Chapter 4 - Magma Generation and Concepts Related to Volcanoes PDF

100 | IGNEOUS PROCESSES AND VOLCANOES The most important aspect of Bowen's Reaction Series is to notice the relationships between minerals and temperature. Norman L. Bowen (1887-1956) was an early 20th Cen- tury geologist who studied igneous rocks. He noticed that in igneous rocks, certain minerals...

100 | IGNEOUS PROCESSES AND VOLCANOES The most important aspect of Bowen's Reaction Series is to notice the relationships between minerals and temperature. Norman L. Bowen (1887-1956) was an early 20th Cen- tury geologist who studied igneous rocks. He noticed that in igneous rocks, certain minerals always occur together and these mineral assemblages exclude other minerals. Curious as to why, and with the hypothesis in mind that it had to do with the temperature at which the rocks cooled, he set about conducting experiments on igneous rocks in the early 1900s. He conducted experiments on igneous rock—grinding combinations of rocks into powder, seal- ing the powders into metal capsules, heating them to various temperatures, and then cooling them. When he opened the quenched capsules, he found a glass surrounding mineral crys- tals that he could identify under his petro- graphic microscope. The results of many of Figure 4.21: Norman L. Bowen. these experiments, conducted at different temperatures over a period of several years, showed that the common igneous minerals crystallize from magma at different temperatures. He also saw that minerals occur together in rocks with others that crystallize within similar temperature ranges, and never crystallize with other miner­ als. This relationship can explain the main difference between mafic and felsic igneous rocks. Mafic igneous rocks contain more mafic minerals, and therefore, crystallize at higher temperatures than felsic igneous rocks. Figure 4.22: Norman L. Bowen and his colleague This is even seen in lava flows, with felsic lavas erupting hundreds of working at the Carnegie Institution of Washington degrees cooler than their mafic counterparts. Bowen’s work laid the foun- Geophysical Laboratory. dation for understanding igneous petrology (the study of rocks) and resulted in his book, The Evolution of the Igneous Rocks in 1928. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 4.2 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=331#h5p-25 4.3 Magma Generation Magma and lava contain three components: melt, solids, and volatiles. The melt is made of ions from minerals that have liquefied. The solids are made of crystallized minerals floating in the liquid melt. These may be minerals that have already cooled Volatiles are gaseous components—such as water vapor, carbon dioxide, sulfur, and chlorine—dissolved in the magma. The presence and amount of these three components affect the physical behavior of the magma and will be dis- cussed more below.  IGNEOUS PROCESSES AND VOLCANOES | 101 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. So for every kilometer of depth, the temperature increases by about 25°C. The depth-temperature graph (see figure 4.23) illustrates the relationship between the geot­ hermal gradient (geotherm, red line) and the start of rock melting (solidus, green line). The geot­ Figure 4.23: Geothermal gradient. hermal gradient changes with depth (which has a direct rela- tionship to pressure) through the crust into upper mantle. The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing temperature with depth makes Figure 4.24: Pressure-temperature diagram showing temperature in degrees Celsius on the the depth of about 125 kilometers (78 miles) where the natural geothermal x-axis and depth below the surface in kilometers gradient is closest to the solidus. (km) on the y-axis. The red line is the geothermal gradient and the green solidus line represents the temperature and pressure regime at which melting The temperature at 100 km (62 mi) deep is about 1,200°C (2,192°F). At bot- begins. Rocks at pressures and temperatures left of tom of the crust, 35 km (22 mi) deep, the pressure is about 10,000 bars. A the green line are solid. If pressure/temperature bar is a measure of pressure, with 1 bar being normal atmospheric pressure conditions change so that rocks pass to the right of the green line, then they will start to melt. at sea level. At these pressures and temperatures, the crust and mantle are solid. To a depth of 150 km (93 mi), the geothermal gradient line stays to the left of the solidus line. This relationship continues through the mantle to the core–mantle boundary, at 2,880 km (1,790 mi). The solidus line slopes to the right because the melting temperature of any substance depends on pressure. The higher pressure created at greater depth increases the temperature needed to melt rock. In another example, at sea level with an atmospheric pressure close to 1 bar, water boils at 100°C. But if the pressure is lowered, as shown on the video below, water boils at a much lower temperature. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=Ks4VuXTTKmo Video 4.1: Boiling water at room temperature. If you are using an offline version of this text, access this YouTube video via the QR code. 102 | IGNEOUS PROCESSES AND VOLCANOES There are three principal ways rock behavior crosses to the right of the green solidus line to create molten magma: 1) decompression melting caused by lowering the pressure, 2) flux melting caused by adding volatiles (see more below), and 3) heat-induced melting caused by increasing the temperature. The Bowen’s Reaction Series shows that minerals melt at different temperatures. Since magma is a mixture of different minerals, the solidus boundary is more of a fuzzy zone rather than a well-defined line; some minerals are melted and some remain solid. This type of rock behavior is called partial melting and represents real-world magmas, which typically contain solid, liquid, and volatile components. 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. This sec­ tion describes volcano location, type, hazards, and monitoring. 4.4.1. Distribution and Tectonics Figure 4.25: Association of volcanoes with plate boundaries. Most volcanoes are interplate volcanoes. Interplate volcanoes are located at active plate boundaries created by vol­ canism at mid-ocean ridges, subduction zones, and continental rifts. The prefix “inter-“ means between. Some volcanoes are intraplate volcanoes. The prefix “intra-“ means within, and intraplate volcanoes are located within tectonic plates, far removed from plate boundaries. Many intraplate volcanoes are formed by hotspots.  IGNEOUS PROCESSES AND VOLCANOES | 103 Volcanoes at Mid-ocean Ridges Figure 4.26: Map of spreading ridges throughout the world. Most volcanism on Earth occurs on the ocean floor along mid-ocean ridges, a type of divergent plate boundary (see chapter 2). 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, Figure 4.27: Pillow basalt on sea floor near Hawai’i. 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. Figure 4.28: Black smoker hydrothermal vent with a colony of giant (6’+) tube worms. 104 | IGNEOUS PROCESSES AND VOLCANOES Figure 4.29: Distribution of hydrothermal vent fields. Without sunlight to support photosynthesis, these organisms instead utilize a process called chemosynthesis. Certain bac- teria are able to turn hydrogen sulfide (H2S), a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies. The videos show some of the ecosystems found around deep-sea hydrothermal vents. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=a5aQ4W9GbpU Video 4.2: Updating the deep–diving submarine at 50-years-old. If you are using an offline version of this text, access this YouTube video via the QR code.  IGNEOUS PROCESSES AND VOLCANOES | 105 One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=dXOQFnU-49k Video 4.3: Incredible views on board the deep-sea vessel. If you are using an offline version of this text, access this YouTube video via the QR code. Volcanoes at Subduction Zones The second most commonly found location for volcanism is adja- cent to subduction zones, a type of convergent plate boundary (see chapter 2). The process of subduction expels water from hydrated minerals in the descending slab, which causes flux melt­ ing in the overlying mantle rock. Because subduction volcanism occurs in a volcanic arc, the thickened crust promotes partial melt­ ing 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-gener- ated eruptions of mostly silica-rich lava; the volcanoes and plutons consist largely of intermediate-to-felsic rock such as andesite, rhy­ Figure 4.30: Distribution of volcanoes on the planet. Click here for an interactive map of volcano distributions. olite, pumice, and tuff. Volcanoes at Continental Rifts Some volcanoes are created at continental rifts, where crustal thin- ning 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 ris- ing 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 (see video). Relatively young cinder cones of basaltic lava Figure 4.31: Basaltic cinder cones of the Black Rock Desert can be found in south-central Utah, in the Black Rock Desert Volcanic near Beaver, Utah. Field, which is part of the zone of Basin and Range crustal extension. These Utah cinder cones and lava flows started erupting around 6 million years ago, with the last eruption occurring 720 years ago. 106 | IGNEOUS PROCESSES AND VOLCANOES One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=4VgMe-JXOAM Video 4.4: Basin and range volcanic processes. If you are using an offline version of this text, access this YouTube video via the QR code. Hotspots Hotspots are the main source of intraplate volcanism. Hotspots occur when lithos- pheric 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 lithos- pheric 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 vol­ canism typically take place underwater. Over time, basaltic volcanoes may build up from the Figure 4.32: Diagram showing a sea floor into islands, non-moving source of magma (mantle plume) and a moving such as the Hawaiian overriding plate. Islands. Where a hotspot is found under a conti­ nental plate, contact with the hot mafic magma may Figure 4.33: The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago. 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.  IGNEOUS PROCESSES AND VOLCANOES | 107 A zone of actively erupting volcanism connected to a chain of extinct vol­ canoes indicates intraplate volcanism located over a hotspot. These vol­ canic chains are created by the overriding oceanic plate slowly moving over a hotspot mantle plume. These chains are seen on the seafloor and continents and include volcanoes that have been inactive for millions of years. The Hawaiian Islands on the Pacific Oceanic plate are the active end of a long volcanic chain that extends from the northwest Pacific Ocean to the Emperor Seamounts, all the way to the to the subduction zone beneath the Kamchatka Peninsula. The overriding North American continental plate moved across a mantle plume hotspot for several million years, creating a chain of volcanic calderas that extends from Southwestern Idaho to the presently active Yellowstone caldera in Wyoming. The three-minute video (below) illustrates hotspot volcanoes. Figure 4.34: The Hawaiian–Emperor seamount and island chain. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=AhSaE0omw9o Video 4.5: What is a volcanic hotspot? If you are using an offline version of this text, access this YouTube video via the QR code. 4.4.2 Volcano Features and Types There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other aspects. Complete this interactive activity to check your understanding. If you are using an offline version of this text, access this interactive activity via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=331#h5p-28 108 | IGNEOUS PROCESSES AND VOLCANOES The figure shows the main features of a typical stratovolcano: 1) magma chamber, 2) upper layers of lithosphere, 3) the conduit or narrow pipe through which the lava erupts, 4) the base or edge of the volcano, 5) a sill of magma between layers of the volcano, 6) a diapir or feeder tube to the sill, 7) layers of tephra (ash) from previous erup­ tions, 8 & 9) layers of lava erupting from the vent and flowing down the sides of the volcano, 10) the crater at the top of the volcano, 11) layers of lava and tephra on (12), a parasitic cone. A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber, 13) the vents of the parasite and the main volcano, 14) the rim of the crater, 15) clouds of ash blown into the sky by the eruption; this settles back onto the volcano and surrounding land. Figure 4.35: Mt. Shasta in Washington state with Shastina, its parasitic cone. The largest craters are called calderas, such as the Crater Lake Caldera in Oregon. Many volcanic features are pro­ duced by viscosity, a basic property of a lava. Viscosity is the resistance to flowing by a fluid. Low viscosity magma flows easily more like syrup, the basaltic volcanism that occurs in Hawai’i on shield volcanoes. High viscosity means a thick and sticky magma, typically felsic or inter­ mediate, that flows slowly, similar to toothpaste. Figure 4.36: Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama. Shield Volcano The largest volcanoes are shield volcanoes. They are characterized by broad low-angle flanks, small vents at the top, and mafic magma cham- bers. 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 effu- sively, meaning the eruptions are small, localized, and predictable. Figure 4.37: Kilauea in Hawai’i.  IGNEOUS PROCESSES AND VOLCANOES | 109 Typically, shield volcano eruptions are not much of a hazard to human life—although non-explosive eruptions of Kilauea (Hawai’i) in 2018 produced uncharacteristically large lavas that damaged roads and structures. Mauna Loa (see USGS page) and Kilauea (see USGS page) in Hawai’i are examples of shield volcanoes. Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift. The largest volcanic edifice in the Solar System is Olympus Mons on Mars. This (possibly extinct) shield volcano covers an area the size of the state of Arizona. This may indicate the Figure 4.38: Eruption of Kiluea in 2018 produced high viscosity lava shown here crossing a road. volcano erupted over a hotspot for This eruption caused much property damage. millions of years, which means Mars had little, if any, plate tectonic activity. Basaltic lava forms special landforms based on magma temperature, compo­ Figure 4.39: Olympus Mons, an enormous sition, and content of dissolved gases shield volcano on Mars, the largest volcano and water vapor. The two main types of in the solar system, standing about two and a half times higher than Everest is basaltic volcanic rock have Hawaiian above sea level. names—pahoehoe and aa. Pahoehoe might come from low-viscosity lava that flows easily into ropey strands. Aa (sometimes spelled a’a or ʻaʻā Figure 4.40: Ropey pahoehoe lava. and pronounced “ah-ah”) is more viscous and has a crumbly blocky appearance. The exact details of what forms the two types of flows are still up for debate. Felsic lavas have lower temperatures and more silica, and thus are higher viscosity. These also form aa-style flows. Low-viscosity, fast-flowing basaltic lava tends to harden on the outside into a tube Figure 4.41: Blocky a’a lava. and continue to flow inter- nally. Once lava flow sub- sides, the empty outer shell may be left as a lava tube. Lava tubes, with or without collapsed roofs, make famous caves in Hawai’i, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monu- Figure 4.42: Volcanic fissure and flow, which ment in Idaho. could eventually form a lava tube. 110 | IGNEOUS PROCESSES AND VOLCANOES Fissures are cracks that commonly originate from shield-style erup- tions. Lava emerging from fissures is typically mafic and very fluid. The 2018 Kiluaea eruption included fissures associated with the lava flows. Some fissures are caused by the volcanic seismic activity rather than lava flows. Some fissures are influenced by plate tectonics, such as the common fissures located parallel to the divergent boundary in Ice- land. Cooling lava can con- tract into columns with semi-hexagonal Figure 4.43: Devils Tower in Wyoming has columnar cross sections called jointing. columnar jointing. This feature forms the famous Devils Tower in Wyoming, possibly an ancient volcanic vent from which the surrounding layers of lava and ash have been removed by ero­ sion. Another well-known exposed example of columnar jointing is the Giant’s Causeway in Ireland. Figure 4.44: Columnar jointing on Giant’s Causeway in Ireland. Stratovolcano A stratovolcano, also called a composite cone volcano, has steep flanks, a sym- metrical cone shape, distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of pyroclastic frag- ments like ash and bombs, and solidified lava flows of varying composition. Exam- ples include Mount Rainier in Washington state and Mount Fuji in Japan. Stratovolcanoes usually have felsic to intermediate magma chambers, but can even produce mafic lavas. Figure 4.45: Mount Rainier towers over Tacoma, Washington. Stratovolcanoes have viscous lava flows and domes, punctuated by explosive eruptions. This produces volcanoes with steep flanks. Lava Domes Lava domes are accumula- Figure 4.46: Mt. Fuji in Japan, a typical stratovolcano, tions of silica-rich volcanic symmetrical, increasing slope, visible crater at the top. 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 col- lapsed 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. Examples of stand-alone lava domes are Chaiten in Chile and Mammoth Mountain in Figure 4.47: Lava domes have started the rebuilding California. process at Mount St. Helens, Washington.  IGNEOUS PROCESSES AND VOLCANOES | 111 Caldera Calderas are steep-walled, basin-shaped depressions formed by the collapse of a vol­ canic edifice into an empty magma chamber. Calderas are generally very large, with diameters of up to 25 km (15.5 mi). The term caldera specif- ically refers to a volcanic vent; however, it is fre- quently used to describe a volcano type. Caldera volcanoes are typically formed by eruptions of high-viscosity felsic lava having high volatiles content. Crater Lake, Yellowstone, and the Long Valley Figure 4.49: Wizard Island sits in the caldera at Crater Caldera are good examples of this type of vol­ Lake. canism. The caldera at Crater Lake National Park in Oregon was created about 6,800 years ago when Mount Mazama, a composite volcano, erupted in a huge explosive blast. The volcano ejected large amounts of volcanic ash and rapidly drained the magma chamber, causing the top to collapse into a large depression that later filled with water. Wizard Island in the middle of the lake is a later resurgent lava dome that formed within the caldera basin. The Yellowstone volcanic system erupted three times in the recent geologic past—2.1, 1.3, and 0.64 million years ago—leaving behind three caldera basins. Each eruption created large rhyolite lava Figure 4.48: Timeline of flows as well as pyroclastic flows that events at Mount Mazama. solidified into tuff formations. These extra-large eruptions rapidly emptied the magma chamber, causing the roof to collapse and form a caldera. The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent lava domes. The calderas are difficult to see today due to the amount of time since their erup- tions and subsequent erosion and glaciation. Figure 4.50: Map of calderas and related rocks around Yellowstone volcanism started about 17-million years ago as a Yellowstone. hotspot under the North American lithospheric plate near the Ore- gon/Nevada border. As the plate moved to the southwest over the stationary hotspot, it left behind a track of past vol­ canic activities. Idaho’s Snake River Plain was created from volcanism that produced a series of calderas and lava flows. The plate eventually arrived at its current location in northwestern Wyoming, where hotspot volcanism formed the Yel- lowstone calderas. 112 | IGNEOUS PROCESSES AND VOLCANOES The Long Valley Caldera near Mammoth, California, is the result of a large volcanic eruption that occurred 760,000 years ago. The explosive eruption dumped enormous amounts of ash across the United States, in a manner similar to the Yellowstone eruptions. The Bishop Tuff deposit near Bishop, California, is made of ash from this eruption. The current caldera basin is 17 km by 32 km (10 mi by 20 mi), large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent dome, and several hot springs. Cinder Cone Cinder cones are Figure 4.51: Several prominent ash beds found in North America, small volcanoes including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed with steep sides, (brown dashed line), and the modern May 18th, 1980 ash fall and made of (yellow). pyroclastic frag- ments 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. Cin­ Figure 4.52: Sunset Crater, Arizona is a cinder cone. der cones are found throughout western North America. A recent and striking example of a cinder cone is the eruption near the village of Parí­ cutin, Mexico that started in 1943. The cinder cone started explosively shooting cinders out of the vent in the middle of a farmer’s field. The volcanism quickly built up the cone to a height of over 90 m (300 ft) within a week, and 365 m (1,200 ft) within the first 8 months. After the initial explosive eruption of gases and cinders, basaltic lava poured out from the base of the cone. This is a common Figure 4.54: Lava from Parícutin covered the local church and destroyed the town of San Juan, Mexico. order of events for cinder cones: violent eruption, cone and crater formation, Figure 4.53: Soon after the birth low-viscosity lava flow from the base. The cinder cone is not strong enough to support a of Parícutin in 1943. column of lava rising to the top of the crater, so the lava breaks through and emerges near the bottom of the volcano. During nine years of eruption activity, the ashfall covered about 260 km2 (100 mi2) and destroyed the nearby town of San Juan.  IGNEOUS PROCESSES AND VOLCANOES | 113 Flood Basalts Figure 4.55: World map of flood basalts. Note the largest is the Siberian Traps. 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 mech- anisms 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 (see chapter 8). Figure 4.56: Igneous rock types and related volcano types. Mid-ocean ridges and shield volcanoes represent more mafic compositions, and strato (composite) volcanoes generally represent a more intermediate or felsic composition and a convergent plate tectonic boundary. Note that there are exceptions to this generalized layout of volcano types and igneous rock composition. 114 | IGNEOUS PROCESSES AND VOLCANOES 4.4.3 Volcanic Hazards and Monitoring While the most obvious volcanic hazard is lava, the dangers posed by volca­ noes 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 pyroclas- tic flow moved at speeds of 80-130 kph (50-80 mph), flattening trees and eject- ing clouds of ash into the air. The USGS video provides an account of this explosive eruption that killed 57 people. Figure 4.57: General diagram of volcanic hazards. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=Ec30uU0G56U Video 4.6: Mout St. Helens. If you are using an offline version of this text, access this YouTube video via the QR code. Figure 4.58: Human remains from the 79 CE eruption of Vesuvius.  IGNEOUS PROCESSES AND VOLCANOES | 115 In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman coun- tryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century. Pompeii famously contains the remains (casts) of people suffocated by ash and covered by 10 feet (3 m) of ash, pumice lapilli, and collapsed roofs. Figure 4.59: Mount St. Helens, the day before the May 18th, 1980 eruption (left) and 4 months after the major eruption (right). Figure 4.60: Image from the May 18, 1980, eruption of Mt. Saint Helens, Washington. Pyroclastic Flows The most dangerous volcanic hazard are pyroclastic flows (video). 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 vol­ canoes such as composite cones usually have pyroclastic flows. The rock tuff and welded tuff is often formed from these pyroclastic flows. Figure 4.61: The material coming down from the eruption column is a pyroclastic flow. 116 | IGNEOUS PROCESSES AND VOLCANOES There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs. Two short videos below document eye-witness video of pyroclastic flows. In the early 1990s, Mount Unzen erupted several times with pyro­ clastic flows. The pyroclastic flow shown in this famous short video killed 41 peo­ ple. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 peo­ ple in moments. Figure 4.62: The remains of St. Pierre. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=Cvjwt9nnwXY Video 4.7: Dome collapse and pyroclastic flow at Unzen Volcano. If you are using an offline version of this text, access this YouTube video via the QR code. Landslides and Landslide–Generated Tsunamis The steep and unstable flanks of a volcano can lead to slope failure and dangerous landslides. These landslides can be triggered by magma movement, explosive eruptions, large earthquakes, and/or heavy rainfall. During the 1980 Mount St. Helens eruption, the entire north flank of the volcano col- lapsed and released a huge landslide that moved at speeds of 160-290 kph (100-180 mph). If enough landslide material reaches the ocean, it may cause a tsunami. In 1792, a landslide caused by the Mount Unzen eruption reached the Ariaka Sea, generating a tsunami that killed 15,000 people (see USGS page). When Mount Krakatau in Indonesia erupted in 1883, it generated ocean waves that towered 40 m (131 ft) above sea level. The tsunami killed Figure 4.63: Sequence of events for Mount St. Helens, May 18th, 1980. Note that an earthquake caused a landslide, which caused the 36,000 people and destroyed 165 villages. “uncorking” of the mountain and started the eruption.  IGNEOUS PROCESSES AND VOLCANOES | 117 Tephra Volcanoes, especially composite volcanoes, eject large amounts of tephra (ejected rock materials), most notably ash (tephra frag­ ments less than 0.08 inches [2 mm]). Larger tephra is heavier and falls closer to the vent. Larger blocks and bombs pose hazards to those close to the eruption such as at the 2014 Mount Ontake dis­ aster in Japan discussed earlier. Hot ash poses an immediate danger to people, animals, plants, machines, roads, and buildings located close to the eruption. Ash is fine grained (< Figure 4.64: Aman sweeps ash from an eruption of Kelud, 2mm) and can travel airborne Indonesia. long distances away from the eruption site. Heavy accumu- lations of ash can cause buildings to collapse. In people, it may cause respiratory Figure 4.65: Micrograph of silica particle in volcanic ash. A cloud of these is issues like silicosis. Ash is destructive to aircraft and automobile engines, which can capable of destroying an aircraft or disrupt transportation and shipping services. In 2010, the Eyjafjallajökull volcano in Ice- automobile engine. land emitted a large ash cloud into the upper atmosphere, causing the largest air- travel disruption in northern Europe since World War II. No one was injured, but the service disruption was estimated to have cost the world economy billions of dollars. Volcanic Gases As magma rises to the surface the confining pressure decreases, and allows dissolved gases to escape into the atmos­ phere. Even volcanoes that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO2), sulfur diox- ide (SO2), hydrogen sulfide (H2S), and hydrogen halides (HF, HCl, or HBr). Carbon dioxide tends to sink and accumulate in depressions and basins. In volcanic areas known to emit carbon dioxide, low-lying areas may trap hazardous concentrations of this colorless and odorless gas. The Mammoth Mountain Ski Resort in California, is located within the Long Valley Caldera, is one such area of carbon dioxide-producing volcanism. In 2006, three ski patrol members died of suffocation caused by carbon dioxide after falling into a snow depression near a fumarole (info). In rare cases, volcanism may create a sudden emission of gases without warning. Limnic eruptions (limne is Greek for lake), occur in crater lakes associated with active volcanism. The water in these lakes is supercharged with high concentrations of dissolved gases. If the water is physically jolted by a landslide or earthquake, it may trigger an immediate and massive release of gases out of solution. An analogous example would be what happens to vigorously shaken bottle of carbon- ated soda when the cap is opened. An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon. Almost 2,000 people were killed by a massive release of carbon dioxide. 118 | IGNEOUS PROCESSES AND VOLCANOES 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 resem- bling 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 Figure 4.66: Mud line shows the extent of lahars lahar flows that extend around Mount St. Helens. for many kilometers across the flood plains surrounding Mount Rainier in Washington (see map). A map of Mount Baker in Oregon shows a similar potential hazard for lahar flows (see map). A tragic scenario played out recently, in 1985, when a lahar from the Nevado del Ruiz volcano in Colombia buried the town of Armero and killed an estimated 23,000 people. 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 1) earthquake activity; 2) increases in gas emission; and 3) changes in land surface orientation and elevation. Figure 4.67: Old lahars around Tacoma, Washington. 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. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=nlo-2JoNHrw Video 4.8: Earthquake signals. If you are using an offline version of this text, access this YouTube video via the QR code.  IGNEOUS PROCESSES AND VOLCANOES | 119 One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=owk4fWbw4qM Video 4.9: Measuring gas emissions. If you are using an offline version of this text, access this YouTube video via the QR code. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=sNYQkxxd_0Q Video 4.10: Using tiltmeters and GPS to monitor a volcano. If you are using an offline version of this text, access this YouTube video via the QR code. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 4.4 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=331#h5p-29 Summary Igneous rock is divided into two major groups: intrusive rock that solidifies from underground magma, and extrusive rock formed from lava that erupts and cools on the surface. Magma is generated from mantle material at several plate tec­ tonics situations by three types of melting: decompression melting, flux melting, or heat-induced melting. Magma com­ position is determined by differences in the melting temperatures of the mineral components (Bowen’s Reaction Series). The processes affecting magma composition include partial melting, magmatic differentiation, assimilation, and col­ 120 | IGNEOUS PROCESSES AND VOLCANOES lision. Volcanoes come in a wide variety of shapes and sizes, and are classified by a multiple factors, including magma composition, and plate tectonic activity. Because volcanism presents serious hazards to human civilization, geologists carefully monitor volcanic activity to mitigate or avoid the dangers it presents. Take this quiz to check your comprehension of this chapter. If you are using an offline version of this text, access the quiz for chapter 4 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=331#h5p-30 URLs Listed Within This Chapter USGS page of Mauna Loa: https://www.usgs.gov/volcanoes/mauna-loa USGS page on Kilauea: https://www.usgs.gov/volcanoes/kilauea Pyroclastic flows video: https://volcanoes.usgs.gov/vsc/movies/movie_101/PF_Animation.mp4 USGS page on volcano landslides that trigger waves and tsunamis: https://volcanoes.usgs.gov/Imgs/Jpg/Unzen/ MayuyamaSlide_caption.html Ski patrol’s fatal fall into a volcanic fumarole: https://pubmed.ncbi.nlm.nih.gov/19364170/ Text References 1. Arndt, N.T., 1994, Chapter 1 Archean Komatiites, in K.C. Condie, editor, Developments in Precambrian Geology: Else- vier, p. 11–44. 2. Bateman, P.C., and Chappell, B.W., 1979, Crystallization, fractionation, and solidification of the Tuolumne Intrusive Series, Yosemite National Park, California: Geological Society of America Bulletin, v. 90, no. 5, p. 465–482., https://doi.org/10.1130/0016-7606(1979)902.0.CO;2 3. Boehler, R., 1996, Melting temperatures of the Earth’s mantle and core: Earth’s thermal structure: Annual Review of Earth and Planetary Sciences, v. 24, no. 1, p. 15–40., doi: 10.1146/annurev.earth.24.1.15. 4. Bowen, N.L., 1922, The Reaction Principle in Petrogenesis: J. Geol., v. 30, no. 3, p. 177–198. 5. Bowen, N.L., 1928, The evolution of the igneous rocks: Dover Publications, 334 p. 6. Carr, M.H., 1975, Geologic map of the Tharsis Quadrangle of Mars: IMAP. 7. Earle, S., 2015, Physical geology OER textbook: BC Campus OpenEd. 8. EarthScope, 2014, Mount Ontake Volcanic Eruption: Online, http://www.earthscope.org/science/geo-events/mount- ontake-volcanic-eruption, accessed July 2016. 9. Frankel, C., 2005, Worlds on Fire: Volcanoes on the Earth, the Moon, Mars, Venus and Io: Cambridge University Press, 396 p. 10. Glazner, A.F., Bartley, J.M., Coleman, D.S., Gray, W., and Taylor, R.Z., 2004, Are plutons assembled over millions of years by amalgamation from small magma chambers? GSA Today, v. 14, no. 4, p. 4., DOI: 10.1130/ 1052-5173(2004)0142.0.CO;2 11. Luongo, G., Perrotta, A., Scarpati, C., De Carolis, E., Patricelli, G., and Ciarallo, A., 2003, Impact of the AD 79 explosive  IGNEOUS PROCESSES AND VOLCANOES | 121 eruption on Pompeii, II. Causes of death of the inhabitants inferred by stratigraphic analysis and areal distribution of the human casualties: J. Volcanol. Geotherm. Res., v. 126, no. 3–4, p. 169–200. 12. Mueller, S., and Phillips, R.J., 1991, On the initiation of subduction: J. Geophys. Res. [Solid Earth], v. 96, no. B1, p. 651–665. 13. Peacock, M.A., 1931, Classification of Igneous Rock Series: The Journal of Geology, v. 39, no. 1, p. 54–67. 14. Perkins, S., 2011, 2010’s Volcano-Induced Air Travel Shutdown Was Justified: Online, http://www.sciencemag.org/ news/2011/04/2010s-volcano-induced-air-travel-shutdown-was-justified, accessed July 2016. 15. Peterson, D.W., and Tilling, R.I., 1980, Transition of basaltic lava from pahoehoe to aa, Kilauea Volcano, Hawaii: Field observations and key factors – ScienceDirect: J. Volcanol. Geotherm. Res., v. 7, no. 3–4, p. 271–293. 16. Petrini and Podladchikov, 2000, Lithospheric pressure–depth relationship in compressive regions of thickened crust: Journal of Metamorphic Geology, v. 18, no. 1, p. 67–77., doi: 10.1046/j.1525-1314.2000.00240.x. 17. Reid, J.B., Evans, O.C., and Fates, D.G., 1983, Magma mixing in granitic rocks of the central Sierra Nevada, California: Earth and Planetary Science Letters, v. 66, p. 243–261., doi: 10.1016/0012-821X(83)90139-5. 18. Rhodes, J.M., and Lockwood, J.P., 1995, Mauna Loa Revealed: Structure, Composition, History, and Hazards: Washing- ton DC American Geophysical Union Geophysical Monograph Series, v. 92. 19. Scandone, R., Giacomelli, L., and Gasparini, P., 1993, Mount Vesuvius: 2000 years of volcanological observations: Jour- nal of Volcanology and Geothermal Research, v. 58, p. 5–25. 20. Stovall, W.K., Wilkins, A.M., Mandeville, C.W., and Driedger, C.L., 2016, Fact Sheet. 21. Thorarinsson, S., 1969, The Lakagigar eruption of 1783: Bull. Volcanol., v. 33, no. 3, p. 910–929. 22. Tilling, R.I., 2008, The critical role of volcano monitoring in risk reduction: Adv. Geosci., v. 14, p. 3–11. 23. United States Geological Survey, 1999, Exploring the deep ocean floor: Online, http://pubs.usgs.gov/gip/dynamic/ exploring.html, accessed July 2016. 24. United States Geological Survey, 2012, Black Rock Desert Volcanic Field: Online, http://volcanoes.usgs.gov/volca- noes/black_rock_desert/, accessed July 2016. 25. USGS, 2001, Dual volcanic tragedies in the Caribbean led to founding of HVO: Online, http://hvo.wr.usgs.gov/vol- canowatch/archive/2001/01_05_03.html, accessed July 2016. 26. USGS, 2011, Volcanoes: Principal Types of Volcanoes: Online, http://pubs.usgs.gov/gip/volc/types.html, accessed July 2016. 27. USGS, 2012a, USGS: Volcano Hazards Program: Online, https://volcanoes.usgs.gov/vhp/hazards.html, accessed July 2016. 28. USGS, 2012b, Yellowstone Volcano Observatory: Online, https://volcanoes.usgs.gov/volcanoes/yellowstone/yellow- stone_geo_hist_52.html, accessed July 2016. 29. USGS, 2016, Volcanoes General – What are the different types of volcanoes? Online, https://www2.usgs.gov/faq/cat- egories/9819/2730, accessed March 2017. 30. USGS, 2017, The Volcanoes of Lewis and Clark – Mount St. Helens: Online, https://volcanoes.usgs.gov/observatories/ cvo/Historical/LewisClark/Info/summary_mount_st_helens.shtml, accessed March 2017. 31. Wallace, P.J., 2005, Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and vol- canic gas data: Journal of Volcanology and Geothermal Research, v. 140, no. 1–3, p. 217–240., doi: 10.1016/j.jvolgeo- res.2004.07.023. 32. Williams, H., 1942, The Geology of Crater Lake National Park, Oregon: With a Reconnaissance of the Cascade Range Southward to Mount Shasta: Carnegie institution. Figure References Figure 4.1: Lava flow in Hawai’i. Brocken Inaglory. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/ File:P%C4%81hoehoe_and_Aa_flows_at_Hawaii.jpg Figure 4.2: Half Dome, an intrusive igneous batholith in Yosemite National Park. Jon Sullivan. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Yosemite_20_bg_090404.jpg 122 | IGNEOUS PROCESSES AND VOLCANOES Figure 4.3: Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. James St. John. 2019. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Granite_47_(49201189712).jpg Figure 4.4: Basalt is a classic fine-grained extrusive igneous rock. James St. John. 2019. CC BY 2.0. https://commons.wiki- media.org/wiki/File:Basalt_3_(48674276863).jpg Figure 4.5: Porphyritic texture. Jstuby. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Olearyandesite.jpg Figure 4.6: Pegmatitic texture. Jstuby. 2007. Public domain. https://commons.wikimedia.org/wiki/File:We-pegmatite.jpg Figure 4.7: Scoria. Jonathan Zander (Digon3). 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Sco- ria_Macro_Digon3.jpg Figure 4.8: Pumice. deltalimatrieste. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Pomice_di_veglia.jpg Figure 4.9: Obsidian (volcanic glass). Note conchoidal fracture. Ji-Elle. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/ wiki/File:Lipari-Obsidienne_(5).jpg Figure 4.10: Welded tuff. Wilson44691. 2010. Public domain. https://commons.wikimedia.org/wiki/File:HoleInThe- WallTuff.JPG Figure 4.11: Mineral composition of common igneous rocks. Woudloper. 2009. Public domain. https://commons.wikime- dia.org/wiki/File:Mineralogy_igneous_rocks_EN.svg Figure 4.12: Igneous rock classification table with composition as vertical columns and texture as horizontal rows. Kindred Grey. 2022. Adapted from Belinda Madsen, An Introduction to Geology. OpenStax. Salt Lake Community College. CC BY- NC-SA 4.0. Table 4.1: Aphanitic and phaneritic rock types with images. Quartz monzonite 36mw1037 by B.W. Hallett, V. F. Paskevich, L.J. Poppe, S.G. Brand, and D.S. Blackwood via USGS (Public domain, https://commons.wikimedia.org/wiki/File:Quartz_mon- zonite_36mw1037.jpg). PinkRhyolite by Michael C. Rygel, 2014 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/ File:PinkRhyolite.tif). Diorite MA by Amcyrus2012, 2015 (CC BY 4.0, https://commons.wikimedia.org/wiki/File:Dior- ite_MA.JPG). Andesite by James St. John, 2014 (CC BY 2.0, https://flic.kr/p/oBkKSy). GabbroRockCreek1 by Mark A. Wilson, 2008 (Public domain, https://commons.wikimedia.org/wiki/File:GabbroRockCreek1.jpg). VesicularBasalt1 by Jstuby, 2008 (Public domain, https://commons.wikimedia.org/wiki/File:VesicularBasalt1.jpg). Figure 4.13: Dike of olivine gabbro cuts across Baffin Island in the Canadian Arctic. Mike Beauregard. 2012. CC BY 2.0. https://en.wikipedia.org/wiki/File:Franklin_dike_on_northwestern_Baffin_Island..jpg Figure 4.14: Igneous sill intruding between Paleozoic strata in Nova Scotia. Mikenorton. 2010. CC BY-SA 3.0. https://com- mons.wikimedia.org/wiki/File:Horton_Bluff_mid-Carboniferous_sill.JPG Figure 4.15: Quartz monzonite in the Cretaceous of Montana, USA. James St. John. 2010. CC BY 2.0. https://commons.wiki- media.org/wiki/File:Butte_Quartz_Monzonite_(Late_Cretaceous,_76_Ma;_Rampart_Mountain,_northeast_of_Butte,_Mon- tana,_USA)_1.jpg Figure 4.16: Half Dome in Yosemite National Park, California, is a part of the Sierra Nevada batholith which is mostly made of granite. Jon Sullivan. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Yosemite_20_bg_090404.jpg Figure 4.17: The Henry Mountains in Utah are interpreted to be a laccolith, exposed by erosion of the overlying layers. Steven Mahoney. 2005. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Henry_Mountains,_Utah,_2005-06-01.jpg Figure 4.18: Laccolith forms as a blister in between sedimentary strata. Erimus and Stannered. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Laccolith.svg  IGNEOUS PROCESSES AND VOLCANOES | 123 Figure 4.19: Bowen’s Reaction Series. Colivine. 2011. Public domain. https://commons.wikimedia.org/wiki/ File:Bowen%27s_Reaction_Series.png Figure 4.20: Olivine, the first mineral to crystallize in a melt. S kitahashi. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/ wiki/File:Peridot2.jpg Figure 4.21: Norman L. Bowen. Unknown author. 1909. Public domain. https://commons.wikimedia.org/wiki/File:NormanL- Bowen_1909.jpg Figure 4.22: Norman L. Bowen and his colleague working at the Carnegie Institution of Washington Geophysical Laboratory. Smithsonian Institution. 2010. Public domain. https://commons.wikimedia.org/wiki/File:(left_to_right)-_Nor- man_Levi_Bowen_(1887-1956)_and_Orville_Frank_Tuttle_(1916-1983)_(4730112454)_(cropped).jpg Figure 4.23: Geothermal gradient. Bkilli1. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Tempera- ture_schematic_of_inner_Earth.jpg Figure 4.24: Pressure-temperature diagram showing temperature in degrees Celsius on the x-axis and depth below the surface in kilometers (km) on the y-axis. Kindred Grey. 2022. CC BY-SA 3.0. Adapted from Partial melting asthenosphere EN by Woudloper, 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File%3APartial_melting_asthenosphere_EN.svg). Figure 4.25: Association of volcanoes with plate boundaries. Jose F. Vigil via USGS. 1997. Public domain. https://com- mons.wikimedia.org/wiki/File:Tectonic_plate_boundaries.png Figure 4.26: Map of spreading ridges throughout the world. Eric Gaba. 2006. CC BY-SA 2.5. https://commons.wikimedia.org/ wiki/File:Spreading_ridges_volcanoes_map-fr.svg Figure 4.27: Pillow basalt on sea floor near Hawai’i. NOAA. 1988. Public domain. https://commons.wikimedia.org/wiki/ File:Nur05018-Pillow_lavas_off_Hawaii.jpg Figure 4.28: Black smoker hydrothermal vent with a colony of giant (6’+) tube worms. NOAA. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Main_Endeavour_black_smoker.jpg Figure 4.29: Distribution of hydrothermal vent fields. DeDuijn. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/ File:Distribution_of_hydrothermal_vent_fields.png Figure 4.30: Distribution of volcanoes on the planet. USGS. 2007. Public domain. https://commons.wikimedia.org/wiki/ File:Map_plate_tectonics_world.gif Figure 4.31: Basaltic cinder cones of the Black Rock Desert near Beaver, Utah. Lee Siebert via Smithsonian Institution. 1996. Public domain. https://commons.wikimedia.org/wiki/File:Black_Rock_Desert_volcanic_field.jpg Figure 4.32: Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate. Los688. 2008. Public domain. https://en.wikipedia.org/wiki/File:Hotspot(geology)-1.svg Figure 4.33: The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago. Kelvin Case. 2013. CC BY 3.0. https://commons.wikimedia.org/wiki/File:HotspotsSRP_update2013.JPG Figure 4.34: The Hawaiian–Emperor seamount and island chain. Ingo Wölbern. 2008. Public domain. https://commons.wiki- media.org/wiki/File:Hawaii-Emperor_engl.png Figure 4.35: Mt. Shasta in Washington state with Shastina, its parasitic cone. Don Graham. 2013. CC BY-SA 2.0. https://com- mons.wikimedia.org/wiki/File:Mt._Shasta_and_Mt._Shastina,_CA_9-13_(26491330883).jpg Figure 4.36: Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama. Zainubrazvi. 2006. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Crater_lake_oregon.jpg 124 | IGNEOUS PROCESSES AND VOLCANOES Figure 4.37: Kilauea in Hawai’i. Quinn Dombrowski. 2007. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/ File:Kilauea_Shield_Volcano_Hawaii_20071209A.jpg Figure 4.38: Eruption of Kiluea in 2018 produced high viscosity lava shown here crossing a road. USGS. 2018. Public domain. https://commons.wikimedia.org/wiki/File:USGS_K%C4%ABlauea_multimediaFile-1955.jpg Figure 4.39: Olympus Mons, an enormous shield volcano on Mars, the largest volcano in the solar system, standing about two and a half times higher than Everest is above sea level. NASA. 1978. Public domain. https://en.wikipedia.org/wiki/ File:Olympus_Mons_alt.jpg Figure 4.40: Ropey pahoehoe lava. Bbb. 2010. GNU Free Documentation License 1.2. https://de.wikivoyage.org/wiki/ Datei:ReU_PtFournaise_Lavastr%C3%B6me.jpg Figure 4.41: Blocky a’a lava. Librex. 2009. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Lava_del_Vol- can_Pacaya_2009-11-28.jpg Figure 4.42: Volcanic fissure and flow, which could eventually form a lava tube. NPS. 2004. Public domain. https://com- mons.wikimedia.org/wiki/File:Volcano_q.jpg Figure 4.43: Devils Tower in Wyoming has columnar jointing. Colin.faulkingham. 2005. Public domain. https://commons.wiki- media.org/wiki/File:Devils_Tower_CROP.jpg Figure 4.44: Columnar jointing on Giant’s Causeway in Ireland. Udri. 2014. CC BY-NC-SA 2.0. https://flic.kr/p/2j1mgwE Figure 4.45: Mount Rainier towers over Tacoma, Washington. Lyn Topinka via USGS. 1984. Public domain. https://com- mons.wikimedia.org/wiki/File:Mount_Rainier_over_Tacoma.jpg Figure 4.46: Mt. Fuji in Japan, a typical stratovolcano, symmetrical, increasing slope, visible crater at the top. Alpsdake. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Numazu_and_Mount_Fuji.jpg Figure 4.47: Lava domes have started the rebuilding process at Mount St. Helens, Washington. Willie Scott via USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:MSH06_aerial_crater_from_north_high_angle_09-12-06.jpg Figure 4.48: Timeline of events at Mount Mazama. USGS and NPS. 2006. Public domain. https://commons.wikimedia.org/ wiki/File:Mount_Mazama_eruption_timeline.PNG Figure 4.49: Wizard Island sits in the caldera at Crater Lake. Don Graham. 2006. CC BY-SA 2.0. https://commons.wikime- dia.org/wiki/File:Crater_Lake_National_Park,_OR_2006_(6539577313).jpg Figure 4.50: Map of calderas and related rocks around Yellowstone. USGS. 1905. Public domain. https://www.usgs.gov/ media/images/simplified-map-yellowstone-caldera Figure 4.51: Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow). USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Yellowstone_volcano_-_ash_beds.svg Figure 4.52: Sunset Crater, Arizona is a cinder cone. NPS. Unknown date. Public domain. https://commons.wikimedia.org/ wiki/File:Sunset_Crater10.jpg Figure 4.53: Soon after the birth of Parícutin in 1943. K. Segerstrom via USGS. 1943. Public domain. https://commons.wiki- media.org/wiki/File:Paricutin_30_612.jpg Figure 4.54: Lava from Parícutin covered the local church and destroyed the town of San Juan, Mexico. Sparksmex. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Paricutin2.jpg  IGNEOUS PROCESSES AND VOLCANOES | 125 Figure 4.55: World map of flood basalts. Williamborg. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/ File:Flood_Basalt_Map.jpg Figure 4.56: Igneous rock types and related volcano types. Unknown author. Unknown date. “Personal use.” https://www.seekpng.com/ipng/u2t4y3r5o0r5r5w7_table-of-igneous-rocks-and-related-volcano-types/ Figure 4.57: General diagram of volcanic hazards. USGS. 2008. Public domain. https://pubs.usgs.gov/fs/fs002-97/ old%20html%20files/ Figure 4.58: Human remains from the 79 CE eruption of Vesuvius. Gary Todd. 2019. Public domain. https://commons.wiki- media.org/wiki/File:Pompeii_Ruins_Cast_of_Human_Victim_at_Villa_of_the_Mysteries_(48445486616).jpg Figure 4.59: Mount St. Helens, the day before the May 18th, 1980 eruption (left) and 4 months after the major eruption (right). Mount St. Helens, one day before the devastating eruption by Harry Glicken, USGS/CVO, 1980 (Public domain, https://commons.wikimedia.org/wiki/File:Mount_St._Helens,_one_day_before_the_devastating_eruption.jpg). MSH80 st helens from johnston ridge 09-10-80 by Harry Glicken via USGS (Public domain, https://commons.wikimedia.org/wiki/ File:MSH80_st_helens_from_johnston_ridge_09-10-80.jpg). Figure 4.60: Image from the May 18, 1980, eruption of Mt. Saint Helens, Washington. Austin Post via USGS. 1980. Public domain. https://commons.wikimedia.org/wiki/File:MSH80_eruption_mount_st_helens_05-18-80-dramatic-edit.jpg Figure 4.61: The material coming down from the eruption column is a pyroclastic flow. C.G. Newhall via USGS. 1984. Public domain. https://en.wikipedia.org/wiki/File:Pyroclastic_flows_at_Mayon_Volcano.jpg#:~:text=English%3A%20Pyroclas- tic%20flows%20at%20Mayon,50%20km%20toward%20the%20west. Figure 4.62: The remains of St. Pierre. Angelo Heilprin. 1902. Public domain. https://commons.wikimedia.org/wiki/ File:Pelee_1902_3.jpg Figure 4.63: Sequence of events for Mount St. Helens, May 18th, 1980. Lyn Topinka via USGS. 1998. Public domain. https://commons.wikimedia.org/wiki/File:Msh_may18_sequence.gif Figure 4.64: Aman sweeps ash from an eruption of Kelud, Indonesia. Crisco 1492. 2014. CC BY-SA 3.0. https://commons.wiki- media.org/wiki/File:Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01.jpg Figure 4.65: Micrograph of silica particle in volcanic ash. USGS. 1980. Public domain. https://volcanoes.usgs.gov/vol- canic_ash/components_ash.html Figure 4.66: Mud line shows the extent of lahars around Mount St. Helens. USGS. 1980. Public domain. https://www.usgs.gov/media/images/lahars-resulting-may-18-1980-eruption-mount-st-helens Figure 4.67: Old lahars around Tacoma, Washington. USGS. 1905. Public domain. https://www.usgs.gov/media/images/ lahar-pathways-events-heading-mount-rainier-map-showing-t