Plate Tectonics: Convergent Plate Boundaries


Subduction zones: where crustal rocks are consumed

For a variety of reasons, the majority of the earth's most violent earthquakes and volcanoes occur in these areas

Good example: Pacific "Ring of Fire"

Chains of active earthquakes/volcanoes around the Pacific Ocean

Relatively narrow, linear zones of activity

Zone of collision between two plates

Results in compression & crustal shortening

Reverse faulting - hanging wall up

Thrust fault - Low angle reverse fault


Three possible situations

1) Continental vs. oceanic plates

Oceanic plate is subducted due to density differences

Location of many shallow to deep focus quakes

Extend to depth beneath the overriding plate

Up to 300 miles deep!

A deep trench commonly occurs where the subducting plate plunges beneath the overriding plate

Collects sediments which are also subducted

Remelting generates magma which forces its way to the surface

Volcanic arc chains on-shore (EXAMPLE: Andes, Cascades)

2) Oceanic vs. oceanic collision

Same basic idea as above

Also results in subduction and trench formation

Volcanic chain usually off-shore

EXAMPLE: Japanese Islands, Aleutian Islands

3) Continent vs. continent collisions

A special case of convergence

Massive slabs of granitic crust collide

No subduction possible

Major mountain-building events

EXAMPLE: Himalaya Mountains; Ural Mountains


Subduction and the formation of igneous rock

Remember I said that the crust was essentially 2 types of igneous rock

Basalt and Granite

Basaltic crust is created at spreading centers

Discussed this under Divergent Plate Boundaries

Granite is intrusive: forms deep within the crust

Associated with Convergent boundaries

Heavier oceanic basalt is subducted beneath the lighter continental materials

Along with debris which falls into the trench

Subjected to increasing heat and pressure

Can cause it to undergo metamorphic changes

In extreme conditions, completely remelt to form new magma

Allows additional differentiation as the felsic materials are selectively re-melted

Bowen's Reaction Series strikes again

If this magma can find a path of weakness to the surface

Generally not a problem where two plates are grinding into each other

Erupt from the surface as lava

Form volcanic mountains (called an "island arc")

The vast majority cools at depth forming large granitic "plutons"

These are sutured to the "continental margin" (added to craton)

Oceanic basalt is created and destroyed in a relatively short time

Continental crust, however, is much more durable

This lighter "scum" is basically the end product of the earth's differentiation

Granitic crust has been accumulating for 4 billion years

Generally too light to be subducted

Floats around in a "sea of basalt"

Edges get beaten up, blocks split and join, but it basically isn't destroyed


Go to rock display


Folding - stress at depth

DIGRESS TO: Brittle-Ductile Transition Zone

Local conditions affect this!

Folding: "Directed compression, resulting in a semi-plastic deformation"

Immense stresses at an extremely slow rate

Remember Strickler's 2nd Law of Geofantasy

Usually occurs at depth in the crust

Increasing heat and pressure cause rocks to bend and fold instead of break

Most easily observed in sedimentary rocks

Generally related to compression of the crust

Result of regional directed pressure: DEFINE

Several broad divisions


Upwarping of crust

Oldest beds in center

Usually accompanied by a...


Downwarping of the crust

Youngest beds in center

Both tell the story of directed stress within the crust


Cascadia Subduction Zone

An example of oceanic/continental convergence

Results in Cascade Mountains

DESCRIBE in general terms

What is potential for future activity

2 Japanese geologists

Correlate quake with tsunami in Japan

Size of wave indicates 9.0+ event in 1700 A.D.

Very strong event, indeed!

What would happen today?



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