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Name: Aura
Status: 
Grade: 6-8
Location: CA
Country: USA
Date: Spring 2012


Question:
Why do so many of Earth's volcanoes occur along plate boundaries?



Replies:
Melissa,

Aura,

There are three general types of plate boundaries: 1) Transcurrent plate boundaries (like the San Andreas Fault system in your state) 2) Divergent plate boundaries (commonly referred to as mid-ocean spreading ridges) 3) Convergent plate boundaries (associated with subduction zones, for example, the Cascade Mountain Range in northern CA, OR, and WA).

Plate boundaries are defined by seismicity and volcanism. All plate boundaries exhibit seismic events, not all plate boundaries are associated with volcanism. Approximately 80% of the volcanoes are associated with convergent plate boundaries, 15% divergent plate boundaries, and none are associated with transcurrent plate boundaries; the remaining volcanoes (5%) are associated with mantle plumes (hot spots that are commonly within a plate and not at a plate boundary).

At convergent plate boundaries, in subduction zones, when oceanic lithosphere is subducted to depths of about 100 km, the water tied up in the mineral structures (part of the mineral chemistry) and the water between the minerals (in pores) is released from the lithosphere and melts the surrounding hot rock of the upper mantle. This molten rock buoyantly rises to the surface and is expressed as volcanoes. This process accounts for about 80% of all volcanoes on Earth.

Les

--- Leslie Kanat, Ph.D. Professor of Geology


Aura,

The weakest points along the Earth's crust is at the plate boundaries. Essentially this is where there are sufficient cracks in the crust to allow for the magma to pass through and develop into volcanoes.

Greg (Roberto Gregorius) Canisius College



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