texture (Phaneritic) Close up of crystals. Note the variety of crystal sizes.

  9. Texture Texture a. size b. shape c. arrangement of interlocking crystals

  10. cooling rate crystal size Slow Cooling Larger Crystals Fast Cooling Smaller or no Crystals

  11. Coarse-grained Phaneritic Fine-grained Aphanitic Glassy Large crystals in matrix Porphyritic

  12. Aphanitic or Fine-grained Phaneritic or Coarse-grained • Fast cooling lava. • Forms at the surface. • Sometimes holes are present due to escaping gases. • Cannot see individual crystals • Forms far • below the surface. • Slow cooling magma. • Intergrown larger crystals. Porphyritic Glassy • Magma cooled slowly for a while then erupted and became lava. • Minerals crystallize at different temperatures and therefore, rates. • Large crystals (phenocrysts) in a matrix (groundmass). • Very rapid cooling. • Ions unable to unite in an orderly crystalline structure so therefore, no crystals.

  13. Porphyritic • Texture

  14. Aphanitic Texture Or Fine-grained

  15. Texture Rock Types Coarse-grained Peridotite, Gabbro, Diorite, Granite Fine-grained Komatiite, Basalt, Andesite, Rhyolite Glassy Obsidian, Pumice, Scoria Vesicular Pumice, Scoria Porphyritic Andesitic porphyry

  16. Igneous Compositions • Mainly silicate minerals. • Determined by the composition of magma or lava from which it crystallizes. • Magma or lava mainly consists of 8 elements, which include: Si, O, Al, Ca, Na, K, Mg, and Fe. • WHEN YOU THINK OF IGNEOUS COMPOSITIONS, BE SURE TO THINK OF BOWEN’S REACTION SERIES.

  17. Igneous Compositions Magma cools slowly. Lava cools fastly. Solidification occurs due to the process of crystallization. Silicate minerals form and there are two groups, which include: dark and light.

  18. Silicate Group Poor In Examples Rich In DARK (Simatic) Olivine Pyroxene Amphibole Biotite Fe and/or Mg Si Si, K, Na, Ca LIGHT (Sialic) Fe and/or Mg Quartz Muscovite Feldspars

  19. Magmatic Differentiation In a magma body: Crystals form as magma cools. Heavy crystals sink to the bottom. There is more than one type of magma as it continuously evolves as minerals crystallize. A variety of igneous rocks crystallize from the same source due to the evolving magma.

  20. Bowen’s Reaction Series – Different Version Of The Same Thing Figure 4.23

  21. Igneous Compositions Assimilation As magmas migrate upwards, they may incorporate surrounding host rock. Magma Mixing Magma bodies could collide with other magma bodies and mixing could therefore occur.

  22. Assimilation Example Big chunks of olivine!

  23. Naming Igneous Rocks Igneous rocks are classified based on: • Texture • Mineral Composition Depends on: how fast/slow lava/magma cools. Depends on: chemical makeup of parent lava/magma.

  24. magnesium + ferrum feldspar + silica (quartz) Igneous Rocks Types • mafic • felsic high in Mg, Fe. Dark, dense high in Si. Lighter, less dense

  25. granite rhyolite Naming Igneous Rocks zooming in volcanic f e l s i c plutonic

  26. Naming Igneous Rocks obsidian volcanic f e l s i c volcanic pumice

  27. Naming Igneous Rocks zooming in andesite volcanic i n t e r m e d i a t e plutonic diorite

  28. Naming Igneous Rocks zooming in basalt volcanic m a f i c gabbro plutonic

  29. volcanic m a f i c scoria

  30. magma recipes 3 main types magnesium + ferrum high in Mg, Fe. Dark, dense “mafic” • basaltic • andesitic • granitic feldspar + silicate high in Si, lighter, less dense “felsic”

  31. magma recipes 3 main types magnesium + ferrum high in Mg, Fe. Dark, dense Most common volcanic Igneous rock. “mafic” • basaltic • andesitic • granitic Most common plutonic Igneous rock “felsic” “felsic” feldspar + silicate high in Si, lighter, less dense

  32. Kimberlite Kimberlite is a volatile-rich, potassic, ultramafic, igneous rock that occurs as small volcanic pipes, dykes, and sills. Kimberlite commonly contains inclusions of upper mantle-derived ultramafic rocks (due to formation and upward travel through the mantle). Minerals associated with kimberlite include: olivine, ilmenite, pyrope, almandine-pyrope, diopside, phlogopite, enstatite, and chromite.

  33. The general consensus reached on kimberlites is that they are formed deep within the mantle, between 150 and 450 kilometers deep, from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and volatile components. It is this depth of melting and generation that makes kimberlites prone to hosting diamonds.

  34. Most kimberlites are confined to the ancient cratons (or areas underlain by the cratons). The ages of most kimberlites are in the Late Mesozoic Era (Jurassic-Cretaceous periods).

  35. Diamonds in the Kimberlite

  36. Kimberlite Indicator Minerals Cr-pyrope (purple colour,), eclogitic garnet (orange-red), Cr-diopside (pale to emerald green), Mg-ilmenite (black, conchoidal fracture), chromite (reddish-black), and olivine (pale yellow-green) are the most commonly used kimberlite indicator minerals.

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