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Chapter+7+Volcanos+-+The+Basics0.pdf

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Department
Geography
Course
Geography 2240A/B
Professor
Philip Egberts
Semester
Winter

Description
Chapter 7 Volcano Basics 1.0 Introduction The second process we study that results from Earth‟s internal energy is volcanic eruption. Dormant or extinct volcanoes look great (Fig.1; Mt. Fuji in Japan)! There‟s also a great deal of good that‟s associated with volcanoes: many volcanic rocks decompose very rapidly to form wonderfully rich agricultural soil; a large proportion of the world‟s richest copper, gold and silver deposits are associated with volcanoes; and the heat associated with the deep magma and hot rocks supplies a significant proportion of geothermal energy to consumers. Also, of course, the slopes of extinct volcanoes are used in many parts of the world for recreational enjoyment such as down-hill skiing. But, of course, what we‟re in this course to consider is not the good but the very bad! Volcanoes are testament to Earth‟s huge internal energy, and the release of that energy can be catastrophic. Imagine a volcanic eruption in which an area the size of Greater New York City collapses, a region the size of Nova Scotia is suddenly buried under meters of hot ash that snuffs out all life, and farmer‟s fields as far as 100 to 2000 km away (depending upon winds) are covered in so much ash that all crops are killed. Imagine that so much ash and gas is blown up into the atmosphere that Sun‟s light is dimmed to the point that there‟s no summer for two years. Unbelievable? It happened at least three times in the western USA when a huge volcano erupted at Yellowstone National Park. We‟ll consider those catastrophic events and others, but first we have to look at the source of energy and material, and consider a few „normal‟ eruptions before we look at the deadly ones. Just as most ( but not a) earthquakes are tied to current plate tectonics of Earth, so most (but not al) volcanic eruptions are clearly tied to plate tectonics – as we‟ll see later in this chapter. In Figure 2 you see the strong correlation of plate boundaries, earthquakes and volcanoes. Want to see an eruption? Well, while almost every day you can expect a new or continuing eruption around the rim of the Pacific Ocean (due to the strong subduction of the Pacific Plate and the melting associated with), for sure there‟s going to be eruptions at sea floor spreading centers (say, the Mid-Atlantic Ridge of Fig. 2) – but flying back and forth around the volcanic “Ring of Fire” of the Pacific rim can be expensive, and you can‟t see the eruptions under oceans. So you‟d better head for an island volcano called Stromboli (Fig.3) off the coast of Sicily; it erupts every 20 minutes to 1 hour every day! 2.0 Magma Characteristics Magma is defined as molten rock, and may be a mix of actual hot liquid, gases and scattered mineral crystals. When magma spills out on surface it produces hot lava (see lava running down the side of the cone in Figu) When magma blows out in a violent eruption, the stuff thrown into the air is usually called pyroclastic material (Fig.4; safe to say the driver of the jeep has „the pedal-to-the-meta) and may be in the form of blobs of hot magma ( sometimes called „bombs‟),very large solid blocks, or very fine pulverized material commonly referred to ( somewhat misleadingly) as „ash‟. Initially, pyroclastic material may be very, very hot because it is being blown out by very hot gases. All of the material erupted from a volcano forms volcanic rock of various types ( we‟ll look at some of them bel) when cooled and solidified. Magma can also solidify (i.e. cool and crystal) beneath surface ( in which case it can‟t be called „volcanic), where it produces plutonic rocks. All types of rock produced from magma – whether volcanic or plutonic - are classified as igneous rock. 2.1 Viscosity and Volatiles Viscosity is defined as that property of materials that provides resistance to flow; the opposite of viscous is, therefore, fluid. Viscosity/fluidity involves interaction of many properties; this can be difficult to explain. As far as magma is concerned, one factor over- rides all others: temperature; the higher the temperature of any material, the more fluid (i.e. less viscous) it is. Reason: the higher the temperature, the greater the rate of atom vibration – thus bonds between atoms (which would form structures in the liquid inhibiting fluidity) tend not oo form very readily. In fact, based on nothing but temperature, a liquid at 900 C would be about 5 orders of magnitude (i.e. 100,000 times) more fluid than the same liquid at 600 C.o Of course, there are other factors that influence viscosity, and next most important is chemical composition, in particular the content of two elements: silicon (Si) and oxygen (O). Reason: Si and O atoms have a very strong affinity for each other and tend to form little tetrahedral structures where each silicon atom ( small central atom of Fi) is surrounded by 4 oxygen atoms ( large surrounding atoms of Fig.); the more of these little structures in a liquid, the more viscous the liquid. But it‟s actually worse than that: the more of these SiO (4 1 Si and 4 Os called silica tetrahe) in a liquid, the more they will tend to bond together to form long strings, sheets and frameworks of Si and O atoms by sharing corner atoms of those tetrahedral (Fig.6). This can really quickly affect viscosity. Of course, the tendency to form these silica tetrahedra is offset by temperature: few at high temperature, rapidly increasing number at lower temperatures. [For all you Science people: please understand that I’m using the word ‘atom’ for simplicity; if I wanted to use proper terminology, I’d have to consider ‘ions’.] If increasing content of silica structures increases viscosity, magma with increasing content of crystals would also become increasingly viscous. Certainly as magma cools, more and more crystals form – eventually turning the material into a solid rock. Take a look at Figure 7; start with a magma we‟ll call basalt (defined later) that‟s really hot and contains no crystals, and now slowly cool it. The first crystals that form will be those that can form stable bonds between atoms at high temperatures (olivine and pyroxene); at ever lower temperatures other minerals become stable, until finally, at the lowest temperature of the magma (just before the whole thing becomes solid) quartz (SiO ) 2 forms. There‟s one final complication to the matter of viscosity, and that‟s the volatile content of the magma – particularly water since it‟s by far the most common volatile compound. The water molecule (1 atom of oxygen and 2 of hydrogen) is unique. The water molecule is not symmetrical; the two hydrogen atoms are tacked on to the oxygen atom in such a way that one end of the molecule (the hydrogen end) is relatively positively charged while the oxygen end is slightly negatively charged. In other words, we have a dipolar molecule (Fig.8). Why is that significant? At magma temperatures of some moderate value, where silica tetrahedral would normally start forming and making structures (where the oxygen of one tetrahedron shares as the oxygen of another tetrahedron and on and on), the slightly positive ends of water molecules attach lightly to the oxygen instead – thus breaking up the chain effect. The result is to keep the magma more fluid than it would be if it contained no water. When water is attached to other molecular structures in this way, it is called dissolved water. But there has to be a limit. You can readily understand that if the magma happens to contain more water than there are opportunities for attachment to silica tetrahedral structures, that extra water will simply be left as contained bubbles of water – that is, exsolved water. It so happens that as the temperature of the magma lowers, water is less able to attach to structures, and the amount of exsolved water increases (we’ll see how that leads to explosions ).ter 2.2 Types of Magma We can get a good idea of how the properties of magma pretty much control the type of volcanic activity we see by restricting our discussion to those magmas that lead to three types of volcanic rock: basalt, andesite and rhyolite. Geologists go to great lengths to determine the source of a melt or magma that produced particular sequences of rock. They have determined that as far as volcanic rocks are concerned, nearly all magma has begun with a partial melt of Earth‟s asthenosphere; sometimes that partial melt is spewed out of a volcano with virtually no modifications to the initial composition; that‟s going to be called basalt. Most basalt flows from a volcano as very hot and very fluid lava. There are two basic textural types of basalt lava: aa (which is a rubbly-looking rock) and pahoehoe (which is a smooth-surfaced, ropey-looking rock (Fig 9). Both are well-exposed in Hawaii, thus the Hawaiian names. The most obvious (and only significant) chemical difference in them is that aa contains much less water than does pahoehoe. Andesite is volcanic rock produced from a modified partial melt. In other words, the primary melt that would ordinarily yield basalt has been changed in composition somehow, the result being a magma that‟s more Si-rich and more volatile-rich than the original. Rhyolite is a volcanic rock even more modified (still higher Si and even higher volatile content) from the original partial melt that yields basalt. Table 1 shows the essential differences in resultant rock appearances and mineral content, in both chemical and physical properties of the magma, and finally in the style of eruption expected from volvanoes fed by the different magmas. I strongly recommend you review this table carefully and thoroughly; not only will you be better able to understand the following discussions, but the content is also appropriate for exam questions! 3.0 Volcano Settings Over 90% of volcanism is associated with edges of tectonic plates (look back at Fig. 2). Of the remaining 10%, most is erupted from hot spots marking the tops of mantle plumes. Figure 10 is a hypothetical cross section through Earth‟s lithosphere and asthenosphere illustrating volcanic activity at a spreading center (right side) and at a subduction zone (left side). We‟ll look at those settings very briefly; we‟re going to delay our discussion of volcanic eruptions associated with mantle plumes until the next chapter. 3.1 Spreading Centers Most active plate spreading takes place underwater. From the point of view of volcanic eruptions, the mid-ocean ridges constitute a comparatively simple environment (Fig.10). As we learned in Chapter 3 (Plate Tectonics), at oceanic spreading centers the asthenosphere - at temperatures usually between 1200 C and 1300 C - is as close as it ever gets to the surface, and the cool lithosphere can be as thin as 3 km. As the plates diverge, the hot asthenosphere rises toward the crest of the mid-ocean ridges. So how does a melt form from the hot mantle/asthenosphere rock? The answer comes from Figure 11. Figure 11 ( Pressure decreases upward on the vertical axiwas constructed to discuss a different tectonic environment, but the principle is the same at a spreading ridge in the middle of an ocean. The asthenosphere is so hot it‟s just at the point of melting; it is commonly described as „hot plastic‟ rather than either solid or liquid. Let‟s say that we are considering asthenosphere at the position A of Figure 11. The point A indicated rock that is stable just inside the „solid‟ field. If we increased the temperature of the rock at point A (i.e. moved it horizontally to the right), obviously it would begin to melt because it‟s now in the „melt‟ field of stability. On the other hand, if we decreased the pressure on point A rock (i.e. moved A vertically upward), it would also begin to melt as it crossed over from solid to melt stability fields. As you can see from Figure 10, at an ocean spreading center the asthenosphere is very close to the ocean floor, thus here we are looking at the situation of reducing the pressure on the asthenosphere rock rather than increasing the temperature on it. OK, we have a valid mechanism to make a melt from asthenosphere rock. So how much melts? All of it? Nope! Not even close! At most, 30-40% of the asthenosphere rock will melt, and that melt will be basalt in composition. Figure 12 is a simple-minded illustration of the melting of selective minerals, while the ones with higher melting temperatures resist breakdown. The magma we produce at spreading centers under the oceans is called MORB for Mid- Ocean Ridge Basalt. Whatever the nomenclature, note that the rock produced is basalt. At ocean spreading centers, the asthenosphere rock that‟s labeled „solid‟ in Figure 11 ( the rock that’s going to partially me) is called peridotite; it has a density of about 3.3 g/ci.e. grams per cubic centimete). The partial melt (that will go to form basalt) that develops from the peridotite has a density of about 2.9 g/cc; obviously, the melt is relatively buoyant. Assuming any fractures or vents exist (which they certainly will at spreading centers) up the partial melt magma goes! And 80% of all volcanic rock is formed just like that. As you can readily see, there‟s nothing catastrophic about eruptions in this environment. 3.2 Subduction Zones Let‟s change our focus to the left end of Figure 10 – the subduction zone. Here, the production of volcanic magma and its eruption is not at all simple, and the resulting activity can most definitely be catastrophic. A quick look back at Figure 2 is in order here – note the many, many volcanoes located at the subduction ends of plates ( labeled with a saw-tooth pattern and called „convergent zones‟ in the). For example, look at the grouping around the Pacific Ocean: up the coast of South America, all along the west coast of North America, across the Aleutian Islands to Asia, then all the way back down into the South Pacific again. There are so many active volcanoes in this belt it is commonly called “The Ring of Fire”. Just so you don‟t have to keep flipping back all the time, I‟ve reproduced the left end of Figure 10 and called it Figure 13. The initial magma begins the same way as for spreading centers – as a partial melt of asthenosphere. But the reason for generating that melt is not because of either rising temperature or decreasing pressure – it‟s because we‟re adding water to system. In Figure 11 there‟s a broad solid line between “solid” and “melt”; if we added water to that system, it would have the immediate effect of moving that solid line toward lower temperatures. In other words, if we add water to a relatively dry rock (at high temperatures, of course), the rock will begin to melt at much lower temperature than in a dry system. Where does the water come from? The slab of material being subducted (in Fig.13) is ocean floor lithosphere – and the top layer is completely saturated with water. As that slab drops further and further into the hot asthenosphere (and you’ll recall that it drops because cold ocean lithosphere is more dense than hot asthenosphere), the increased heat it‟s subjected to drives water off – and straight up into that wedge of hot asthenosphere between the top of the ocean lithosphere slab and the base of the continental crust (Fig.13). And once again, just as at oceanic spreading centers, a partial melt of basalt- equivalent is produced, and up it goes. At the base of the continental crust the basaltic melt comes in contact with some rocks of dramatically different composition. The „roots‟ of continental crust blocks are composed of very old and very evolved rocks that are granitic in content. That means they contain an abundance of those minerals we saw at the low temperature end of the mineral sequence shown in Figure 7 ( please go back and lo): potassium-rich feldspars, some micas and quartz. The result when a high-temperature basaltic magma encounters a low temperature granitic rock is melting of the granitic rock and, thus, modification of the magma. Depending upon how much continental crust it has to move through to reach surface, the magma will be increasingly modified. It‟s important to learn what those modifications are because they change a volcano from a peaceful eruption to a dramatically explosive eruption. The schematic of Figure 13 shows blobs of partial melt magma gradually moving from the point of origin (just above the subducting slab) upward toward surface. These blobs are meant to represent „magma chambers‟ where the magma makes room for itself in the continental crust rocks as it moves in „spurts and stops‟ toward surface. Figure 14 ( ignore the terminology right now, just look at the proc) shows some of the detail right at the walls of a hypothetical magma chamber. The hot magma works it way into fractures in the surrounding solid rock, loosens blocks of that rock which gradually fall into the magma, and melts some/all of those blocks (depending on their composition). This is a process called assimilation. By this process, the originally basaltic magma changes composition to some intermediate composition between the original and the composition of the rock it melts. In fact, there are a few really important processes like this simultaneously changing the magma composition. Another is illustrated in Figure 15; as magma gradually moves toward surface, it cools. At cooler temperatures crystals begin to form in the melt; if they are crystals of minerals that are relatively heavier (i.e. higher specific gravit) than the melt, they‟ll fall through the magma chambers toward the base, where they may form a rock consisting almost entirely of these minerals (plus whatever magma is trapped interstitially). This is a process called fractional crystallization (i.e. a fraction of the melt crystallizes and thus removes itself from the bulk of the magma that sooner or later will move further up toward surface). OK, let‟s look at one more process that modifies magma composition. Figure 16 shows one bleb of supposedly basaltic magma joining and mixing with a larger bleb of magma that‟s been sitting in a magma chamber for some time, thus already quite modified in composition. This is a process called magma mixing. The result is simply to produce magma of some composition intermediate to that of the two „pots‟ that mix. The result of these modification processes has been to produce magma dramatically different in all properties to those of the original partial melt. Certainly, the magma that finally makes it to surface will be cooler, more silica-rich, more volatile-rich and thus considerably more viscous and explosive than a basaltic magma. The rocks produced from such magma are commonly andesite or rhyolite; take another look at the basic properties of those rock types in Table 1. 4.0 Volcanic Eruption Hazards We‟re not going to waste time discussion potential hazards around spreading center volcanoes; they‟re non-existent or insignificant. We‟ll move straight to the dangerous category: subduction zone volcanoes, where we‟ll look at three major volcanic products: lava, pyroclastic products and gas. There also are some secondary products which we‟ll mention as we proceed; see Figure 17 for a very
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