Check Your Understanding
1. A lahar is a mixture of fine and coarse volcanic material and water that flows down the slope of a volcano. A pyroclastic flow is a dense mixture of hot gases, ash, and rock fragments. A caldera is a hole or depression where a volcanic vent has collapsed.
2. Tephra is the general term for particles of volcanic rock and lava erupted into the air. Tephra is divided into various sizes: volcanic blocks and bombs (the largest particles), lapilli (medium-sized particles), and ash (the finest particles).
3. The height of the ash plume, and the speed of the wind. See answer to 3 c) in the Investigate section.
4. The higher the silica content of a magma, the more viscous the magma is. In more viscous magma, gas that comes out of solution, in the form of bubbles, cannot escape as easily from the magma, thus allowing gas pressure to build up, which leads to explosive eruptions.
5. a) The VEI is a scale or measure of the potential for explosive eruption. It is based on the volume of erupted material and the height the plume of erupted material reaches. The students are not given the algorithm used to convert these effects into a value of the index.
b) It is not a perfect indicator, because other factors, not built into the index, like the wind field near the eruption, the possible occurrence of pyroclastic flows, or the possible later occurrence of lahars, are either not built into the index or are included only indirectly. |