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Answers to Chapter 11 Review Questions
The three main tectonic settings for volcanism are (1) subduction zones at convergent plate boundaries, (2) divergent plate boundaries, and (3) mantle plumes (a.k.a. hot spots).
The primary mechanism for partial melting at a convergent plate boundary is the addition of water to hot mantle rock. The water reduces the melting temperature of the rock (flux melting).
The explosiveness of a volcanic eruption depends on the pressure of the magma. Gases create that pressure, and if the magma is viscous those gases cannot escape easily. Felsic and intermediate magmas tend to have more gas than mafic magmas, and are also more viscous, trapping the gas in.
When magma is deep within the crust the pressure is too high for the gases to bubble out of solution.
Pillow lavas form where mafic lava erupts in water. When the magma oozes out into the water the outside cools first forming a hard skin that maintains the pillow shape.
Composite volcanoes are formed of layers of lava flows and tephra (volcanic fragments ranging from fine ash to blocks and bombs) from explosive eruptions.
A lahar is a mud flow or debris flow on a volcano. Lahars are common on composite volcanoes because they are steeper than shield volcanoes, they typically have ice and snow, and they are not as strong as shield volcanoes.
Some lahars form during an eruption when snow and ice melt quickly, while others may form from heavy rain.
The lava that forms shield volcanoes is typically low viscosity. It can flow easily and also tends to form lava tubes. As a result, it is able to travel a long way from the vent, forming a low broad shield.
Cinder cones erode rapidly because they are mostly piles of tephra. Composite volcanoes are more resistant to weathering and erosion than cinder cones because lava flows help hold together tephra, but composite volcanoes still don’t last as long as shield volcanoes. Shield volcanoes are stronger because they consist more of lava flows than tephra.
Weak seismic activity is associated with all stages of a volcanic eruption. In the early stages magma is moving at depth and pushing rock aside, creating small earthquakes. The flow of magma can also produce special type of seismic response known as a harmonic tremor.
GPS technology is used to determine if there is any slow deformation of the flanks of a volcano related to movement of magma toward the surface.
The Nazko Cone is thought to be related to a mantle plume.
One hypothesis to explain the lower rate of volcanism in British Columbia than in adjacent Washington and Oregon is that the northern part of the Juan de Fuca Plateis not subducting as quickly as the rest of the plate.