Crust and Mantle Flashcards
In what region of the mantle does the zone of low velocity occur?
This area is located just under the lithosphere and is a low velocity zone because there is 1% magma in this region which causes the material to act much more like a liquid.
What type of rock is at the lower part of the continental crust?
This area contains granulates with basic composition which has a Vp of 7km/s, which makes it the most viable answer since it can exist at those temps and pressures.
What was the process by which granulite was determined to be the type of rock at the lower continental crust? Rocks here have a Vp of 6.5 to 7.2 km/s
1) First it was assumed to be basalt, but under those temps and pressures it transforms to eclogites.
2) However eclogites with basaltic composition have too high of a Vp (8.4 km/s)
3) eclogites made of diorite have a lower Vp (7.3 km/s) which is still too high, also temps and pressures are not right
4) Amphiboles have Vp = 7km/s but they do not exist at those temps and Pressures.
5) The only option left is Granulites with a basic composition (7 km/s).
What are the maximum ages for continental and ocean crusts?
Oceanic = 200 million Years Continental = 4.3 billion years
What is the dominante type of rock in the upper part of the mantle?
Peridotite
< 45% silica (Ultramafic)
Mostly made up of Olivine and Pyroxene
High in Mg
What are the three layers of oceanic crust?
1) Sediment - Max Depth (1km) but usually (.5km) Vp = 2km/s
2) Volcanic Layer - Basalt from Pillow Lavas (Vp = 3.4-6 km/s)
3) Oceanic Layer - Average depth of 5km, has dikes (basic)
Who were important people in the creation of tectonic theory?
Holmes (1931) - Proposed convection currents were causing drift.
Hess and Dictz (1960) - Formed the basis of modern plate tectonics
Vine, Matthews, Morley (1963) - Proposed the creation of oceanic crust at ridges
What is the average thickness of a plate?
100 km (this includes the crust and upper mantle)
What is the Wilson Cycle
1) Formation of rift valley
2) Formation of an expansive area
3) Formation of an ocean
4) Beginning of subduction
5) Subduction of the ridge
6) Continental collision
How much of the earth’s mass is the mantle?
67.2%
How much of the earth’s volume is the mantle?
83%
Who were some important people to first describe the mantle?
Bowen(1928) - Said that the mantle was composed of peridotite
Ringwood(1962,1975) - Said that a primitive mantle underwent partial melting and changed from a basaltic composition to a dunite-peridotite composition.
Different estimates show that 90% of the mantle is composed of
FeO, MgO, SiO2
What six components make up the mantle
FeO, MgO, SiO2, Na2O, CaO, Al2O3
What are the ways we can study the mantle?
1) Stratigraphic intrusions - Intrusions of pyroxene, peridotite, sometimes gabros, basalts, and anortosites.
2) Orogenic formations (mountains) - They have a high probability of representing the mantle, although they are generally metamorphic
3) Ophiolites - A portion of the oceanic crust that did not subduct
4) Ultramafic Oceanic Crust
5) Ultramafic Nodules in Basalt
