Lecture 4_Igneous rocks.pdf

Evolution of Earth and life ECS 1213 Lecture – 4 Igneous rocks  Questions Why does the rock melt? If they are forming by melting of rocks, why do we expect to find different compositions? Can we identify ancient plate boundaries? Photo...

Evolution of Earth and life ECS 1213 Lecture – 4 Igneous rocks  Questions Why does the rock melt? If they are forming by melting of rocks, why do we expect to find different compositions? Can we identify ancient plate boundaries? Photo credit  What is rock cycle? Three different types of rocks are closely related in their formation. Pressure Igneous Metamorphic rock Temperature rocks Melting Sedimentary Magma rocks Melting Crystallization 1. Form igneous rocks 2. Rocks that you get from igneous rocks (formation of metamorphic and igneous rocks) 3. Relationship between metamorphic and sedimentary rocks  Magma: The parent material Magma = Molten rock that comes from the Earth’s interior. Lava = Magma that reaches the surface. 1. Extrusive/Volcanic igneous rocks: When molten rocks solidify above the ground. 2. Intrusive/Plutonic igneous rocks: When molten rocks solidify below the ground. 1. Extrusive rocks: Cascade Range 2. Intrusive rocks: Stone Mountain  Magma to Rock: Composition and Texture Volatile H2O, CO2, SO2 Magma Melt Composed of mainly silicon, oxygen Solid Composed of mainly silicate minerals - As magma cools down, solids crystallizes. - It settles down from the magma. -The remaining magma changes the composition. Texture of igneous rocks: Three factors decide how an igneous rock going to look like (texture): 1. The rate at which magma cools 2. The amount of silica 3. Amount of dissolved gas  Rate of cooling & crystallization If it cools rapidly, the crystals could not grow for very long time --- small crystals. If it cools very slowly, the crystals grow over a long period --- large crystals. Slow cooling Rapid cooling Magma Magma After 3hours After 5 hours After 5 hours After 20 hours After 7 hours Solid After 30 hours Solid What would be the crystal size of an extrusive igneous rock? What about intrusive ones? When rocks only have unordered ions, it is referred to as glass.  Types of igneous texture 1. Aphanitic (fine-grained) texture: Crystals too small to see. 2. Phaneritic (Coarse-grained) texture: Crystals large enough to see. 3. Porphyritic (mixed) texture: Both large and small crystals. 4. Glassy texture: No crystal structure. 5. Pyroclastic (fragmented) texture: Consolidation of rocks fragments. 6. Pegmatitic texture: Unusually large crystals because of the volatile content. 2. Phaneritic 3. 1. Aphanitic Porphyritic 4. Glassy 5. 6. Pyroclastic Pegmatititic  Igneous composition - Mainly composed of silicate minerals. - Other ions include Al, Ca, Na, K, Mg, Fe - As magma cools elements combine to form two major groups of silicates -- 1. Dark silicates – high in Mg, Fe; low in silica 2. Light silicates – high in silica; also has Na, K, Ca Mafic igneous rocks: - Composed of dark colored minerals - Heavy in Mg, Fe (hence the name) - Basaltic composition Felsic igneous rocks: - Composed of light colored minerals - Heavy in feldspar, silica (hence the name) - Granitic composition  Classification of igneous rocks Two main components: 1. Composition: Tells you something about the parent magma 2. Texture: Tells you something about the cooling history Composition Texture Classification of igneous rocks  What determines the composition? - Magma does not freeze at one particular temperature, rather over a range. -With progressive cooling different minerals form at different temperatures. - As a result the composition of the melt is continually changing. - Bowen’s reaction series explains the formation sequence of different minerals.  How do you get various composition? -Igneous rocks greatly vary in their composition. Magma mixing - Parent magma has a fairly homogeneous composition. (Younger magma body intrudes older Main mechanisms responsible behind formation of one) different rocks from same magma: Old magma 1. Magmatic differentiation due to crystal settling. 2. Assimilation New magma 3. Magma mixing Assimilation Host rock (Host rocks incorporated into magma) Magma Younger magma Crystallization & settling A composite picture