Understanding Earth Chapter 4: IGNEOUS ROCKS Solids from Melts 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 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

How Do Igneous Rocks Differ from One Another? Texture – size of crystals Coarse-grained rocks Fine-grained rocks Mixed texture rocks

How Do Igneous Rocks Differ from One Another?

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

How Do Igneous Rocks Differ from One Another? Texture is related to rate of cooling. Intrusive igneous rocks Extrusive igneous rocks

How Do Igneous Rocks Differ from One Another?

Igneous Textures Pyroclasts Extrusive rocks Porphyry Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive rocks Porphyry Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Porphyry Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Porphyry Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Intrusive rocks

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Intrusive rocks Porphyry

Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Intrusive rocks Some phenocrysts grow large, but the remaining melt cools faster, forming smaller crystals during an eruption. Porphyry

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

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 processes create the unequal sizes of crystals in porphyries?

2. How Do Magmas Form? Why do rocks melt?

A rising mass of magma that pushes aside crustal rocks as 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.

A temperature of about 1000C is required for partial melting of 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

3. Magmatic Differentiation BOWEN’S REACTION SERIES Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Simultaneous crystallization Ultramafic (low silica) ~1200°C Olivine Calcium- rich

BOWEN’S REACTION SERIES As magma temperature decreases… Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Simultaneous crystallization Ultramafic (low silica) ~1200°C Olivine Calcium- rich

BOWEN’S REACTION SERIES As magma temperature decreases… …materials crystallize in an ordered series… Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Simultaneous crystallization Ultramafic (low silica) ~1200°C Olivine Calcium- rich

BOWEN’S REACTION SERIES …while plagioclase feldspar crystallizes, from calcium-rich sodium-rich form… As magma temperature decreases… …materials crystallize in an ordered series… Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Simultaneous crystallization Ultramafic (low silica) ~1200°C Olivine Calcium- rich

BOWEN’S REACTION SERIES …while plagioclase feldspar crystallizes, from calcium-rich sodium-rich form… As magma temperature decreases… …materials crystallize in an ordered series… Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Simultaneous crystallization Ultramafic (low silica) ~1200°C Olivine Calcium- rich …and the composition of magma changes from ultramafic to andesitic.

3. Magmatic Differentiation GRANITE AND BASALT Magma chamber A Crystallizing minerals Magma chamber B Partial melting of country rock Basaltic magma

Partial melting creates a magma of a particular composition. chamber A Crystallizing minerals 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. Magma chamber A Crystallizing minerals 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. Magma chamber A Magma chamber A Crystallizing minerals Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Magma chamber A Magma chamber A Crystallizing minerals Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Mixing results in andesitic magma. Magma chamber A Magma chamber A Crystallizing minerals Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Mixing results in andesitic magma. Magma chamber A Magma chamber A Crystallizing minerals Crystals may accumulate on the sides and roof of the chamber due to turbulence. Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

Thought questions for this chapter What observations would show that a pluton solidified during fractional crystallization?

4. Forms of Igneous Intrusions Plutons Discordant intrusions Batholiths Stocks Concordant intrusions Sills Dikes Veins

4. Forms of Igneous Intrusions

Ash falls and pyroclasts Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dike Sill Dike Sill Pluton Batholith

Dikes cut across layers of country rock… Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dikes cut across layers of country rock… Dike Sill Dike Sill Pluton Batholith

Dikes cut across layers of country rock… Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dikes cut across layers of country rock… Dike Sill Dike Sill Pluton Batholith …but sills run parallel to them.

Dikes cut across layers of country rock… Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dikes cut across layers of country rock… Dike Sill Dike Sill Pluton Batholith …but sills run parallel to them. Batholiths are the largest forms of plutons, covering at least 100 km2.

Magma factories: Spreading centers Subduction zones Mantle plumes 5. Igneous Processes and Plate Tectonics Magma factories: Spreading centers Subduction zones Mantle plumes

5. Igneous Processes and Plate Tectonics

Origin of magma in magma factories: 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 Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Mantle Peridotite layer Spreading center

Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Mantle Peridotite layer Spreading center

A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Mantle Peridotite layer Spreading center

A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, Dikes Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Mantle Peridotite layer Spreading center

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Mantle Peridotite layer Spreading center

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro Moho Mantle Peridotite layer Spreading center

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center Cold seawater Heated seawater carrying dissolved minerals Sheeted dikes

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center Cold seawater Heated seawater carrying dissolved minerals Sheeted dikes Seawater filters through the basalt layer, where it is heated.

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center Cold seawater Heated seawater carrying dissolved minerals Sheeted dikes Seawater filters through the basalt layer, where it is heated. The heated seawater then rises. Dissolved minerals precipitate in the ocean.

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center Cold seawater Heated seawater carrying dissolved minerals Magma chamber Sheeted dikes Peridotite layer Mantle Seawater filters through the basalt layer, where it is heated. The heated seawater then rises. Dissolved minerals precipitate in the ocean.

Dikes intrude dikes to form sheeted dikes. Dikes intruding dikes A thin dike erupts, spilling lava in “pillows.” Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center Cold seawater Heated seawater carrying dissolved minerals Magma chamber Sheeted dikes Peridotite layer Mantle Seawater filters through the basalt layer, where it is heated. The heated seawater then rises. Dissolved minerals precipitate in the ocean. Crystals settle out of the magma, forming the peridotite layer.

Fluid-Induced Melting – Subduction Zones Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere

Subducting oceanic crust carries sediments with it. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it.

Subducting oceanic crust carries sediments with it. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it. Sediment grains Water

Subducting oceanic crust carries sediments with it. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it. Water remains trapped as the pressure and temperature increase. Sediment grains Water

Subducting oceanic crust carries sediments with it. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O H2O Water remains trapped as the pressure and temperature increase. Sediment grains The trapped water is released as the temperature increases,… Water

Subducting oceanic crust carries sediments with it. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O …causing the sedimentary rocks to melt at lower temperatures. H2O Water remains trapped as the pressure and temperature increase. Sediment grains The trapped water is released as the temperature increases,… Water

and molten sediments melt parts of the overlying plate. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere The water and molten sediments melt parts of the overlying plate. Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O …causing the sedimentary rocks to melt at lower temperatures. H2O Water remains trapped as the pressure and temperature increase. Sediment grains The trapped water is released as the temperature increases,… Water

and molten sediments melt parts of the overlying plate. combine with lithospheric magma. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere The water and molten sediments melt parts of the overlying plate. Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O …causing the sedimentary rocks to melt at lower temperatures. H2O Water remains trapped as the pressure and temperature increase. Sediment grains The trapped water is released as the temperature increases,… Water

Magma of intermediate composition is erupted to form arc volcanoes. Molten sediments combine with lithospheric magma. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere The water and molten sediments melt parts of the overlying plate. Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O …causing the sedimentary rocks to melt at lower temperatures. H2O Water remains trapped as the pressure and temperature increase. Sediment grains The trapped water is released as the temperature increases,… Water

Thought questions for this chapter 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 Basalt Batholith Bomb Concordant intrusion 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 Intrusive igneous rock Lava Mafic rock 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 Sill Stock Tuff Ultramafic rock Vein Viscosity Pyroclast Rhyolite Sill Stock Tuff Ultramafic rock Vein Viscosity Volcanic ash