Chapter 4 Flashcards
What are the three main tectonic settings for volcanism on Earth?
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).
What is the primary mechanism for partial melting at a convergent plate boundary?
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).
Why are the viscosity and gas content of a magma important in determining the type of volcanic rocks that will be formed when that magma is extruded?
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.
Why do the gases in magma not form gas bubbles when the magma is deep within the crust?
When magma is deep within the crust the pressure is too high for the gases to bubble out of solution.
Where do pillow lavas form? Why do they form and from what type of magma?
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.
What two kinds of rock textures are typically found in a composite volcano?
Composite volcanoes can produce rocks with a wide range of textures, including (1) aphanitic or porphyritic rock from lava flows, (2) pyroclastic rock (with textures ranging from fine ash to coarse fragments) from explosive eruptions, and (3) sedimentary rock from lahars.
What is a lahar, and why are lahars commonly associated with eruptions of composite volcanoes?
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.
Under what other circumstances might a lahar form?
Some lahars form during an eruption when snow and ice melt quickly, while others may form from heavy rain.
Explain why shield volcanoes have such gentle slopes.
The magma at shield volcanoes is typically non-viscous. It can flow easily and also tends to form lava tubes, and thus is able to extend a long way from the vent, forming a low broad shield.
In very general terms, what is the lifespan difference between a composite volcano and a shield volcano?
Shield volcanoes tend to have much longer lives than composite volcanoes. Most of the Hawaiian shields, for example lasted 1 million years, while most composite volcanoes are younger than 100,000 years.
Why is weak seismic activity (small earthquakes) typically associated with the early stages of a volcanic eruption?
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 harmonic tremor.
How can GPS technology be used to help monitor a volcano in the lead-up to an eruption?
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.
What type of eruption at Mount St. Helens might have produced columnar basalts?
The Mt. St. Helens columnar basalts were formed by a flow of mafic lava.
What is the likely geological origin of the Nazko Cone?
The Nazko Cone is thought to be related to a mantle plume.
What might be the explanation for southwestern B.C. having much less subduction-related volcanism than adjacent Washington and Oregon?
No one is certain why there a lower rate of volcanism in B.C. than in adjacent Washington and Oregon, but one theory is that the northern part of the Juan de Fuca Plate (the Explorer Plate) is not subducting as quickly as the rest of the plate.
