Earthquakes and Volcanoes Week 5 Reading
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Geology
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Dec 6, 2023
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CHAPTER 6. Volcanic Eruptions: Plate Tectonics and Magmas
1.
Mount Vesuvius is one of the world’s most active volcanoes, yet it has quiet intervals lasting how
long? Compare the times between major eruptions to a human life span.
An eruption occurred long back and buried cities of Pompeii and Herculaneum. This happened
for more than 1500 years ago. It is expected that earthquakes may occur in the next 17 years but
not as the intensity and frequency of the earlier on. Thus, the eruptions may take place at a very
long time between two major eruptions.
2.
What percentage of magma erupted each year comes out at spreading centers, subduction
centers, and hot spots?
80% comes out at spreading centers. 7-13% comes out at subduction centers, and 0-55 comes
out at hot spots.
3.
What changes in temperature, pressure, and water content cause hot rock to partially melt?
What are the two most relevant melting agents?
An increase in temperature, a decrease in pressure, a higher water content. The two most
relevant melting agents are lava and pyroclastic.
4.
What common elements combine to form most igneous rocks?
Silica, Iron, and Sodium
5.
Contrast the differences between basaltic and rhyolitic magma in terms of SiO
2
percentage,
water content, temperature, viscosity, and mode of gas escape.
Basaltic has lower SiO2 % while Rhyolitic has higher SiO2 %. Basaltic has high water content
while Rhyolitic has low water content. Basaltic is thinner with lower viscosity while Rhyolitic is
thicker with high velocity.
6.
How does explosiveness vary between magmas that have a low versus high water content?
Low weight % content of water leads to peaceful explosions, high weight % content of water
leads to deadly explosions.
7.
Why do volcanoes above subduction zones erupt more explosively than volcanoes at spreading
centers?
Volcanoes is subduction zones have more viscous, stickier magmas, which provide more
resistance as the bubbles grow.
8.
What determines whether volcanic activity will be a lava flow or a pyroclastic eruption?
Lava Flows: Molten rock that moves over the ground
Pyroclastic Debris: Fragments blown out of a volcano
9.
Explain the factors controlling the volcanic explosively index (VEI).
The factors that control the volcanic explosivity index include the volume of the materials
ejected from the volcano, height of the eruption column, and the duration of the major eruptive
blast.
10.
Describe how each of the following magmas might erupt: (a) low viscosity, low volatility; (b) low
viscosity, high volatility; (c) high viscosity, low volatility; (d) high viscosity, high volatility.
A: Would be a peaceful eruption.
B: Would be an explosive eruption.
C: Would be a peaceful eruption.
D: Would be an explosive eruption.
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