There are two major factors that control

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There are two major factors that control decompression of magmatic gases. The first is the amount of gas present in the magma. If a magma has a lot of gas, it would be more likely to have an explosive eruption. The second controlling factor is the how fast magma is able to rise and how easily bubbles are able to rise within the magma. The formation of bubbles is controlled by the thickness, or viscosity, of the magma (see Lab #2). Highly viscous magma is very thick and difficult for bubbles to move through, and gases become trapped. Low viscosity magmas are runny and bubbles have an easier time migrating through the liquid- rock. Viscosity in magma is controlled by the composition and the temperature of the magma. Felsic magma is much more viscous than mafic magma due to high silica contents and cooler temperatures. Therefore felsic eruptions are more violent than mafic eruptions (e.g. Kilauea vs. Mount St. Helens). Volcanic Hazards Volcanoes pose many hazards such as lahars (volcanic mudflows; Fig. 5) and volcanic ash (tephra) fall (bits of pulverized rock and glass that are expelled during a volcanic eruption; Fig. 4). Lahars are a mixture of water and loose ash that flow down stream valleys very quickly. Because streams are an important source of water, agriculture, and transportation in most regions of the world, the banks of most streams are densely populated. For this reason lahars are responsible for the most volcano-related fatalities worldwide. Lahars are a common geologic occurrence, and form when loose volcanic material on the flanks of the volcano is washed downstream by a sudden pulse of water. During an explosive volcanic eruption, hot ash causes snow to melt and generate large quantities of water. Once in a river valley, the water is incorporated in the lahar and it is able to pick up more sediment and rock, often until it has the consistency of wet cement. This flood of thick, muddy water (which can travel at speeds exceeding 30 mph) contains much more mass than a normal flood, so it can easily wipe out structures near rivers, like bridges or dams. Lahars can flow great distances, sometimes up to 40 miles, but usually travel less than 25 miles. Figure 4 : The most common volcanic hazards (by USGS). Figure 5. Mount St. Helens lahar after a small eruption.
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Lab #7: Geologic Hazards of Whatcom County 101 In-Class Activities Activity 1: Angle of Repose 1. Predict the stability of the materials in your experiment by determining which sediment is most stable and which sediment is least stable. Stability Reason Sediment #1 __________________ ________________________________________________ Sediment #2 __________________ ________________________________________________ 2. Measure the steepest angle you can create with each sediment. a. On your sheet of wax paper, hold the protractor with the bottom edge flat along the bottom of the wax paper so you can read the centimeter scale along the top of the protractor.
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