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
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
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
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
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?
You are GeoManiac number since April 1, 1997