Igneous and Metamorphic Rock Forming Minerals Department of Geology Mr. Victor Tibane 1 SGM 210_2013
Grotzinger Jordan Understanding Earth Sixth Edition Chapter 4: IGNEOUS ROCKS Solids from Melts 2011 by W. H. Freeman and Company
Chapter 4: Igneous Rocks: Solids from Melts
About Igneous Rocks Igneous rocks form from liquid rock (magma) in several different ways. Igneous processes within the Earth produce intrusive igneous rocks. Igneous processes on or near Earth s surface produce extrusive igneous rocks.
Lecture Outline 1. How do igneous rocks differ from one another? 2. How do magmas form? 3. Magmatic differentiation 4. Forms of igneous intrusions 5. Igneous processes and plate tectonics
1. How Do Igneous Rocks Differ from One Another? Texture size of crystals: Coarse-grained rocks Fine-grained rocks Mixed texture rocks
1. How Do Igneous Rocks Differ from One Another?
1. How Do Igneous Rocks Differ from One Another? Clues about significance of texture Early studies of volcanic rocks Laboratory crystallization studies Studies of slow cooling in granite
1. How Do Igneous Rocks Differ from One Another? Texture is related to rate of cooling. Intrusive igneous rocks Extrusive igneous rocks
Pyroclasts 1. Igneous Rocks Differ Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Basalt Felsic Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Intrusive rocks Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Some phenocrysts grow large, but the remaining melt cools faster, forming smaller crystals during an Porphyry eruption.
Chemical and Mineral Composition of Igneous Rocks Two basic compositional groups: Felsic igneous rocks Mafic igneous rocks
Chemical and Mineral Composition of Igneous Rocks Four compositional groups: Felsic igneous rocks Intermediate igneous rocks Mafic igneous rocks Ultramafic igneous rocks
2. How Do Magmas Form? Why do rocks melt?
2. How Do Magmas Form? What is a magma chamber? A rising mass of magma that pushes aside crustal rocks as it rises through the crust.
2. How Do Magmas Form? A temperature of about 1000 C is required for partial melting of crustal rocks. A depth of at least 40 km is required for temperatures of 1000 C to occur.
3. Magmatic Differentiation A process by which rocks of varying composition can arise from a uniform parent magma. The first minerals to crystallize from a cooling magma are the ones that are the last to melt.
3. Magmatic Differentiation Fractional crystallization is the process by which the crystals are formed in a cooling magma and are segregated from the remaining liquid. Example: basaltic intrusion like Palisades, New Jersey
3. Magmatic Differentiation The Palisades Intrusion An excellent example from South Africa is the Bushveld Intrusion
3. Magmatic Differentiation Bowen s Reaction Series Temperature ~600 C Orthoclase feldspar Muscovite mica Quartz Magma composition Felsic, Rhyolitic (high silica) Biotite mica Amphibole Sodiumrich Intermediate, andesitic Pyroxene Mafic, basaltic ~1200 C Olivine Simultaneous crystallization Calciumrich Ultramafic (low silica)
Bowen s Reaction Series As magma temperature decreases Temperature ~600 C Orthoclase feldspar Muscovite mica Quartz Magma composition Felsic, Rhyolitic (high silica) Biotite mica Amphibole Sodiumrich Intermediate, andesitic Pyroxene Mafic, basaltic ~1200 C Olivine Simultaneous crystallization Calciumrich Ultramafic (low silica)
Bowen s Reaction Series Temperature ~600 C As magma temperature decreases materials crystallize in an ordered series Orthoclase feldspar Muscovite mica Quartz Magma composition Felsic, Rhyolitic (high silica) Biotite mica Sodiumrich Intermediate, andesitic ~1200 C Olivine Amphibole Pyroxene Simultaneous crystallization Calciumrich Mafic, basaltic Ultramafic (low silica)
Bowen s Reaction Series Temperature ~600 C As magma temperature decreases materials crystallize in an ordered series Orthoclase feldspar Muscovite mica Quartz while plagioclase feldspar crystallizes, from calcium-rich to sodium-rich form Magma composition Felsic, Rhyolitic (high silica) Biotite mica Amphibole Sodiumrich Intermediate, andesitic Pyroxene Mafic, basaltic ~1200 C Olivine Simultaneous crystallization Calciumrich Ultramafic (low silica)
Bowen s Reaction Series Temperature ~600 C As magma temperature decreases materials crystallize in an ordered series Orthoclase feldspar Muscovite mica Quartz while plagioclase feldspar crystallizes, from calcium-rich sodium-rich form Magma composition Felsic, Rhyolitic (high silica) Biotite mica Amphibole Sodiumrich Intermediate, andesitic Pyroxene Mafic, basaltic ~1200 C Olivine Simultaneous crystallization Calciumrich Ultramafic (low silica) and the composition of magma changes from ultramafic to andesitic.
3. Magmatic Differentiation Granite and Basalt Crystallizing minerals Magma chamber A Magma chamber B Partial melting of country rock Basaltic magma
Partial melting creates a magma of a particular composition. Cooling causes minerals to crystallize and settle. Crystallizing minerals Magma chamber A Magma chamber B Magma chamber A A basaltic magma chamber breaks through. Mixing results in andesitic magma. Magma chamber B Crystals may accumulate on the sides and roof of the chamber due to turbulence. Partial melting of country rock Basaltic magma
4. Forms of Igneous Intrusions Plutons: Discordant intrusions Batholiths Stocks Concordant intrusions Sills Dikes Veins
4. Forms of Igneous Intrusions
Country rock Volcano Lava flow (extrusive) Ash falls and pyroclasts (effusive) Volcanic neck with radiating dikes Stock Sill Sill Sill Dikes cut across layers of country rock but sills run parallel to country rock. Batholiths are the largest forms of plutons, covering at least 100 km 2.
5. Igneous Processes and Plate Tectonics Magma factories: Spreading centers Subduction zones Mantle plumes
5. Igneous Processes and Plate Tectonics
5. Igneous Processes and Plate Tectonics Origin of magma in magma factories: Decompression melting in spreading centers Fluid-induced melting in subduction zones
Decompression Melting: Spreading Centers
Decompression Melting: Spreading Centers
Decompression Melting: Spreading Centers
Fluid-Induced Melting Subduction Zones
Thought questions for this chapter How would you classify a coarse-grained igneous rock that contains about 50 percent pyroxene and 50 percent olivine? What kind of rock would contain some plagioclase feldspar crystals about 5 mm long floating in a dark gray matrix of crystals less than 1 mm? What differences in crystal size might you expect to find between two sills, one intruded at a depth of about 12 km, where country rock was very hot, and the other at a depth of 0.5 km, where the country rock was moderately warm?
Thought questions for this chapter Assume that a magma with a certain ratio of calcium to sodium starts to crystallize. If fractional crystallization occurs during the solidification process, will the plagioclase feldspars formed after complete crystallization have the same ratio of sodium that characterized the magma? Why are plutons more likely than dikes to show the effects of fractional crystallization? What might be the origin of a rock composed almost entirely of olivine?
Thought questions for this chapter What observations would show that a pluton solidified during fractional crystallization? If you were to drill a hole through the crust of a mid-ocean ridge, what intrusive or extrusive igneous rocks might you expect to encounter at or near the surface? What intrusive or extrusive igneous rocks might you expect at the base of the crust? Water is abundant in the sedimentary rocks and oceanic crust of subduction zones. How would the water affect melting in these zones? Why are granitic and andesitic rocks so plentiful?
Key terms and concepts Andesite Basalt Batholith Bomb Concordant intrusion Country rock Dacite Decompression melting Dike Discordant intrusion Extrusive igneous rock Felsic rock Fluid-induced melting Fractional crystallization Gabbro
Key terms and concepts Granodiorite Intermediate igneous rock Intrusive igneous rock Lava Mafic rock Magma chamber Magmatic differentiation Obsidian Ophiolite suite Partial melting Pegmatite Peridotite Pluton Porphyry Pumice
Key terms and concepts Pyroclast Rhyolite Sill Stock Tuff Ultramafic rock Vein Viscosity Volcanic ash