Why does subduction produce explosive volcanoes?

Water driven from the descending slab lowers the melting point of the overlying mantle. The magma produced is andesitic, silica-rich and viscous, so gas builds pressure until it erupts violently.

What is a Benioff zone?

A Benioff zone is the inclined plane of earthquake foci that marks a descending slab. Foci deepen away from the trench and can reach around 700 km.

Why do the deepest earthquakes occur only at subduction zones?

Only a subducting slab stays cold and brittle to great depth. Elsewhere the mantle is too warm and ductile to fracture below about 70 km.

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Tectonic Hazards

Convergent margins, subduction and collision

3 things to know
Component 3 Section A

Overview

Three types of convergent plate margin, plates and the features of the Andean margin and benioff zone.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Name the three types of convergent plate margin.

Key term

What happens at an oceanic-continental margin?

Key term

Name the plates and the features of the Andean margin.

Questions from this subtopic

Each one with the model answer underneath.

1
Name the three types of convergent plate margin.
Model answer
Oceanic meeting continental, oceanic meeting oceanic, and continental meeting continental are the three types. Only the first two involve subduction.
2
What happens at an oceanic-continental margin?
Model answer
The denser oceanic plate subducts beneath the continental plate. A deep ocean trench, a zone of earthquakes and a chain of composite volcanoes result.
3
Name the plates and the features of the Andean margin.
Model answer
The Nazca Plate subducts beneath the South American Plate at around 7 cm yr⁻¹. The Peru-Chile Trench and the Andean volcanic arc mark the margin.
4
Why does subduction produce explosive volcanoes?
Model answer
Water driven from the descending slab lowers the melting point of the overlying mantle. The magma produced is andesitic, silica-rich and viscous, so gas builds pressure until it erupts violently.
5
What is a Benioff zone?
Model answer
A Benioff zone is the inclined plane of earthquake foci that marks a descending slab. Foci deepen away from the trench and can reach around 700 km.
6
Why do the deepest earthquakes occur only at subduction zones?
Model answer
Only a subducting slab stays cold and brittle to great depth. Elsewhere the mantle is too warm and ductile to fracture below about 70 km.

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