Physical Geography - Version 1 Unit 15 Earths Dynamic Surface Volcanoes

Figure Geothermal Hot Springs in Hot Creek , Mammoth California . Image by Jeremy is used under a license . UNIT 15 EARTHS DYNAMIC SURFACE VOLCANOES Goals Objectives of this unit Explain the relationships between plate tectonics , the formation of magma , and volcanism Describe the range of magma compositions formed in differing tectonic environments , and discuss the relationship between magma composition ( and gas content ) and eruption style Explain the geological and differences between different types of volcanoes , especially shield volcanoes , composite volcanoes , and cinder cones Understand the types of hazards posed to people and infrastructure by the different types of volcanic eruptions GEOGRAPHY

VOLCANISM A volcano is any location where magma comes to the surface or has done so within the past several million years . This can include eruptions on the ocean floor ( or even under the water of lake ) where they are called subaqueous eruptions , or on land , where they are called subaerial eruptions . Not all volcanic eruptions produce the volcanic mountains with which we are familiar in fact , most of Earth volcanism takes place along the spreading ridges on the seafloor and does not produce volcanic mountains at all , not even mountains . The study of volcanoes is critical to our understanding of the geological evolution of Earth , and our understanding of significant changes in climate . But , most important of all , understanding volcanic eruptions allows us to save lives and property . Over the past few decades , volcanologists have made great strides in their ability to forecast volcanic eruptions and predict the consequences , this has already saved thousands of lives . Plate Tectonics Volcanism Magma is formed at three main settings divergent boundaries ( decompression melting ) convergent boundaries ( flux melting ) and mantle plumes ( decompression melting ) Composite volcanoes form at subduction zones , either on convergent boundaries or convergent boundaries . Both shield volcanoes and cinder cones form in areas of continental . Shield volcanoes form above mantle plumes but can also form at other tectonic settings . volcanism can take place at divergent boundaries , mantle plumes , and boundaries . Figure The Settings of Common Volcanism . Image by is in the public domain . GEOGRAPHY

The mantle and crustal processes that take place in areas of volcanism . At a spreading ridge , hot mantle rock moves slowly upward by convection ( and within about 60 of the surface , partial melting starts because of decompression . Nearly 10 of the mantle rock melts , producing mafic magma that moves upward toward the axis of spreading ( where the two plates are moving away from each other ) The magma fills vertical fractures produced by the spreading and spills out onto the seafloor to form basaltic pillows ( more on that later ) and lava flows . At an or convergent boundary , oceanic crust is pushed far down into the mantle . It is heated up , and while there is enough heat to melt the crust , there is enough to force the water out of some of its minerals . This water rises into the overlying mantle where it contributes to the flux melting ofthe mantle rock . The mafic magma produced rises through the mantle to the base ofthe crust . There it contributes to the partial melting of crustal rock , and thus it much more material . That magma , now intermediate in composition , continues to rise and assimilate crustal material in the upper part of the crust , it accumulates into . From time to time , the magma from the rises toward the surface , leading to volcanic eruptions . Mount Rainier is an example of related volcanism . segment TNE GLACIER SYSTEM MOUNT . Figure Relief Map . 1896 . Plate by is in the public domain . GEOGRAPHY

Pin It ! Volcano Monitoring Videos ! View this website for videos on how volcanoes are monitored and learn more about some of the research fro and Mount Rainier . A mantle plume is an ascending column of hot rock ( not magma ) that originates deep in the mantle , above the boundary . Mantle plumes are thought to rise at approximately 10 times the rate of mantle convection . The ascending column may be on the order of kilometers to tens of kilometers across , but near the surface , it spreads out to create a head that is several tens to over 100 kilometers across . Near the base of the lithosphere ( the rigid part of the mantle ) the mantle plume ( and possibly some ofthe surrounding mantle material ) partially melt to form mafic magma that rises to feed volcanoes . Since most mantle plumes are beneath the oceans , the early stages of volcanism typically take place on the seafloor . Over time , islands may form like those in Hawaii . Magma Composition Eruption Style As noted in the previous section , the types of magma produced in the various volcanic settings can differ significantly . At divergent boundaries and oceanic mantle plumes , where there is little interaction with crustal materials and magma fractionation to create melts does not take place , the magma tends to be consistently mafic . At subduction zones , where the magma ascends through significant of crust , the interaction between the magma and the crustal rock , some of which is quite leads to increases in the character of the magma . Several processes can make magma that is stored in a chamber within the crust more and can also contribute to the development of vertical zonation from more mafic at the bottom to more at the top . Partial melting of country rock and increases the overall character of the magma first , because the country rocks tend to be more than the magma , and second , because the more components of the melt preferentially . Settling of ferromagnesian crystals from the upper part of the magma , and possible of those crystals in the lower part can both contribute to the vertical zonation from relatively mafic at the bottom to more at the top . GEOGRAPHY

( Possible melting of olivine or pyroxene can make the lO ' magma Figure Changes in the Composition Stored Within a Chamber . Image by Steven , From the perspective of volcanism , there are some important differences between and mafic . First , as we already discussed , tend to be more viscous because they have more silica , and hence more polymerization . Second , tend to have higher levels of that is , components that behave as gases during volcanic eruptions . The most abundant volatile in magma is water ( followed typically by carbon dioxide ( and then by sulphur dioxide ( 02 ) The general relationship between the of magma and the number of is shown in Figure . Although there are many exceptions to this trend , mafic typically have to , intermediate have to , and have to . Differences in viscosity and volatile levels have significant implications for the nature of volcanic eruptions . When magma is deep beneath the surface and under high pressure from the surrounding rocks , the gases remain dissolved . As magma approaches the surface , the pressure exerted on it decreases . Gas bubbles start to form , and the more gas there is in the magma , the GEOGRAPHY

more bubbles form . gas content is low or the magma is runny enough for gases to rise through it and escape to the surface , the pressure will not become excessive . Assuming that it can break through to the surface , the magma will flow out relatively gently . An eruption that involves a steady flow of magma is called effusive . If the magma is , and therefore too viscous for gases to escape easily , or if it has a particularly high gas content , it is likely to be under high pressure . Viscous magma does flow easily , so even if there is a way for it to move out , it may not flow out . Under these circumstances , pressure will continue to build as more magma moves up from beneath and gases continue to . Eventually , some part ofthe volcano will break and then all of that pressure will lead to an explosive eruption . Mantle plume and tend to be consistently mafic , so effusive eruptions are the norm . At subduction zones , the average magma composition is likely to be close to intermediate , but as we seen , magma chambers can become zoned and so compositions ranging from to mafic are possible . Eruption styles can be correspondingly variable . TYPES OF VOLCANOES The sizes and shapes of a typical shield , composite , and cinder cone volcanoes are compared in the table below , although , to be fair , Loa is the largest shield volcano on Earth all others are smaller . Loa rises from the surrounding flat seafloor , and its diameter is in the order of 200 . Its elevation is above sea level . a composite volcano , rises above the surrounding hills of the Cascade Range . Its diameter is about , and its height is above sea level . Cinder cones are much smaller . On this drawing , even a large cinder cone is just a dot . GEOGRAPHY

Table Types of Volcanoes Type Tectonic Size and Shape Magna and Eruption Example Setting Characteristics Cinder Cone Various some Small 10 to Most are mafic and . form on the 100 of meters . form from the flanks of larger Steep ( early stages of a shield volcanoes . or rift associated eruption . Composite Almost all are Medium 1000 Magma composition . Volcano at subduction of meters . varies from to zones . Moderate mafic , and from steepness ( explosive to effusive . to ) Shield Most are at Large , Magna is almost Volcano mantle plumes high and 200 always mafic , and Hawaii some are on across . eruptions are typically spreading Not steep ( to effusive , although ridges . cinder cones are common on the flanks of shield volcanoes . Large Igneous Associated with Enormous Magma is always mafic Columbia Provinces super mantle Millions of and individual flows River plumes and 1005 of can be 105 of meters thick . thick . Seafloor Generally Large areas of A typical eruption Juan de Volcanism associated with seafloor rates , pillows form at Ridge spreading associated with faster rates , lava flows ridges but also spreading ridges . develop . with mantle plumes . Cinder Cones Cinder cones , like Red Mountain in the Eastern Sierra , are typically only a few hundred meters in diameter , and few are more than 200 high . Most are made up of fragments of vesicular mafic rock ( scoria ) that were expelled as the magma boiled when it approached the surface , creating fire fountains . In many cases , these later became effusive ( lava flows ) when the gases were depleted . Most cinder cones are monogenetic , meaning that they formed during a single GEOGRAPHY

eruptive phase that might have lasted weeks or months . Because cinder cones are made up almost exclusively of loose fragments , they have very little strength . They can be easily , and relatively quickly , eroded . Composite Composite volcanoes , like . in Washington State , are almost all associated with subduction at convergent plate boundaries either or boundaries . They can extend up to several thousand meters from the surrounding terrain , and , with slopes ranging up to , are typically up to 10 across . At many such volcanoes , magma is stored in a magma chamber in the upper part of the crust . For example , at . there is evidence of a magma chamber that is approximately wide and extends from about to 14 below the surface . Systematic variations in the composition of volcanism over the past several thousand years at . imply that the magma chamber is zoned , from more at the top to more mafic at the bottom . Figure Columnar Basalt at The Devils Near Mammoth , California . Image by Jeremy is used under a license . Mafic eruptions ( and some intermediate eruptions ) on the other hand , produce lava flows the one shown below is thick enough ( about 10 in total ) to have cooled in a columnar jointing pattern . Lava flows both flatten the profile of the volcano ( because the lava typically flows farther than pyroclastic debris falls ) and protect the fragmental deposits from erosion . Even so , composite volcanoes tend to erode quickly . Patrick Pringle , a volcanologist with the Washington State Department of Natural Resources , describes . as a pile GEOGRAPHY

The rock that makes up . ranges in composition from rhyolite to basalt this implies that the types of past eruptions have varied widely in character . As already noted , magma does flow easily and does allow gases to escape easily . Under these circumstances , pressure builds up until a conduit opens , and then an explosive eruption results from the upper part of the magma chamber , producing pyroclastic debris . This type of eruption can also lead to the rapid melting of ice and snow on a volcano , which typically triggers large known as . Hot , pyroclastic flows and are the two main causes of casualties in volcanic eruptions . Pyroclastic flows killed approximately people during the 1902 eruption of . on the Caribbean island of . Most were incinerated in their homes . In 1985 a massive , triggered by the eruption of del Ruiz , killed people in the Colombian town of , about 50 from the volcano . In a geological context , composite volcanoes tend to form relatively quickly and do not last very long . for example , is made up of rock that is all younger than years most of it is younger than years . If its volcanic activity ceases , it might erode within a few tens of thousands of years . This is largely because of the presence of pyroclastic eruptive material , which is not strong . Shield Most shield volcanoes are associated with mantle plumes , although some form at divergent boundaries , either on land or on the seafloor . Because of their mafic magma , they tend to have relatively gentle slopes ( to ) and the larger ones can be over 100 in diameter . The shield volcanoes are those that make up the Hawaiian Islands , and of these , the only active ones are on the big island of Hawaii . Loa , the world largest volcano and the world largest mountain ( by volume ) last erupted in 1984 . arguably the world most active volcano , has been erupting , virtually without interruption , since 1983 . is an underwater volcano on the southeastern side of Hawaii . It is last known to have erupted in 1996 but may have erupted since then without being detected . All of the Hawaiian volcanoes are related to the mantle plume that currently lies beneath Loa , and . In this area , the Pacific Plate is moving northwest at a rate of about . This means that the earlier formed , and now extinct , volcanoes have now moved well away from the mantle plume . There is evidence of crustal magma chambers beneath all three active Hawaiian volcanoes . At , the magma chamber appears to be several kilometers in diameter and is situated between and 11 below the surface Although it is not a prominent mountain , volcano has a large caldera in its summit area . A caldera is a volcanic crater that is more than in diameter this one is long and GEOGRAPHY

wide . It contains a smaller feature called uma ' crater , which has a total depth of over 200 below the surrounding area . Most volcanic craters and are formed above magma chambers , and the level of the crater floor is influenced by the amount of pressure exerted by the magma body . During historical times , the floors of both caldera and uma ' crater have moved up during the expansion of the magma chamber and down during deflation of the chamber . The two main types of textures created during effusive subaerial eruptions are pahoehoe and aa . Pahoehoe , ropy lava that forms as lava , flows gently , forming a skin that gels and then wrinkles because of the ongoing flow of the lava below the surface . Pin It ! Lava Flow View this Lava flow video to get a closer look at a Pahoehoe flow . Aa , or blocky lava , forms when magma is forced to flow faster than it can ( down a slope for example ) lava fragments , is produced during explosive eruptions and accumulates in the vicinity of cinder cones . Below is a view of an extinct lava tube in Mojave , California . Very thick , gooey rhyolitic lava does flow very far . The runny basaltic sort that characterizes some of the lava flows of Mojave National Preserve , however , spreads out as smoothly as hot maple syrup . It flowed from the sides of the cones or pooled near their bases . As the lava streamed out across the land , it slowly began to cool . Often , the top of flow would cool while liquid lava continued moving underneath , creating a tunnel . When the eruption ended , the flowing lava in the tunnel either cooled in place or emptied the end , leaving a hollow lava tube . A lava tube is accessible via a drive from Road . Climbing through a collapsed hole in the tube roof , visitors have a rare opportunity to view this river of rock from the lava perspective . GEOGRAPHY

Figure Photo from Inside the Mojave Volcanic Tubes . image by Jeremy is used under a license . Volcanism Some large igneous provinces eruptions occur on the seafloor , the largest being the one that created the Java Plateau in the western Pacific Ocean at around 122 Ma . But most floor volcanism originates at divergent boundaries and involves relatively eruptions . Under these conditions , hot lava that oozes out into the cold seawater quickly cools on the outside and then behaves a little like toothpaste . The resulting blobs of lava are known as pillows , and they tend to form piles around a lava vent . In terms of area , there is very likely more pillow basalt on the seafloor than any other type of rock on Earth . VOLCANIC HAZARDS There are two classes of volcanic hazards , direct and indirect . Direct hazards are forces that directly kill or injure people or destroy property or wildlife habitat . Indirect hazards are environmental changes that lead to distress , famine , or habitat destruction . Indirect effects of volcanism have accounted for approximately million deaths during historical times , while direct effects have accounted for fewer than , or of the total . GEOGRAPHY

Volcanic Gas Emissions Large volumes of ( rock fragments , mostly pumice ) and gases are emitted during major eruptions ( large explosive eruptions with hot gas a column extending into the stratosphere ) at composite volcanoes , and a large volume of gas is released during some very effusive eruptions . One of the major effects is cooling of the climate by to for several months to a few years because the dust particles and tiny droplets and particles of sulfur compounds block the sun . The last significant event of this type was in 1991 and 1992 following the large eruption of . in the Philippines . A drop of to may not seem like very much , but that is the global average amount of cooling , and cooling was much more severe in some regions . Over eight months in 1783 and 1784 , a massive effusive eruption took place at the volcano in Iceland . Although there was relatively little volcanic ash involved , a massive amount of sulfur dioxide was released into the atmosphere , along with a significant volume of hydrofluoric acid ( The sulphate aerosols that formed in the atmosphere led to dramatic cooling in the Northern Hemisphere . There were serious crop failures in Europe and North America , and a total of million people are estimated to have died from famine and respiratory complications . In Iceland , poisoning from the resulted in the death of 80 of sheep , 50 of cattle , and the ensuing famine , along with poisoning , resulted in more than human deaths , about 25 of the population . Volcanic ash can also have serious implications for aircraft because it can destroy jet engines . For example , over million airline passengers had their travel disrupted by the 2010 volcanic eruption in Iceland . Pyroclastic Density Currents In a typical explosive eruption at a composite volcano , the and gases are ejected with explosive force and are hot enough to be forced high up into the atmosphere . As the eruption proceeds and the amount of gas in the rising magma starts to decrease , parts will become heavier than air , and they can then flow downward along the flanks of the volcano . As they descend , they cool more and flow faster , reaching speeds up to several hundred . A pyroclastic density current ( consists of ranging in size from boulders to microscopic shards of glass ( made up of the edges and junctions of the bubbles of shattered pumice ) plus gases ( dominated by water vapor , but also including other gases ) The temperature of this material can be as high as . Among the most famous are the one that destroyed in the year 79 CE , killing an estimated people , and the one that destroyed the town of Pierre , in 1902 , killing an estimated . GEOGRAPHY

Figure 15 . Example of Pyroclastic Debris , Saint 1980 . Image by Bob , is in the public domain . Pyroclastic Fall Most of the from an explosive eruption ascends high into the atmosphere , and some of it is distributed around Earth by winds . The larger components ( larger than ) tend to fall relatively close to the volcano , and the amount produced by large eruptions can cause serious damage and casualties . The large 1991 eruption of . in the Philippines resulted in the accumulation of tens of centimeters of ash in fields and on rooftops in the surrounding populated region . Heavy typhoon rains that hit the island at the same time added to the weight of the , leading to the collapse of thousands of roofs and at least 300 of the 700 deaths attributed to the eruption . A is any mudflow or debris flow that is related to a volcano . Most are caused by melting snow and ice during an eruption , as was the case with the that destroyed the Colombian town of in 1985 ( described earlier ) can also happen when there is no volcanic eruption , and one ofthe reasons is that , as we seen , composite volcanoes tend to be weak and easily eroded . In October 1998 , category Hurricane Mitch slammed into the coast of Central America . The damage was extensive and people died , not so much because of high winds but because GEOGRAPHY

of intense rainfall , some regions received almost of rain over a few days ! and debris flow occurred in many areas , especially in Honduras and . An example is Casita Volcano in , where the heavy rains weakened rock and volcanic debris on the upper slopes , resulting in a debris flow that rapidly volume as it raced down the steep slope , and then ripped through the towns of El and Rodriguez killing more than people . El and Rodriguez were new towns that had been built without planning approval in an area that was known to be at risk of . UNIT 15 SUMMARY Volcanism is closely related to plate tectonics . Most volcanoes are associated with convergent plate boundaries ( at subduction zones ) and there is also a great deal of volcanic activity at divergent boundaries and areas of continental . At convergent boundaries magma is formed where water from a plate acts as a flux to lower the melting temperature of the adjacent mantle rock . At divergent boundaries magma forms because of decompression melting . Decompression melting also takes place within a mantle plume . The initial in most volcanic regions are mafic in composition , but they can evolve into more types through interaction with crustal rock , and as a result of crystal settling within a magma chamber . tend to have higher gas contents than mafic , and they are also more viscous . The higher viscosity prevents gases from escaping from the magma , and so are more pressurized and more likely to erupt explosively . Cinder cones , which can form in various volcanic settings , are relatively small volcanoes that are composed mostly of mafic rock fragments that were formed during a single eruptive event . Composite volcanoes are normally associated with subduction , and while their magma tends to be intermediate on average , it can range from to mafic . The corresponding differences in magma viscosity lead to significant differences in eruptions style . Most shield volcanoes are associated with mantle plumes and have consistently mafic magma which generally erupts as lava flows . Most direct volcanic hazards are related to volcanoes that erupt explosively , especially composite volcanoes . Pyroclastic density currents , some as hot as can move at hundreds of and will kill anything in the way . can be large enough to destroy entire towns . Lava flows will destroy anything in their paths but tend to move slowly enough so that people can get to safety . GEOGRAPHY