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Lavas. Understanding Volcanism Provides a Background for Theories

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Lavas. Understanding Volcanism Provides a Background for Theories lavas. Understanding volcanism provides a background for theories relating to mountain building, the development and evolution of continental and oceanic crt~st,and how the crust is deformed. Our observations of volcanic activity fit nicely into plate-tectonic theory as described in chapter 11. The May 18, 1980 eruption of Mount St. Helens (box 10.1) was a spectacular release of energy from the earth's interior. The plate-tectonic explanation is that North America, moving weritwa~d,is overriding a portion of the Pacific Ocean Aoor. Melting of previously solid rock takes place at depth, just above the subduction zone. (Thii was described briefly in chapter 1 and more thoroughly in chapter 11.) At least some of the magma (molten rock or liquid that is mostly silica) works its way upward to the earth's surface to erupt. Magma does not always reach the earth's surface before solidifying, but when it does it is called lava. At Mount St. Helens the lava solidified quickly as it was blasted explosively by gases into the air, producing rock frag- ments known as pyrodasw (from the Greekpyro, "fire," and clart, "broken"). Pyroclastic debris is also known as tcphra. Elsewhere in the world (notably Hawaii) lava extrudes out of Flgulr 10.1 fissures in the ground as lava flows. Pyrodastic debris and Volcanic eruptions in Hawaii, 1969. A lava fountain is supply11 rock formed by solidification of lava are collectively regarded the lava cascading over the cliff. as emwive rock, surface rock resulting from volcanic Photo by D A Swanson, US Geolog~calSurvey acrivity. The most obvious landform created by volcanism is a vol- cano, a hill or mountain formed by the extrusion of lava or fertile soil. Moreover, Hawaii's periodically erupting volc ejection of rock fragments from a vent. However, volcanoes are (which are relatively safe to watch) are great spectacle not the only volcanic landforms. Very fluid lava may flow out attract both tourists and scientists, benefiting the is. of the earth and flood an area, solidifying into a nearly hori- economy (figure 10.1). zontal layer of extrusive rock. Successive layers of lava flows Were it not for volcanic activity, Hawaii would not may accumulate into a lava plateau. The islands are the crests of a series of volcanoes that have Volcanic acrivity is important to geology for several rea- built up from the bottom of the Pacific Ocean over millic sons. Landforms are created and portions of the earth's surface years (the vertical distance from the summit of Mauna Lo built up. Less commonly, as at Mount St. Helens, landforms can0 to the ocean floor greatly exceeds the height abm are destroyed by violent eruptions. Volcanoes are important to level of Mount Everest). When lava flows into the sc the science of geology because they provide clues about the solidifies, more land is added to the islands. Hawaii is, nature of the earth's inaccessible interior and help us under- literally, growing. stand how the earth's internal processes work. By studying the magma, gases, and rocks from eruptions, we can infer the chemical conditions as well as the temperatures and pressures Geothermal Energy within the earth's crust or underlying mantle. In some other areas of geologically recent volcanic acl underground heat generated by igneous activity is harn for human needs. In Italy, Mexico, New Zealand, Arger Effects on Humans Japan, and California, geothermal installations produce Volcanism is also significant in human affiirs. Its effects can be tric power. Steam or superheated water trapped in lay catastrophic or, surprisingly, beneficial. hot volcanic rock is tapped by drilling and then piped c the ground to power turbines that generate electricity. I The Growth of Hawaii rally heated geothermal fluids can also be tapped for spa domestic water heating or industrial use, as in paper man The overall effects of volcanism have been favorable to turing. humans in Hawaii, although occasionally a field or village is overrun by outpourings of lava. Kilauea volcano is especially active. During the 1980s and 1990s, over 1.5 billion cubic Effect on Climate meters of lava erupted--enough to build a highway that cir- Occasionally, a volcano will spew large amounts of cles the world four times. There were 181 houses destroyed in .volcanic dust and gas into the high atmosphere. U the 1980s and 1990s, but no one was killed or injured. Never- can keep fine particles suspended over the earth for 1 theless, the weathered volcanic ash and lava produce excellent The 1991 eruption of Mount Pinatubo in the Philip Chapter I0 lrokanirm and Exmiw Rocks Lake. Collapse of the volcano, as kc11 as ex, sions, accounts for the depression the present- lake occupies (figures 10.3 and 10.4). The southern Cascade Mountains, wt Crater Lake is located, have been built up eruptions over the past 30 to 40 million ye&( urc 10.5; sec also thc geologic map, inside cov Only the youngest peaks (those built within past 2 million years), such as Mount St. Helc Mount Rainier, Mount Shwta, and Mount He still stand out as cones. As we know from Mo St. Helens, any of these could again bccc active. Eruptive Violence and Physica characteristics of Lava What determines the degree of violence associa 8 - r- 10.3 with volcanic activity? Why can we state co Crater Lake, Oregon. Figure 10.4 showi Its geologlc hlstory. dendy that active volcanism in Hawaii poses c Photo by @ Greg VaughnTTom Stack & Assoolates slight danger to humans, but expect violent ex1 C D Flgum 10.4 The development of Crater Lake. (A) Cluster of overlapping volcanoes form. (8)Collapse into the partially emptied magma chamber is accompanied by violent eruptions. (C) Volcanic activity ceases, but steam explosions take place In the caldera. (D)Water fills the calder to become Crater Lake, and minor renewed volcanism builds a cinder cone (Wizard Island). After C Bacon U S Gsologlcal Survey "greater the dissolved ws content, the more fluid t the lava.) If the lava !&ing extruded is consider- ably hotter than its solidification temperature, the lava is less viscous (more fluid) than when its temperature is near its solidification point. Temperatures at which lavas solidify range from about 700°C for silicic rocks to 1,200°C for mafic rocks. Volcanic rocks, and the magma from which they formed, have a silica content that ranges from 45% to 75% by weight. Sdicic (or felsic) rocks are silica-rich (65% or more SiO,) rocks. Rhyolite is the most abundant silidc volcanic rock. M&c rocks are silica- @n'cnt rocks. Their silica content is close to 50%. Baralt is the most common mafic rock. Intermediate rocks have a chemical content between that of felsic and mafic rocks. The most common intermediate rock is andrrite. A more complete description of the chemistry of igneous rocks and their relation- ship to the mineral content of rocks is given in chapter 11. Mafic lavas, which are relatively low in SiO,, tend to flow easily. Conversely, felsic lavas are very viscous and flow slugg'ihly. Law rich in silica are more viscous because even before they have cooled enough to allow crystalliza- tion of minerals, silicon-oxygen tetrahedrons have begun to form small frameworks in the lava. Although too few atoms are involved for the structures to be considered crystals, the total effect of these silicate structures is to make the liquid lava more viscous, much the way that flour or cornstarch thickens gravy. South Because silicic magmas are the most vis- cous, they are associated with the most violent eruptions. Malic magmas are the least viscous Cascade volcanoes. See also the map in chapter 1 (figure 1.51, and commonly erupt as lava flows (such as in Hawaii). Eruption associated with intermediate magma can be violent or can produce lava to occur in the Cascade Mountains?Whether eruptions are flows. (The Cascade volcanoes are predominantly composed of explosive or relatively "quiet" is largely determined by two intermediate rock.) rs: (1) the amount of gas in the lava or magma and (2)the or difficulty with which the gas can escape to the atmo- re. The vkmiy, or resistance to flow, of a lava determines Extrusive Rocks and Gases easily the gas escapes. The more viscous the lava and the er the volume of gas trying to escape, the more violent the Scientific Investigation of Volcanism ion. Later we will show how these factors not only dcter- Volcanoes and lava flows, unlike many other geologic phenom- the degree of violence of an eruption but also influence the ena, can be observed dimly, and samples can be collected and height of a volcano. without great difficulty (at least for the quiet, Hawaiian-type he two most important factors that influence viscosity of eruption). We can measure the temperature of lava flows, the temperature of the lava relative to the cooler tem- collect samples of gases being given off, obse~ethe lava solidi- re at which it solidifies and (2) the silica (SiO,) content fying into rock, and take newly-formed rock samples into the e lava. (A third factor is gas dissolved in magma-the laboratory for analysis and study. By comparing rocks observed Volcanirm and Extrusive Rocks Pyrdastlc flow descending Mayon volcano, Philippines, in 1984. Photo by ChrisNewhall, U.S.Geowical Survey Flgwl. 10.7 The ins of St. Pierre in 1902. Mount Pelbe is in the clouds. "lidifying from lava with Ones from other areas of the photo by Underwood& Underwwd,oourtesy Library of Congress world (and even with samples from the moon) where volcan- ism is no longer active, we can determine the nature of vol- canic activity that took place in the geologic past.
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