Crust and Mantle

Question 1

In what region of the mantle does the zone of low velocity occur?

Answer 1

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.

Question 2

What type of rock is at the lower part of the continental crust?

Answer 2

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.

Question 3

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

Answer 3

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).

Question 4

What are the maximum ages for continental and ocean crusts?

Answer 4

Oceanic = 200 million Years
Continental = 4.3 billion years

Question 5

What is the dominante type of rock in the upper part of the mantle?

Answer 5

Peridotite

< 45% silica (Ultramafic)
Mostly made up of Olivine and Pyroxene
High in Mg

Question 6

What are the three layers of oceanic crust?

Answer 6

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)

Question 7

Who were important people in the creation of tectonic theory?

Answer 7

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

Question 8

What is the average thickness of a plate?

Answer 8

100 km (this includes the crust and upper mantle)

Question 9

What is the Wilson Cycle

Answer 9

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

Question 10

How much of the earth’s mass is the mantle?

Answer 10

67.2%

Question 11

How much of the earth’s volume is the mantle?

Answer 11

83%

Question 12

Who were some important people to first describe the mantle?

Answer 12

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.

Question 13

Different estimates show that 90% of the mantle is composed of

Answer 13

FeO, MgO, SiO2

Question 14

What six components make up the mantle

Answer 14

FeO, MgO, SiO2, Na2O, CaO, Al2O3

Question 15

What are the ways we can study the mantle?

Answer 15

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

Question 16

What happens to pyroxene when it melts

Answer 16

It begins to exchange magnesium for calcium (or vice versa) and can change to another member of its family.

Question 17

What three families of minerals make up the asthenosphere

Answer 17

Garnet, Pyroxene, Olivine.

Question 18

What are the top elements in the continental crust by weight?

Answer 18

1) Oxygen 47%
2) Silicon (27%)
3) Aluminum (8%)
4) Iron (5%)
5) Calcium (3.6%)
6) Sodium (2.8%)
7) Potassium (2.6%)
8) Magnesium (2%)
9) all others (1%)

Question 19

What is an ophiolite and what is it composed of

Answer 19

An ophiolite is a section of the continental crust that did not subduct.

• Sediments
• Pillow Lavas
• Gabbro
• Layered Gabbro
• Layered Peridotite
• Upper Mantle

Question 20

What is the main difference between eclogite and granulates (temp and pressure)

Answer 20

Eclogites form at High pressures but a wide range of temps, and granulates form only at high temperatures but a wider range of pressures.

Question 21

What makes up granulite

Answer 21

Mainly pyroxene and feldspar

Question 22

What is the structure of the continental crust?

Answer 22

Top Top portion – Volcanoes/Sediments/Metamorphic rocks

Top Bottom Portion – Granodiorites (Like Granite – Igneous rocks)

Lower Portion – Least understood, contains granulites (high temp metamorphic rock)