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Igneous Rocks I

Igneous Rocks I. Formed from the cooling and consolidation of magma plutonic (intrusive) — cooled below the surface volcanic (extrusive) — cooled on the surface. Igneous rocks II. Macroscopic-microscopic. 1 cm. 1 mm. Glassy no minerals present

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Igneous Rocks I

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  1. Igneous Rocks I

  2. Formed from the cooling and consolidation of magma plutonic (intrusive) — cooled below the surface volcanic (extrusive) — cooled on the surface Igneous rocks II

  3. Macroscopic-microscopic 1 cm 1 mm

  4. Glassy no minerals present Crystalline rocks made of mineral grains Porphyritic mixture of coarse and fine Vesicular with bubble holes Igneous textures

  5. Composition:higher SiO2; higher viscositylower volatiles; higher viscosity Temperature: lower temperature; higher viscosity Factors controlling the viscosity of magmas Viscosity: The internal friction of a fluid which makes its resist flowing. Higher viscosity larger amount of friction required to flow Newtonian fluid: low viscosity, no resistance to flow.

  6. Granite (g) Intruding Metamorphic (m) Rocks m g Fig. 4.2

  7. Controlled by cooling rate Grain size Degree of crystallinity Vesicularity Magma Texture of Igneous Rocks Usually a silicate melt (liquid) at high temperatures (650 to 1200°C). Mixture of all the elements that make up minerals plus volatile components: H2O, CO2, Cl, F, S These components form gases and will exsolve when pressure is released.

  8. 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

  9. Pyroclastic Igneous Rocks Obsidian Pumice Ash Fig. 4.3

  10. Common minerals

  11. Extrusive Intrusive Fine grained Coarse grained Basalt gabbro Andesite diorite Rhyolite granite Classification by composition • magnesium (Mg) + iron (Fe) = mafic • feldspar + quartz (Si) = felsic Classification by texture Basalt Gabbro Rhyolite Granite

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

  13. Classification of Igneous Rocks Iron, magnesium and calcium content Sodium and potassium content 700oC Temperature 1200oC

  14. The process of complete melting of a rock or complete crystallization of a magma will not change the bulk composition of the system, however it is the exception! If either of these processes goes only part way, the composition of the solid and the liquid can be very different (especially for small amounts of melt, ƒ). Melting takes place when material rises to the surface Melt is more bouyant than matrix and is often able to escape. Melting and crystallization:

  15. Factors Affecting Melting Temperatures Fig. 4.7

  16. 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)

  17. Liquids Squeezed from Crystals

  18. Sill intrusive paralel to the layering Dyke intrusive cross-cutting the layering The Palisades Sill

  19. Palisades explained

  20. Bowen’s reaction series

  21. Magma Differentiation Fig. 4.12

  22. Types of Igneous Structures

  23. Methods of Intruding Magma

  24. Sill and dike

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