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Volcaniism at spreading ridgees, and volccanic eruptions during the early hhistory of th he Earth releasedd gases thatt are thought to have foormed the attmosphere and a surface waters. VOLCA ANISM Volcaniism refers to t the proceesses whereeby magma and its associated gasses rise thro ough the Earth's crust and are a extruded d onto the ssurface or in nto the atmosphere. Cuurrently, mo ore than 550 voolcanoes are active th hat is, theyy have eru upted during historic time. Welll-known examples of activve volcanoees include M Mauna Loaa and Kilau uea on the island of Hawaii. Mount Etna on Sicily, Fujiyaama in Japaan, and Mount St. Hellens in Wasshington. Only O two other bodies in the solar system are thought to possess active volcanoes: 10. a moon of Jupiter. and perhaps Triton, one of Neptune's moons (see Chapter 2 Prologue). In addition to active volcanoes, numerous dormant volcanoes exist that have not erupted recently but may do so again. Mount Vesuvius in Italy had not erupted in human memory until A.D. 79 when it erupted and destroyed the cities of Herculaneum and Pompeii. Some volcanoes have not erupted during recorded history and show no evidence of doing so again; thousands of these extinct inactive volcanoes are known. Volcanic Gases Samples of gases taken from present-day volcanoes indicate that 50 to 80% of all volcanic gases are water vapor. Lesser amounts of carbon dioxide, nitrogen, sulfur gases, especially sulfur dioxide and hydrogen sulfide, and very small amounts of carbon monoxide, hydrogen, and chlorine are also commonly emitted. In areas of recent volcanism, such as Lassen Volcanic National Park in California, gases continue to be emitted. One cannot help but notice the rotten-egg odor of hydrogen sulfide gas in such areas. When magma rises toward the surface, the pressure is reduced and the contained gases begin to expand. In felsic magmas, however, which are highly viscous, expansion is inhibited and gas pressure increases. Eventually, the pressure may become great enough to cause an explosion and produce pyroclastic materials such as ash. In contrast, low viscosity mafic magmas allow gases to expand and escape easily. Accordingly, mafic magmas generally erupt rather quietly. The amount of gases contained in magmas varies, but is rarely more than a few percent by weight. Even though volcanic gases constitute a small proportion of a magma, they can be dangerous and, in some cases, have had far-reaching climatic effects. Most volcanic gases quickly dissipate in the atmosphere and pose little danger to humans, but on several occasions these gases have caused numerous fatalities. In 1783, toxic gases, probably sulfur dioxide, erupted from Laki fissure in Iceland had devastating effects. About 75% of the nation's livestock died, and the haze resulting from the gas caused lower temperatures and crop failures; about 24% of Iceland's population died as a result of the ensuing Blue Haze Famine. The country suffered its coldest winter in 225 years in 1783~ 1784, with temperatures 4.8°C below the long term average. The effects of the 1783 Laki fissure eruption were felt far beyond Iceland. The eruption produced what Benjamin Franklin called a "dry fog" that was responsible for dimming the intensity of sunlight in Europe. The severe winter of 1783-1784 in Europe and eastern North America is attributed to the presence of this "dry fog" in the upper atmosphere. The particularly cold spring and summer of 1816 are attributed to the 1815 eruption of Tambora in Indonesia, the largest and most deadly eruption during historic time. The emption of Mayon volcano in the Philippines during the previous year may have contributed to the cool spring and summer of 1816 as well. Another large historic eruption that had widespread climatic effects was the eruption of Krakatau in 1883. More recently, in 1986. in the African nation of Cameroon 1.746 people died when a cloud of carbon dioxide engulfed them. The gas accumulated in the waters of Lake Nyos. which occupies a volcanic crater. No agreement exists on what caused the h'3S to suddenly burst forth from the lake, but once it did. it Rowed downhill along the surface because It was denser than air. In fact. the density and velocity of the gas cloud we re great enough to flatten vegetarian, includiing trees, a few kilom meters from m the lake. Unfortunaately, thoussands of me as fur as 23 km from m the lake we w re asphyxxiated. animalss and many people. som Lava F Flows and Pyroclastic P Materials Rows are frequently f portrayed iin movies and on teelevision ass fiery streeams of Lava R incandeescent rock material posing a greeat danger to humans. Actually. lava Rows are the least daangerous maanifestation n of volcaniism. althoug gh they may destroy bbuildings an nd cover agriculttural land. Most M lava Rows R do noot move parrticularly fast. and becaause they are a fluid, they folllow existinng low areass. So once a Row eruptts from a vo olcano. deteermining the path it will takke is fairly easy, e and an nyone in areeas likely to be affected d can be evaacuated. Two types of laava flows, both b of whiich were named n for Hawaiian H fllows, are generally g nounced pahh-hoy-hoy) flow has a ropy surfacce almost liike taffy recogniized. A pahooehoe (pron (~ Figuure 5-3a). The T surface of an aa ((pronounced d ah-ah) flo ow is charaacterized by y rough, jagged angular bloocks and fraagments (Fiigure 5-3b). Pahoehoe flows are leess viscous than aa Rows; iindeed, the latter are viscous v enouugh to breaak up into blocks b and move forward as a wall of rubble. Colum mnar jointts are comm mon in manny lava flow ws, especiallly mafic fllows. but th hey also occur inn other kindds of flows and a in somee intrusive igneous rock ks (~ Figuree 5-4). A lav va Flow coontracts as it i cools and d produces fforces that cause c fractures called jooints to open n up. On the surfface of a floow, these jo oints comm monly form polygonal p (often ( six-siided) crackss. These cracks also extendd downward d into the fflow, tannin ng parallel columns w with their lo ong axes perpenddicular to thhe principall cooling suurface. Exceellent examp ples of coluumnar jointss can be seen at Devil's Poostpile Natio onal Monum ment in Caalifornia (Figure 5-4), Devil's Tow wer Na- tional Monument in Wyoming (see Chapter 4 Prologue), the Giant's Causeway in Ireland, and many other areas. Much of the igneous rock in the upper part of the oceanic crust is of a distinctive type; it consists of bulbous masses of basalt resembling pillows, hence the name pillow lava. It was long recognized that pillow lava forms when lava is rapidly chilled beneath water, but its formation was not observed until 1971. Divers near Hawaii saw pillows form when a blob of lava broke through the crust of an underwater lava flow and cooled almost instantly, forming a glassy exterior. The remaining fluid inside then broke through the crust of the pillow, resulting in an accumulation of interconnected pillows (~ Figure 5-5). Much pyroclastic material is erupted as ash, a designation for pyroclastic particles measuring less than 2.0 mm (~ Figure 5-6). Ash may be erupted in two ways: an ash fall or an ash Row. During an ash fall, ash is ejected into the atmosphere and settles to the surface over a wide area. In 1947. .ash erupted from Mount Hekla in Iceland fen 3,800 km away on Helsinki, Finland. Ash is also erupted in ash flows. which are coherent clouds of ash and gas that commonly Row along or close to the land surface. Such Rows can move at more than 100 km per hour, and some of them cover vast areas. Pyroclastic materials larger than ash are also erupted by explosive volcanoes. Particles measuring from 2 to 64 mm are known as lapilli and any particle larger than 64 mm is called a bomb or block depending on its shape. Bombs have twisted, streamlined shapes that indicate they were erupted as globs of fluid that cooled and solidified during their Right through the air (~ Figure 5-7). J3Iocks are angular pieces of rock ripped from a volcanic conduit or pieces of a solidified crust of a magma. Because of their large size, volcanic bomb and block accumulations are not nearly as widespread as ash deposits; instead, they are confined to the immediate area of eruption. Volcan noes Conicall mountainss formed aro ound a ventt where lavaa and pyrocllastic materrials are eru upted are volcanooes. Volcannoes, which are named for vulcan, the Romaan deity of ffire, come in i many shapes and sizes, but geolog gists recognnize severall major cattegories eacch of whicch has a distinctive eruptivee style. Onee must realiize howeveer, that each h volcano haas a uniquee overall history of eruptionns and development. Page 880-81 Lava D Domes. If thhe upward pressure p in a volcanic conduit c is great g enoughh, the most viscous magmas move upw ward and form f bulboous, steep-sided lava domes d (~ FFigure 5-12 2). Lava domes are generrally comp posed of ffelsic lavass although some aree of interrmediate u verry slowly; the t lava compossition. Becaause such magma is so viscous,, it moves upward dome thhat formed in Santa Maria M volcanno in Guateemala in 19 922 took tw wo years to grow to 500 Il1 high and 1,200 m accross. Lavaa domes con ntribute significantly tto many co omposite volcanooes. Beginniing in 1980 0, a number of lava dom mes were em mplaced in tthe crater of Mount St. Heleens; most off these weree destroyedd during sub bsequent eru uptions. Sinnce 1983, Mount M St. Helens has been chharacterized d by sporadiic dome gro owth. In Junne 1991, a dome d in Jap pan's Unzenn volcano co ollapsed, caausing a flow w of debris and hot ash thaat killed 433 people in n .a nearby town. Lav va domes are a also oftten responssible for extremely explosive eruptions. In 1902, viscous magma accumulated beneath the summit of Mount Pelee on the island of Martinique. Eventually, the pressure within the mountain increased to the point that it could no longer be contained, and the side of the mountain blew out in a tremendous explosion. When this occurred, a mobile, dense cloud of pyroclastic materials and gases called a nuee ardente (French for "glowing cloud") was ejected and raced downhill at about 100 km/hr, engulfing the city of St. Pierre (~ Figure 5-13). This nuee ardcnte had internal temperatures of 700°C and incinerated everything in its path. Of the 28,000 residents of St. Pierre, only two survived, a prisoner in a cell below the ground surface and a man on the surface who was terribly burned by the nuee ardente, Monitoring Volcanoes and Forecasting Eruptions According to the U.S. Geological Survey, nearly 500 million people live near the volcanoes on the margins of the Earth's tectonic plates. Many of these volcanoes have erupted explosively during historic time and have the potential to do so again. As a matter of fact, volcanic eruptions are not as unusual as one might think; 376 separate outbursts occurred between 1975 and 1985. Fortunately, none of these compared to the 1815 eruption of Tambora; nevertheless, fatalities occurred in several instances, the worst being in 1985 in Colombia where about 23,000 perished in mudflows generated by an eruption (Table 5-1). Only a few of these potentially dangerous volcanoes are monitored, including some in Italy, Japan, New Zealand, Russia, and the Cascade Range. Many of the methods for monitoring active volcanoes were developed at the Hawaiian Volcano Observatory. These methods involve recording and analyzing various changes in both the physical and chemical attributes of volcanoes. Tiltmeters are used to detect changes in the slopes of .1 volcano when it inflates as magma is injected into it, while a geodimeter uses a laser beam to measure horizontal distances, which also change when a volcano inflates (~ Figure 5-14). Geologists also monitor gas emissions and changes in the local magnetic and electrical fields of volcanoes. Of critical importance in volcano monitoring and eruption forecasting are a sudden increase in earthquake activity and the detection of harmonic tremor. Harmonic tremor is continuous ground motion as opposed to the sudden jolts produced by earthquakes. It precedes all eruptions of Hawaiian volcanoes and also preceded the eruption of Mount St. Helens. Such activity indicates that magma is moving below the surface. The analysis of data gathered during monitoring is not by itself sufficient to forecast eruption to past history of a particular volcano must also be known. To determine the eruptive history of a volcano, the record of previous eruptions as preserved in rocks must be studied and analyzed. Indeed, prior to 1980, Mount St. Helens was considered one of the most likely Cascade volcanoes to erupt because detailed studies indicated that it has had a record of explosive activity for the past 4,500 years. For thhe better monitored m vo olcanoes, suuch as thosse in Hawaiii, it is now w possible to t make accuratee short-term m forecastss of eruptioons. In 1'16 60 the warrning signs of an erup ption of Kilaueaa were recoognized soon n enough too evacuate the residen nts of a smaall village that t was subsequuently burieed by lava flows. f Unfoortunately, current c foreccasting is liimited to ju ust a few months in the futurre. m prediictions. On January For soome volcannoes little orr no informaation is avaiilable for making 14, 1993, for exam mple, Colom mbia's Galeraas volcano erupted witthout warnin ing, killing (6 of 10 volcanoologists on a field trip p and threee Colombiaan tourists. Ironically, the volcan nologists were atttending a coonference on o improvinng methods for f predictin ng volcanicc eruptions. Fissuree Eruption ns During the Miocenne and Plioccene epochss (between about a 17 miillion and 5 million yeaar. ago), 2 n Washingtoon and pam m of Oregon and Idahoo were cov vered by some 164,000 km of eastern overlappping basalt lava flows.. These Collumbia Riveer basalt" ass they art' ccalled, are now n well exposedd in the waalls of the canyons ero ded by the Snake and Columbia rivers (~ Figure 515). Thhese lavas, which w weree erupted frrom long fissures, were so fluid tthat volcaniic cones failed to develop. Such fissure eruptions yield flows that spread alit over large areas and form basalt plateaus (Figure 5-15). The Columbia River basalt flows have an aggregate thickness of .about 1000 m, and some individual flows cover huge areas-the Roza flow, which is 30 m thick, advanced along ,I trout about 100 km wide and covered 40,000 km2", Fissure eruptions and basalt plateaus are not common, .although several large areas with these features are known. Currently, this type of activity is occurring only in Iceland. A number of volcanic mountains art' present in Iceland, but the bulk of the island is composed of basalt flows erupted from fissures. Two large fissure eruptions, one in A.D. 930 and the other in 1783, account for about half of the magma erupted in Iceland during historic time. The 1783 emption occurred along the Laki fissure, which is more than 30 km long; lava flowed several tens of kilometers from the fissure, covering more than 560 km2, and in one place filled a valley to a depth of about 200 m. Pyroclastic Sheet Deposits More than 100 years ago, geologists were aware of vast areas covered by felsic volcanic rocks a few meters to hundreds of meters thick. It seemed improbable that these could have formed as vast lava flows, but it also seemed equally unlikely that they were ash £111 deposits. Based on observations of historic pyroclastic flows, such as the nucc ardcnte erupted by Mount Pelee in 1902, it now seems probable that these ancient rocks originated as pyroclastic flows, hence the name pyroclastic sheet deposits. They cover far greater areas than any observed during historic time and apparently erupted from long fissures rather than from a central vent. The pyroclastic materials of many of these flows were so hot they fused together to form welded tuff. It now appears that major pyroclastic flows issue from fissures formed during the origin of calderas. The Yellowstone Tuff, for instance, was erupted during the formation of a large caldera in the area of present-day Yellowstone National Park in Wyoming. Similarly, the Bishop Tuff of eastern California appear.; to have been erupted shortly before the formation of the Long Valley caldera. Interestingly, earthquake activity in the Long Valley caldera and nearby areas beginning in 1978 may indicate t hat magma is moving upward beneath part of the caldera. Thus, the possibility of future eruptions in that area cannot be discounted. DISTRIBUTION OF VOLCANOES Rather than being distributed randomly around the Earth, volcanoes occur in well-defined zones or belts. More than 60% of all active volcanoes are in the circum-Pacific belt that nearly encircles the margins of the Pacific Ocean basin (~ Figure 5-16). This belt includes the volcanoes along the west coast of South America, those in Central America, Mexico, and the Cascade Range, and the Alaskan volcanoes in the Aleutian Island arc. The belt continues on the western side of the Pacific Ocean basin where it extend, through Japan, the Philippines, Indonesia, and New Zealand. Mount Pinatubo and Mayon volcano, two Philippine volcanoes that have erupted since June 1991, are in this belt. The circumPacific belt also includes the southernmost active volcano. Mount Erebus in Antarctica, and a large caldera at Deception Island that erupted during 1970. About 20% of all active volcanoes are in the Mediterranean belt (Figure 5-16). Included in this belt arc the famous ridges iis the Mid-A Atlantic Rid dge, which is near thee middle of the Atlantiic Ocean baasin and curves aaround the southern tip p of Africa where it co ontinues as the Indian R Ridge. Bran nches of the Indiian Ridge extend into the t Red Seaa and East Africa. A Mou unt Kilimanj njaro in Afriica is on this lattter branch (Figure 5-16). Most of the volccanism along the midd-oceanic ridges is submarine, and muuch of it go oes undeteccted; but in n a few placces, such ass Iceland, it occurs above ssea level. Volcaanism is occcurring in a few other aareas at preesent, most notably on and near th he island of Haw waii (Figure 5-16). Only y two volcaanoes are cu urrently acttive on the island, Mau una Loa and Killauea, althoough a subm marine volccano named d Loihi exissts about 322 km to the south; Loihi rises more than 3,000 m above the sea floor, but its summit is still about 940 m below sea level. PLATE TECTONICS AND IGNEOUS ACTIVITY At this point. two questions might be asked regarding volcanoes: (I) What accounts for the alignment of volcanoes in belts? (2) Why do magmas erupted within ocean basins and magmas erupted at or near continental margins have different compositions? In addition, plutons emplaced within the ocean basins are invariably mafic, mostly gabbro, whereas the vast batholiths emplaced at continental margins are composed of felsic and intermediate rocks such as granite and diorite. Recall from Chapter I that the outer part of the Earth is divided into large plates. which are sections of the lithosphere. Most igneous activity occurs at spreading ridges where plates diverge or along subduction zones where plates converge. Igneous Activity at Spreading Ridges Spreading ridges are areas where new oceanic lithosphere is produced by igneous activity as plates diverge from one another. Mafic magma originates beneath these spreading ridges: some of the magma is erupted at the surface as basalt lava flows and/or pyroclastic materials, but much is simply emplaced at depth as vertical dikes and gabbro plutons (~ Figure 5-17). In tact, the oceanic crust is composed largely of such mafic rocks. The fact that volcanism occurs at spreading ridges is undisputed. but how magma originates beneath the ridges is not fully understood. One explanation is related to the manner in which the Earth's temperature increases with depth. We know from deep mines and deep drill holes that a temperature increase, called the geothermal gradient occurs and that, on average. it is about 25°C/km. Accordingly, rocks at depth are hot, but remain solid because their melting temperature rises with increasing pressure. Beneath spreading ridges, the temperature locally exceeds the melting temperature. at least in part, because pressure decreases. That is, rifting probably causes a decrease in pressure on the hot rocks at depth, thus initiating melting (~ Figure 5-18a). Furthermore, the presence of water can also decrease the melting temperature beneath spreading ridges because water aids thermal energy in breaking the chemical bonds in minerals (Figure 5-18b). Another explanation for spreading-ridge igneous activity is that localized, cylindrical plumes of hot mantle material, called mantle plumes, rise beneath ridges and spread outward in all directions. Perhaps localized concentrations of radioactive minerals within the crust and upper mantle decay and generate the heat responsible for the melting associated with these hot mantle plumes. The lavas erupted at spreading ridges are invariably mafic and cool to form basalt. But the upper mantle, from which these lavas are derived, is composed of ultramafic rock, probably peridotite, which consists largely of ferrornagnesian silicates and lesser amounts of nonferromagnesian silicates. To explain how mafic magma (45-52% silica) originates from ultramafic rock (≤45% silica), geologists propose that the magma is formed from source rock that only partially melts. This phenomenon of partial! melting occurs because various minerals have different melting temperatures. Recall the sequence of minerals in Bowen's reaction series (Figure 4-5). The order in which these minerals melt is the opposite of their order of crystallization. Acccordingly, quartz, pottassium feld dspar, and sodium-ricch plagioclaase melt before m most of thee ferromagn nesian silicaates and thee calcic variieties of plaagioclase. So S when ultramaafic rock beegins to meelt, the minnerals richeest in silica melt first followed by b those containing less sillica. Accord dingly, if m melting is not n complete, a mafic magma con ntaining proportionately moore silica th han the sourrce rock results. Once this mafic magma is formed, some off it rises to the t surface where it is erupted, coo ols, and cry ystallizes to form basaltt. Igneou us Activity y at Subdu uction Zon nes Subducction occurss where an oceanic plaate and a continental c plate conveerge, or wh here two oceanicc plates convverge. In eitther case, a belt of com mposite volccanoes and pplutons occcurs near the leadding edge of o the overrriding platee (~ Figure 5-1.9). As the subduccted plate descends d toward the asthenoosphere, it eventually e reeaches a depth where the temperat ature is high h enough for parttial meltingg to occur, and magm ma is generaated. Additiionally, thee wet ocean nic crust descendds to a depth at which h dewaterinng occurs, and a as the water w rises into the ov verlying mantle, it enhancess melting an nd magma fforms (Figu ure 5-19b). _ Partiaal melting is one pheenomenon accounting for the taact that mag agmas generated at subducttion zones are intermeediate and ffelsic in composition. Recall thatt partial meelting of ultramaafic rock of the upper mantle m yieldds mafic maagma. Likew wise, partiall melting off oceanic crust, w which has a mafic comp position, maay yield maagma richerr in silica thhan the sourrce rock. Additioonally, somee of the silicca-rich sediiments and sedimentary y rocks of ccontinental margins are probably carried downwaard with th e subducted d plate and d contributee their silica to the magma. Also, maffic magma rising r throuugh (he lower continental crust maay be contaaminated with fellsic materiaals, which ch hange its coomposition. Intermeediate and felsic f magm mas are typpically prod duced at con nvergent pllate margin ns where subducttion occurs.. The interm mediate maagma that iss erupted is more viscoous than -c magma and tennds to form composite volcanoes. Much felsiic magma iss intruded iinto the con ntinental crust w where it varrious plutons, especiaally batholiths, but so ome is eruppted as pyrroclastic materiaals or emplaaced as lava domes, acccounting forr the explosive eruptionns that charaacterize converggent plate margins. m Intraplate Volca anism Mauna Loa and Kilauea K on the t island oof Hawaii and a Loihi ju ust to the soouth are wiithin the interior of a rigid plate far fro om any sprreading ridg ge or subduction zone (Figure 5-1 17). It is postulatted that a mantle m plum me creates a local "ho ot spot" beneath Haw waii. The magma is mafic aand relativelly fluid, so it i builds up shield volcanoes. Even though theese Hawaiiaan volcanoees are unreelated to sp preading riddges or sub bduction zones, tthe evolutioon of rhe Hawaiian Islaand. is relatted to plate tectonics. N Notice in Figure F 221 that the ages off the rocks composing c tthe islands in i the Hawaaiian chain iincrease tow ward the northweest; Kauai formed f 3.8 to 5.6 milllion years ago, a whereaas Hawaii bbegan form ming less than onne million yeears ago, an nd Loihi beggan forming g even more recently. C Continuouss motion of the P Pacific platee over the "hot " spot," nnow beneatth Hawaii, has h created the variouss islands in succeession. Mantlle plumes annd "hot spo ots" have alsso been prop posed to explain volcannism in a feew other areas. A mantle pluume may bee beneath Y Yellowstone National Park in Wyom oming. Somee source of heatt at depth is i responsib ble for the present-daay hot sprin ngs and geeysers such as Old Faithfull, but manyy geologists think that tthe source of heat is a body of inntruded mag gma that has not yet compleetely cooled d' rather thann a mantle plume. p Answers Add itional read dings