EAR 105 Chapter 9

List and describe volcanic landforms other than volcanic cones.

Calderas are among the largest volcanic structures. They form when the stiff, cold rock above a magma chamber cannot be supported and collapses to create a broad, bowl- like depression. Fissure eruptions produce massive floods of low- viscosity, silica- poor lava from large cracks in the crust.

Describe the formation, size, and composition of cinder cones.

Cinder cones are steep- sided structures composed mainly of pyroclastic debris, typically having a basaltic composition. Lava flows sometimes emerge from the base of a cinder cone but typically do not flow out of the crater. ¦ Cinder cones are small relative to the other major kinds of volcanoes, reflecting the fact that they form quickly, in single eruptive events. Because they are unconsolidated, cinder cones easily succumb to weathering and erosion.

Explain the formation, distribution, and characteristics of composite volcanoes.

Composite volcanoes are called " composite" because they consist of both pyroclastic material and lava flows. They typically erupt silica- rich lavas that cool to produce andesite or rhyolite. They are much larger than cinder cones and form from multiple eruptions over a million years or longer.

Explain why some volcanic eruptions are explosive and others are quiescent.

One important characteristic that differentiates various lavas is their viscosity ( resistance to flow). In general, the higher silica content of a lava, the more viscous it is. The lower the silica content, the runnier the lava. Another fac-tor that influences viscosity is temperature. Hot lavas are more fluid, while cool lavas are more viscous. High- silica, low- temperature lavas are the most viscous and allow the greatest amount of pressure to build up before they " let go" in an eruption. In contrast, lavas that are hot and low in silica are the most fluid. Because basaltic lavas are less viscous, they produce relatively gentle eruptions, while vol-canoes that erupt felsic lavas ( rhyolite and andesite) tend to be more explosive.

Summarize the characteristics of shield volcanoes and provide one example.

Shield volcanoes consist of many successive layers of low- viscosity basaltic lava and lack significant amounts of pyroclastic debris. Lava tubes help transport lava far from the main vent, resulting in very gentle, shield- like profiles.

Summarize the major processes that generate magma from solid rock.

Solid rock may melt under three geologic circumstances: when heat is added to the rock, raising its temperature; when already hot rock experiences lower pressures ( de-compression, as occurs at mid- ocean ridges); and when water is added to hot rock that is near its melting point ( as occurs at subduction zones).

Compare and contrast these intrusive igneous structures: dikes, sills, batholiths, stocks, and laccoliths.

Tabular intrusions may be concordant ( sills) or discordant ( dikes). Massive plutons may be small ( stocks) or very large ( batholiths). Blister- like intrusions also exist ( lac-coliths). As solid igneous rock cools, its volume decreases. Contraction can produce columnar jointing, a distinctive pattern consisting mainly of six- sided fractures.

Discuss the major geologic hazards associated with volcanoes.

The greatest volcanic hazard to human life is pyroclastic flow, or nuée ardente. This dense mix of hot gas and pyroclastic fragments races down-hill at great speed and incinerates everything in its path. Lahars are volcanic mudflows. These rapidly moving slurries of ash and debris suspended in water can occur even when a volcano isn't actively erupting. Volcanic ash in the atmosphere can be a risk to air travel when it is sucked into airplane engines.

Compare and contrast the 1980 eruption of Mount St. Helens with the eruption of Kilauea, which began in 1983 and continues today.

Volcanic eruptions cover a broad spectrum from explosive eruptions, like Mount St. Helens in 1980, to the quiescent eruptions of Kilauea.

Relate the distribution of volcanic activity to plate tectonics.

Volcanoes occur at both convergent and divergent plate boundaries, as well as in intraplate settings.