Magma Generation Volcanism and Volcanoes Chapter Reading PDF

# Igneous Processes and Volcanoes ## 4.3.1 Geothermal Gradient * Below the surface, the temperature of the Earth rises. * This heat is caused by residual heat left from the formation of Earth and ongoing radioactive decay. * The rate at which temperature increases with depth is called the **geoth...

# Igneous Processes and Volcanoes ## 4.3.1 Geothermal Gradient * Below the surface, the temperature of the Earth rises. * This heat is caused by residual heat left from the formation of Earth and ongoing radioactive decay. * The rate at which temperature increases with depth is called the **geothermal gradient**. * The average geothermal gradient in the upper 100 km (62 mi) of the crust is about 25°C per kilometer of depth. * For every kilometer of depth, the temperature increases by about 25°C. ## 4.4 Volcanism * When magma emerges onto the Earth's surface, the molten rock is called **lava**. * A **volcano** is a type of land formation created when lava solidifies into rock. * Volcanoes have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of plate tectonics has made them less mysterious. ### 4.4.1. Distribution and Tectonics * Most volcanoes are **interplate volcanoes**. * Interplate volcanoes are located at active plate boundaries created by volcanism at mid-ocean ridges, subduction zones, and continental rifts. * Some volcanoes are **intraplate volcanoes**. * Intraplate volcanoes are located within tectonic plates, far removed from plate boundaries. * Many intraplate volcanoes are formed by **hotspots**. ## Volcanoes at Mid-ocean Ridges * Most volcanism on Earth occurs on the ocean floor along **mid-ocean ridges**. * These interplate volcanoes are also the least observed and famous, since most of them are located under 3,000-4,500 m (10,000-15,000 ft) of ocean and the eruptions are slow, gentle, and oozing. * One exception is the interplate volcanoes of Iceland. * The diverging and thinning oceanic plates allow hot mantle rock to rise, releasing pressure and causing decompression melting. * Ultramafic mantle rock, consisting largely of **peridotite**, partially melts and generates magma that is basaltic. * Because of this, almost all volcanoes on the ocean floor are basaltic. * In fact, most oceanic lithosphere is basaltic near the surface, with phaneritic gabbro and ultramafic peridotite underneath. * When basaltic lava erupts underwater it emerges in small explosions and/or forms pillow-shaped structures called **pillow basalts**. * These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around mid-ocean ridges. * This ecosystem exists around tall vents emitting black, hot mineral-rich water called **deep-sea hydrothermal vents**, also known as **black smokers**. ## Volcanoes at Subduction Zones * The second most commonly found location for volcanism is adjacent to subduction zones, a type of convergent plate boundary. * The process of subduction expels water from hydrated minerals in the descending slab, which causes flux melting in the overlying mantle rock. * Because subduction volcanism occurs in a volcanic arc, the thickened crust promotes partial melting and magma differentiation. * These evolve the mafic magma from the mantle into more silica-rich magma. * The **Ring of Fire** surrounding the Pacific Ocean, for example, is dominated by subduction-generated eruptions of mostly silica-rich lava. * The volcanoes and plutons consist largely of intermediate-to-felsic rock such as andesite, rhyolite, pumice, and tuff. ## Volcanoes at Continental Rifts * Some volcanoes are created at **continental rifts**. * Crustal thinning is caused by diverging lithospheric plates, such as the East African Rift Basin in Africa. * Volcanism caused by crustal thinning without continental rifting is found in the Basin and Range Province in North America. * In this location, volcanic activity is produced by rising magma that stretches the overlying crust. * Lower crust or upper mantle material rises through the thinned crust, releases pressure, and undergoes decompression-induced partial melting. * This magma is less dense than the surrounding rock and continues to rise through the crust to the surface, erupting as **basaltic lava**. * These eruptions usually result in **flood basalts**, **cinder cones**, and **basaltic lava flows**. ## Hotspots * Hotspots are the main source of **intraplate volcanism**. * Hotspots occur when lithospheric plates glide over a hot mantle plume, an ascending column of solid heated rock originating from deep within the mantle. * The mantle plume generates melts as material rises, with the magma rising even more. * When the ascending magma reaches the lithospheric crust, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across. * Since most mantle plumes are located beneath the oceanic lithosphere, the early stages of intraplate volcanism typically take place underwater. * Over time, basaltic volcanoes may build up from the sea floor into islands, such as the Hawaiian Islands. * Where a hotspot is found under a continental plate, contact with the hot mafic magma may cause the overlying felsic rock to melt and mix with the mafic material below, forming intermediate magma. * Or the felsic magma may continue to rise, and cool into a granitic batholith or erupt as a felsic volcano. * The Yellowstone caldera is an example of **hotspot volcanism** that resulted in an explosive eruption. ## Volcano Features and Types * There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other aspects. ### Stratovolcano * Also called a **composite cone volcano**. * Has steep flanks, a symmetrical cone shape, distinct crater, and rises prominently above the surrounding landscape. * The term composite refers to the alternating layers of pyroclastic fragments like ash and bombs, and solidified lava flows of varying composition. ### Shield Volcano * The largest volcanoes are **shield volcanoes**. * They are characterized by broad, low-angle flanks, small vents at the top, and mafic magma chambers. * The name comes from the side view, which resembles a medieval warrior's shield. * They are typically associated with hotspots, mid-ocean ridges, or continental rifts with rising upper mantle material. * The low-angle flanks are built up slowly from numerous low-viscosity basaltic lava flows that spread out over long distances. * The basaltic lava erupts effusively, meaning the eruptions are small, localized, and predictable. ### Cinder Cone * Cinder cones are small volcanoes with steep sides, and made of pyroclastic fragments that have been ejected from a pronounced central vent. * The small fragments are called **cinders** and the largest are **volcanic bombs**. * The eruptions are usually short-lived events, typically consisting of mafic lavas with a high content of volatiles. * Hot lava is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the volcano. ### Lava Domes * Lava domes are accumulations of silica-rich volcanic rock, such as rhyolite and obsidian. * Too viscous to flow easily, the felsic lava tends to pile up near the vent in blocky masses. * Lava domes often form in a vent within the collapsed crater of a stratovolcano, and grow by internal expansion. * As the dome expands, the outer surface cools, hardens, and shatters, and spills loose fragments down the sides. * Mount Saint Helens has a good example of a lava dome inside of a collapsed stratovolcano crater. ### Caldera * Calderas are steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber. * Calderas are generally very large, with diameters of up to 25 km (15.5 mi). * The term caldera specifically refers to a volcanic vent; however, it is frequently used to describe a volcano type. * Caldera volcanoes are typically formed by eruptions of high-viscosity felsic lava having high volatiles content. ### Flood Basalts * A rare volcanic eruption type, unobserved in modern times, is the **flood basalt**. * Flood basalts are some of the largest and lowest viscosity types of eruptions known. * They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. * Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been involved in the Earth's largest mass extinction. ## 4.4.3 Volcanic Hazards and Monitoring * While the most obvious volcanic hazard is lava, the dangers posed by volcanoes go far beyond lava flows. * For example, on May 18, 1980, Mount Saint Helens (Washington, United States) erupted with an explosion and landslide that removed the upper 400 m (1,300 ft) of the mountain. * The initial explosion was immediately followed by a lateral blast, which produced a **pyroclastic flow** that covered nearly 600 km² (230 mi²) of forest with hot ash and debris. * The pyroclastic flow moved at speeds of 80-130 kph (50-80 mph), flattening trees and ejecting clouds of ash into the air. * The USGS video provides an account of this explosive eruption that killed 57 people. ### Pyroclastic Flows * The most dangerous volcanic hazard are **pyroclastic flows**. * These flows are a mix of **lava blocks**, **pumice**, **ash**, and **hot gases** between 200°C-700°C (400°F-1,300°F). * The turbulent cloud of ash and gas races down the steep flanks at high speeds up to 193 kph (120 mph) into the valleys around composite volcanoes. * Most explosive, silica-rich, high-viscosity magma volcanoes such as composite cones usually have pyroclastic flows. * The rock **tuff** and **welded tuff** is often formed from these pyroclastic flows. ### Lahars * Lahar is an Indonesian word and is used to describe a **volcanic mudflow** that forms from rapidly melting snow or glaciers. * Lahars are slurries resembling wet concrete, and consist of water, ash, rock fragments, and other debris. * These mudflows flow down the flanks of volcanoes or mountains covered with freshly-erupted ash and on steep slopes can reach speeds of up to 80 kph (50 mph). * Several major cities, including Tacoma, are located on prehistoric lahar flows that extend for many kilometers across the floodplains surrounding Mount Rainier in Washington. ### Monitoring * Geologists use various instruments to detect changes or indications that an eruption is imminent. * The three videos show different types of volcanic monitoring used to predict eruptions: * earthquake activity * increases in gas emission * changes in land surface orientation and elevation. * One video shows how monitoring earthquake frequency, especially special vibrational earthquakes called **harmonic tremors**, can detect magma movement and possible eruption. * Another video shows how gas monitoring may be used to predict an eruption. * A rapid increase of gas emission may indicate magma that is actively rising to surface and releasing dissolved gases out of solution, and that an eruption is imminent. * The last video shows how a GPS unit and tiltmeter can detect land surface changes, indicating the magma is moving underneath it.