Chapter 4 Up from the Inferno: Magma and Igneous Rocks Essentials of Geology, 4th edition by Stephen Marshak © 2013 W.W. Norton & Company 1 PowerPoint slides prepared by Rick Oches, Professor of Geology & Environmental Sciences, Bentley University, Waltham, Massachusetts Up from the Inferno: Magma and Igneous Rocks Introduction ! Volcano—a vent where molten rock comes out of Earth " Example: Kilauea Volcano, Hawaii # Hot (~1,200oC) lava pools around the volcanic vent. # Hot, syrupy lava runs downhill as a lava flow. # The lava flow slows, loses heat, and crusts over. # Finally, the flow stops and cools, forming an igneous rock. Introduction ! Igneous rock is formed by cooling from a melt. " Magma—melted rock below ground " Lava—melted rock once it has reached the surface ! Igneous rock freezes at high temperatures (T). " 1,100 °C–650 °C, depending on composition. ! There are many types of igneous rock. Igneous Rocks ! Melted rock can cool above or below ground. " Extrusive igneous rocks—cool quickly at the surface # Lava flows—streams or mounds of cooled melt # Pyroclastic debris—cooled fragments $ Volcanic ash—fine particles of volcanic glass $ Volcanic rock—fragmented by eruption Igneous Rocks ! Melted rock can cool above or below ground. " Intrusive igneous rocks—cool out of sight, underground " Much greater volume than extrusive igneous rocks " Cooling rate is slower than for extrusives. # Large volume magma chambers # Smaller volume tabular bodies or columns Why Does Magma Form? ! Magma is not everywhere below Earth’s crust. ! Magma only forms in special tectonic settings. " Partial melting occurs in the crust and upper mantle. " Melting is caused by # pressure release. # volatile addition. # heat transfer. Causes of Melting ! Decrease in pressure (P)—decompression " The base of the crust is hot enough to melt mantle rock. " But, due to high P, the rock doesn’t melt. " Melting will occur if P is decreased. # P drops when hot rock is carried to shallower depths. $ Mantle plumes $ Beneath rifts $ Beneath mid-ocean ridges Causes of Melting ! P drops when hot rock is carried to shallower depths. " Mantle plumes " Beneath rifts " Under mid-ocean ridges Causes of Melting ! Addition of volatiles (flux melting) " Volatiles lower the melting T of a hot rock. " Common volatiles include H2O and CO2. " Subduction carries water into the mantle, melting rock. Causes of Melting ! Heat transfer melting " Rising magma carries mantle heat with it. " This raises the T in nearby crustal rock, which then melts. What Is Magma Made Of? ! Magmas have three components (solid, liquid, and gas). " Solid—solidified mineral crystals are carried in the melt. " Liquid—the melt itself is composed of mobile ions. # Dominantly Si and O; lesser Al, Ca, Fe, Mg, Na, and K # Other ions to a lesser extent. " Different mixes of elements yield different magmas. What Is Magma Made Of? " Gas—variable amounts of dissolved gas occur in magma. # Dry magma—scarce volatiles # Wet magma—up to 15% volatiles $ Water vapor (H2O) $ Carbon dioxide (CO2) $ Sulfur dioxide (SO2) $ Nitrogen (N2) $ Hydrogen (H2) Major Types of Magma ! There are four major magma types based on % silica (SiO2). " Felsic (feldspar and silica) 66–76% SiO2 " Intermediate 52–66% SiO2 " Mafic (Mg- and Fe-rich) 45–52% SiO2 " Ultramafic 38–45% SiO2 Magma Movement ! Magma doesn’t stay put; it tends to rise upward. " Magma may move upward in the crust. " Magma may breach the surface—a volcano. ! This transfers mass from deep to shallow parts of Earth. " A crucial process in the Earth System " Provides the raw material for soil, atmosphere, and ocean Magma Movement ! Why does magma rise? " It is less dense than surrounding rocks. # Magma is more buoyant. # Buoyancy lifts magma upward. " Weight of overlying rock creates pressure. # Pressure squeezes magma upward. # It is like mud squeezed between your toes. Magma Movement ! Speed of magma flow governed by viscosity. " Lower viscosity eases movement. " Lower viscosity is generated by # higher T. # lower SiO2 content. # higher volatile content. Magma Movement ! Viscosity depends on temperature, volatiles, and silica. " Temperature: # hot = lower viscosity; cooler = higher viscosity " Volatile content: # More volatiles—lower viscosity # Less volatiles—higher viscosity " Silica (SiO2) content: # Less SiO2 (mafic)—lower viscosity. # More SiO2 (felsic)—higher viscosity. Making Igneous Rock ! Fractional crystallization—settling early formed crystals. " Felsic magma can evolve from mafic magma. " Modeled by Bowen’s reaction series # Experimental results of mineral growth in magmas # A mineral succession proceeds from cooling. Bowen’s Reaction Series ! N. L. Bowen—devised experiments cooling melts (1920s). " Early crystals settled out, removing Fe, Mg, and Ca. " Remaining melt progressively enriched in Si, Al, and Na. ! He discovered that minerals solidify in a specific series. " Continuous—plagioclase changed from Ca-rich to Na-rich. " Discontinuous—minerals start and stop crystallizing. # Olivine # Pyroxene # Amphibole # Biotite Igneous Environments ! Two major categories—based on cooling locale. " Extrusive settings—cool at or near the surface. # Cool rapidly. # Chill too fast to grow big crystals. " Intrusive settings—cool at depth. # Lose heat slowly. # Crystals often grow large. Extrusive Settings ! Lava flows cool as blankets that often stack vertically. ! Lava flows exit volcanic vents and spread outward. ! Low-viscosity lava (basalt) can flow long distances. ! Lava cools as it flows, eventually solidifying. Extrusive Settings ! Explosive ash eruptions " High-viscosity felsic magma erupts explosively. " Yield huge volumes of ash that can cover large regions " Pyroclastic flow—volcanic ash and debris avalanche # Races down the volcanic slope as a density current # Often deadly Intrusive Settings ! Magma invades preexisting wall rock by " percolating upward between grains. " forcing open cracks. ! The wall rock—magma-intrusive contact reveals high heat. " Baked zone—rim of heat-altered wall rock " Chill margin—rim of