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Where do we find igneous rocks? • Divergent and convergent plate boundaries Continental margins above subduction zones Island arc volcanic chains above subduction zones Divergent boundaries: mid-ocean ridges, continental rifts • Hot spots • Hawaii – in an ocean basin • Yellowstone – on the N. American continent • Northern part of the E African Rift Valley (sometimes continental rifts, or divergent boundaries within continents, are started by hotspots) • Note - transform plate boundaries are not generally associated with igneous rocks and volcanism. Where do we find igneous rocks? Where magma is produced: known volcanoes of the world from the Smithsonian Global Volcanism Project Where do we find igneous rocks? Plutonic Rocks Exposed at the Surface Magma injects between or across layers of rock giving rise to the various plutonic rock bodies shown at right. Uplift and erosion over geologic time exposes intrusive bodies of igneous rock. Where do we find igneous rocks? Terminology for Intrusive Features • Dike – a tabular intrusion cutting across layers (usually oriented vertically) • Sill – a tabular intrusion in between layers (usually oriented horizontally) • Stock/batholith - large areas of intrusive rock that represent old “frozen” magma chambers (plutons) that are now exposed at the surface. By definition a batholith covers an area of >100 km2. Where do we find igneous rocks? Fig 4.7 Sills do not cut across older rocks that they intrude into. Dikes cut across older rocks they intrude into. Note geologists on the dike for scale! Where do we find igneous rocks? Shiprock, NM – a volcanic neck Dikes are often “feeders” for lava flows Sierra Nevada Mountains: A Large Batholith Where do we find igneous rocks? Volcanic rocks in the landscape • Volcanic neck - the frozen residual “core” of magma that was rising up through the center of the volcano (actually a very shallow intrusive body!) • Lava domes - a mound of lava that did not flow • Pyroclastic deposits - ash, pumice, lapilli, bombs, may form deposits such as welded tuff Rhyolite ash and pumice deposited very hot and fused into a consolidated mass of tuff – a compact, solid volcanic rock composed of pyroclastic material. What Are Common Volcanoes Like? • The Type of Volcano is Controlled by the Type of Magma That is Erupting. • Felsic (silicic) magmas are cooler, very viscous (thick, sticky) and highly gas charged. • Mafic magmas are hotter, more fluid and flow easily by comparison, and have significantly less gas. • The result is that silicic magma produces lava domes, short thick flows and explosive (pyroclastic) rocks. • In contrast, mafic magma produces long, thin lava flows, and eruptions are generally not as explosive. What Are Common Volcanoes Like? Gases Drive Eruptions Similar to a bottle of champagne, a “corked” volcano builds up pressure as gases come out of solution. Eventually enough pressure builds up that an eruption occurs. What Are Common Volcanoes Like? • Viscosity: defined as the resistance to flow • Controlled by silica tetrahedra polymerization in the melt, so varies dramatically with composition • High silica = high viscosity, thick, sticky magma Gases escape more readily from low-viscosity magma • Low silica = more fluid magma (like the thin basalt lava flows that make up the Hawaiian islands) What Are Common Volcanoes Like? What Are Common Volcanoes Like? Basic Types of Basalt Volcanoes Cinder cone: small, made mainly of pyroclastics. Eruptive phases 1 to 20 years. Shield volcano: vary in size, made of thin flows of basaltic lava. Longlived eruptions. Low profile. What Are Common Volcanoes Like? Intermediate to Felsic Composition Magmas Produce Very Different Volcanic Features Composite volcano: modest size, steep profile. Andesite to dacite lavas interbedded with pyroclastics. Intermittent eruptions. Dome complexes: modest size, overlapping domes. Dacite to rhyolite lavas. What Are Common Volcanoes Like? Tuff is deposited by a pyroclastic flow, these are called a nuee ardente or glowing avalanche. They move at speeds up to 200 kph (120 mph) and have internal temperatures up to 700-800 oC (approx. 1,300-1,500 oF). Pyroclastic flow on Merapi volcano, Indonesia What Are Common Volcanoes Like? Pyroclastic flows are characteristic of eruptions from andesite, dacite and rhyolite volcanoes. Pyroclastic flows on Mt. Unzen volcano in Japan What Are Common Volcanoes Like? Pyroclastic flow deposits may appear to be cold, but can remain very hot for many weeks after they are formed. Thermal camera images capture this. Recently formed pyroclastic flow on Montserrat volcano, Carribean What Are Common Volcanoes Like? Panum Crater, California – A small rhyolite lava dome with surrounding pyroclastic apron from the initial explosive phase of the eruption. What Are Common Volcanoes Like? Felsic (silicic) magma forms short, thick lava flows What Are Common Volcanoes Like? Mt. Fuji, Japan – A classic example of a dacite to andesite composition composite volcano – often called a stratovolcano. These are built up from explosive and effusive eruptions producing alternating layers of pyroclastic rocks and lava flows. What are common volcanoes like? Basalt magma forms pahoehoe lavas (smooth and with a “ropy” surface) or a’a’ lavas (rough and broken into sharp fragments). What sounds would you make if you walked barefoot on these two lavas? Pahoehoe lava in the process of forming…. Formation of Collapse Calderas • Two basic types: smaller ones produced on basalt volcanoes, and much larger ones produced by felsic volcanoes. • In either case withdrawal of magma from a nearsurface chamber leads to collapse of the rock above forming a large crater – called a caldera. • Basalt volcanoes – usually small, ~1-3 km diameter. • Silicic volcanoes – can be very large, the La Garita caldera in the San Juan Volcanic Field is ~30x80 km (20x50 miles). Approx. 5,000 km3 of magma was erupted to form this caldera (in comparison Mt. St. Helens was about 1 km3 of magma erupted) How and why do rocks melt? • If temperature is raised enough rocks will melt. • Silicate minerals melt across a wide range from approximately 800oC to around 1900oC. • Increasing pressure increases the melting temperature – Decreasing pressure does what? • At the surface olivine melts at 1890oC, whereas at 100 km it melts at 2050oC • Mixtures of minerals melt at a different temp than individual minerals (due to reactions between minerals). • Olivine mixed with pyroxene and plagioclase melts at ~1200oC • Adding water decreases the melting temperature. How does magma generation relate to plate tectonics? • Where do conditions for melting occur? • Where decompression occurs due to rising rock • Divergent boundaries: Mid-ocean ridges • Also, at hotspots • Convergent boundaries • Addition of water to the “mantle wedge” as wet ocean crust is subducted and dehydrates How and why do rocks melt? • When a rock melts completely (temperature high enough to melt all minerals) it can cool and recrystallize into the same rock. • However, incomplete melting - or partial crystallization - can yield different composition rocks than the starting composition rock or magma. • Example: partial melting may produce magma from pyroxene and plagioclase, most olivine is still solid. • Example: during partial crystallization olivine forms first, depleting the remaining magma in the chemical components that olivine needs to grow. How and why do rocks melt? What makes igneous rock compositions so diverse? Fractional crystallization: As minerals crystallize out, the remaining melt changes in comparison to the original melt. Example: Olivine crystallizing from a basalt magma depletes the remaining magma in Mg and Fe. What makes igneous rock compositions so diverse? The first crystals to form will either settle downward (if denser), or stay where they form—on the walls (cooling occurs from outside in). If remaining magma solidifies or erupts it’s composition will be different from the original magma composition. Fig 4.23 What makes igneous rock compositions so diverse? • Melting rocks produces magma of differing compositions. The extent to which the rocks melt also determines composition. • Fractional crystallization can change a magma into one with a different composition. • Rocks surrounding a magma chamber may partly melt and become assimilated. • Two or more magmas may mingle to produce a hybrid magma of intermediate composition.