Lecture Three - Convergent boundaries (subduction zones and continental collisions) Flashcards
Do MORs create new lithosphere or just crust?
MORs produce new oceanic lithospheric crust, but not the entrie new lithosphere, just the top portion of it.
The oceanic lithospheric crust gets thicker as you move out from the MOR.
Does the denser or less dense lithosphere subduct?
The more dense lithosphere subducts under the less dense lithosphere.
This means oceanic crust always subducts under continental crust.
Younger crust is less dense than older cruse, thus older crust subducts under younger crust.
Lable the oceanic arc created by oceanic crust subducting under another slab of ocenaic crust.
Definitions:
Forarc: A forearc is the region between an oceanic trench and the associated volcanic arc.
Swell: The area of crust which is pushed up (higher elevation) next to the subduction zones trench.
Backarc: Area behind a volcanic arc.
Accretionary wedge: A mass of sedimentary material scraped off a region of oceanic crust during subduction and piled up at the edge of a continental crustal plate.
Pluton: A body of igneous rock (intrusive).
How does volcanism occur at subduction zones?
The rocks in the subducting slab heat up as the move down into the lower crust/upper mantle.
This heat melts the rocks, and causes them to dehydrate.
The water released moves into the asthenospheric mantle, causing volcanism due to the creation of magma.
Is andersitic or rhyolitic volcanism more explosive?
Rhyollitice volcanism is more explosive due to the higher viscosity, higher water content and higher silica content of rhyolite.
What factors affect the angle of subduction?
If the collision is occuring quickly, the angle will be a more shallow angle - acuteish.
If the collision is occuring slowly, the angle will be a more deep angle - obtuseish.
Slow collision cause extension.
Fast collision cause compression.
If the subducting slab is young, it is buyant and will have a shallow angle of subduction.
If the subducting slab is older, it is less buyant and will have a deeper angle of subduction.
What characteristics of a subduction zone cause the formation of higher mountains.
Usually shallow subduction with young subducting slab which is less dense cause higher mountains.
What is a Benioff-Wadati zone?
A planar zone of seismicity corresponding with the down-going slab in asubduction zone.
Where and what is the MOHO or Mohorovicic discontinuity?
The boundary surface between the earth’s crust and the mantle, lying at a depth of about 10–12 km under the ocean bed and 40–50 km under the continents.
What are the thicknesses of the continental and oceanic crusts?
Continental crust is 20-70 km thick.
Made predominantly of AL, Ca and K rich silicate ~ granit.
BUT, at mountain belts such as the Tibeten plateau, the continental crust is about double the usual thickness (~70-75 km).
Oceanic crust is 7-10 km thick.
Made predominantly of Fe, Mg rich silicate ~ basalt.
What is an ophiolite?
A part of a rock sequence, rather than a rock type.
It corresponds to oceanic lithosphere.
Oceanic lithosphere thrust into continents.
Includes ocean sediments, volcanic rock, dyke swarms.
What are strike slip transform faults?
When two plates move laterally past one another.
Do not creat much topography.
Sometimes there are bends where the plates do not move quite at right angles to one another - may cause the formation of a small rift with a lake, or a small mountain.
Strike slip compressions = transcompression.
Strike slip extensions = transtension.
Lineaments can be formed - long straight valleys.
Transform faults moving right relative to one another are called dextral faults.
Transform faults moving left relative to one another are called sinistral faults.
