Volcanism and Extrusive Rocks
1. The May 18, 1980 eruption of Mount St. Helens was a spectacular reminder of the energy in the earth's interior. If plate tectonic theory is correct, parts of North America are overriding a portion of the Pacific ocean floor. Some of the mechanical energy is transformed to heat energy along the subduction zone.
2. At depth, previously solid rock melts at high temperatures. 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.
3. Lava may erupt quietly or, as in the case of Mount St. Helens, violently. Volcanic activity, or volcanism, is not restricted to the eruption of lava. Rock fragments may erupt instead of lava (as happened during the early 1980 eruptions of Mount St. Helens.
4. Gases also play an important role in volcanism. Gas explosions at Mount St. Helens were particularly violent; in other cases, the escaping gases merely force magma out of the ground and produce lava flows.
5. Material blown out of a volcano is called pyroclastic debris, or tephra, both terms meaning rock fragments produced by volcanic explosions. Tephra, such as the volcanic ash that littered much of the Pacific Northwest in 1980, and rock formed by solidification of lava are collectively regarded as extrusive rock, surface rock resulting from volcanic activity.
6. The most obvious landform created by volcanism is a volcano, a hill or mountain formed by the extrusion of lava or ejection of rock fragments from a vent. However, volcanoes are not the only landforms created by volcanism. Fluid lavas may flow out of the earth and flood an area, solidifying into a nearly horizontal layer of extrusive rock. Successive layers of lava flows may accumulate into a lava plateau.
7. Volcanic activity is important to geology for several reasons. Landforms are created and portions of the earth's surface build up. Less commonly, landforms are destroyed by violent eruptions. Volcanoes are important to the science of geology because they provide clues about the nature of the earth's inaccessible interior and help ups understand how the earth's internal processes work. By studying the magma, gases, and rocks form eruptions, we can infer the chemical conditions as well as the temperatures and pressures within the earth's crust or underlying mantle.
8. Volcanism is also significant in human affairs. Its effects can be catastrophic or, surprisingly, beneficial.
9. One region where the overall effects of volcanism have been favorable to the human inhabitants is Hawaii. Occasionally a field or village is overrun by outpourings of lava. During the 1980s, Kilauea volcano was especially active. Over 800 million cubic meters of lava erupted - enough to cover a four lane highway extending from the east coast to the west coast of North America with a 10 meter thick layer of lava. Nearly forty houses were destroyed during this period, but no one was killed or injured. Nevertheless, the weathered volcanic ash and lava produce excellent fertile soil. Moreover, Hawaii's periodically erupting volcanoes (which are relatively safe to watch) are great spectacles that attract both tourists and scientists, benefiting the islands; economy.
10. Were it not for volcanic activity, Hawaii would not exist. The islands are the crests of a series of volcanoes that have been built up from the bottom of the Pacific Ocean over millions of years (the vertical distance form the summit of Mauna Loa volcano to the ocean floor greatly exceeds the height of Mount Everest). When lava flows into the sea and solidifies, more land is added to the islands. Hawaii is, quite literally, growing.
11. In some other areas of geologically recent volcanic activity, underground heat generated by volcanism is harnessed for human needs. In Italy, Mexico, New Zealand, Argentina, and California, geothermal installations produce electric power. Steam or superheated water trapped in layers of hot volcanic rock is tapped by drilling and then piped out of the ground to power turbines that generate electricity. Naturally heated geothermal fluids an also be tapped for space or domestic water heating or industrial use, as in paper manufacturing.
12. The island of Krakatoa in the western Pacific, composed of three apparently inactive volcanoes, erupted in 1883 with the force of several hydrogen bombs. This Indonesian islands, which formerly rose 800 meters above sea level, was blown apart. Only one-third of the island remained after the eruption. An estimated 13 cubic kilometers of rock collapsed into the subsurface magma chamber that had been emptied by the eruption, leaving an underwater depression 300 meters deep where the major part of the island had been. On nearby Java, tens of thousands of people died as a result of the giant sea waves (tsunamis) generated by the explosion.
13. What determines the degree of violence associated with volcanic activity? Why can we state confidently that active volcanism in Hawaii poses only slight danger to humans when violent explosions such as at Mount St. Helens, occur in the Cascade Mountains? Whether eruptions are very explosive or relatively "quiet" is largely determined by two factors:(1) the amount of gas in the lava or magma and (2) the ease or difficulty with which the gas escapes to the atmosphere.
14. The viscosity, or resistance to flow, of a lava determines how easily the gas escapes. The more viscous the lava and the greater the volume of gas trying to escape, the more violent the eruption will be.
15. Volcanoes and lava flows, unlike many other geologic phenomena, can be observed with great difficulty (at least for the quiet Hawaiian type of eruption). We can measure the temperature of lava flows, collect samples of gases being given off, observe the lava solidifying into rocks, and take rock samples into the laboratory for analysis and study. By comparing rocks observed solidifying from lava with similar ones form other areas of the world( and even with samples from the moon) where volcanism is no longer active, we can determine the nature of volcanic activity that took place in the geologic past.
16. From active volcanoes we have learned that most of the gas released during eruptions is water vapor, which condenses as steam. Other gases such as sulfur dioxide, hydrogen sulfide (which smells like rotten eggs) carbon dioxide and hydrochloric acid, are given off in lesser amounts with the steam.
17. Chemical analysis show that (SiO2) is the most abundant component of virtually all volcanic rocks. The amount of silica, however, can vary from about 45% to about 75% of the total weight of volcanic rocks. The variations between these extremes account for striking differences in the appearance and mineral content of the rocks as well as in the behavior of the parent lava.
18. Rocks with a silica content close to 50% (by weight) are considered silica-poor, even though silica is by far, the most abundant constituent. chemical analyses show that the remainder is composed mostly of oxides of aluminum(Al2O3) calcium (CaO), magnesium (MgO), and iron (FeO and Fe2O3). Rocks in this group are called mafic- silica-poor igneous rocks with a relatively high content of magnesium, iron, and calcium. (The term mafic comes from magnesium and ferric.) The most common mafic extrusive rock is basalt, which is black in color.
19. At the other extreme, the silica-rich(65% or more of silica) rocks tend to have only very small amount of the oxides of calcium, magnesium, and iron. The remaining 25% to 35% of these rocks is mostly aluminum oxides(Al2O3) and oxides of sodium (Na2O) and potassium(K2O). These are called felsic rocks -silica-rich igneous rocks with a relatively high content of potassium and sodium( the name comes form the generally high amount of feldspar, which crystallizes from the potassium, sodium, aluminum and silicon oxides). Rhyolite, the most abundant volcanic rock with a felsic composition, is light in color because of the low iron and magnesium content.
20. Rocks with a chemical content between that of felsic and mafic are classified as intermediate rocks. Andesite, usually medium to dark gray in color, is the most common intermediate volcanic rock.
Types of Volcanoes
1. Volcanic material that is ejected from and deposited around a central vent produces the conical shape typical of volcanoes. The vent is the opening through which an eruption takes place. The crater of the volcano is a basinlike depression over a vent at the summit of the cone. Material is not always ejected from the central vent. In a flank eruption, lava pours form a vent on the side of a volcano.
2. A caldera is a volcanic depression much larger than the original crater. The most famous caldera in the United Sates is misnamed "Crater Lake." A caldera can be caused when a volcano's summit is blown off by exploding gases, as occurred at Mount St. Helens in May 1980, or as in the case of Crater Lake, when the crater floor collapses into a vacated magma chamber beneath the volcano.
3.Shield volcanoes are broad, gently sloping cones constructed of solidified lava flows. During eruptions, the lava spreads widely and thinly due to its low viscosity. Because the lava flows form a central vent, without building up much near the vent, the slopes are usually between 2 and 10 degrees from the horizontal , producing a volcano in the shape of a flattened dome or "shield".
4. Thee are two types of basalt flows, each of which solidifies with a characteristic surface. Both types have Hawaiian names. Pahoehoe (pronounced pah-hoy-hoy) is a lava flow characterized by a ropy or billowy surface. The surface is formed by the quick cooling and solidification from the surface downward of a lava flow or pool of lava that was fully liquid. By contrast, flowing basalt that is cool enough to have partially solidified moves as a slow, pasty mass. Its largely solidified front is shoved forward a s pile of rubble. A flow such as this is called aa(pronouned ah-ah) and solidifies with a spiny, rubbly surface.
5. The surface of a solidifying lava flow sometimes develops a minor feature called a spatter cone, a small, steep-sided cone built form lava sputtering out of a vent. When a small local pocket of gas is trapped in a cooling lava flow, the as seeks to escape and belches the lava up out of a vent through the already hard surface of the flow. Falling lava plasters itself onto the developing cone and solidifies. The sides of a spatter cone can be very steep, but the height rarely exceeds 10 meters.
6. A cinder cone is a volcano constructed of loose rock fragments ejected from a central vent. In contrast to the gentle slopes of shield volcanoes, cinder cones commonly have slopes of about 30 degrees. Most of the ejected material lands near the vent during an eruption, building up the cone to a peak. The steepness of slopes of accumulating loose material is limited by gravity to about 33 degrees. Cinder cones tend to be very much smaller than shield volcanoes. Few of them exceed a height of 500 meters.
7. The fragments of volcanic rock that make up the cinder cones are pyroclastic. Pyroclasts, or tephra, the fragments formed by volcanic explosion, can be almost any size. Dust and ask are the finest particles; cinders range from about 4 to 32 millimeters; bombs and blocks are large pyroclasts. When solid rock has been blasted apart by a volcanic explosion, the pyroclastic fragments are angular with no rounded edges or corners. If lava is ejected into the air, a molten blob becomes streamlined during flight, solidifies, and falls to the ground as a bomb, a spindle- or lens-shaped pyroclast.
8. A composite volcano (also called stratovolcano) is one constructed of alternating layers of pyroclastics and rock solidified form lava flows. The slopes are intermediate in steepness compared with cinder cones and shield volcanoes. Pyroclastic layers build up steep slopes as debris collects near the vent, just as in cinder cones. However, subsequent lava flows partially flatten the profile ofthe cone as the downward flow builds up the height of the flanks more than the summit area. The solidified lava acts as a protective cover over the loose pyroclastic layers, making composite volcanoes less vulnerable to erosion than cinder cones.