ArcVolcano Flashcards
Where do arc volacnoes form?
Above subduction zones
What is an example of a continental arc?
Cascades
How is magma created at a subduction zone?
Water is added to hot mantle which reduces the temp of the rock at high pressures
Because water is involved, this magma has a high _____ content
volatile
Where does melting take place?
in the mantle above the subducting plate
What releases H2O?
Metamorphism at ~100 km depth
When does the composition of the magma change?
When it interacts with the overlying crust
Why doesn’t magma from island arcs change much chemically?
It rises through low silica ocean crust on the way to the surface
What does island arc lava start as?
Basalt
When does fractional crystallization occur?
When magma has a harder time going through new, thick crust
What happens to magma during fractional crystallization?
It becomes more silicic
What type of lava occurs from fractional crystallization?
Andesite lava
The crystals tend to be ____ in composition
mafic
What is the main difference between continental and island arcs?
Magma must work through crust of silicic composition to get to surface
How does going through silicic composed crust change magma?
Through assimilation and secondary melting
What is the crust of high silicia called that continental arc magma must go through?
granitic
What melts at a lower temperature - granite or basalt?
Granite
Assimilation
Magma melts through existing rock on its way up
Where can secondary melting occur?
An overheated piece of lower continental crust
Continental arc lavas have silica percentages from ____ to ___
50% (basalt) to 75% (Rhyolite)
What rises with magma during hydration melting at subduction zones
Water
Why are arc lavas more explosive than rift lavas?
It has water and other gases
What types of hazards are more common with arc volcanoes?
Lahars and pyroclastic flows
What is a Phreatomagmatic eruption?
A steam and ash eruption
When can a Phreatomagmatic eruption occur?
When any volcano comes in contact with ground water
What do Phreatomagmatic eruptions make?
Maars - low-relief volcanic craters with a lake in the middle
