2.13 Magma generation

Question 1

What is the average geothermal gradient in the lithosphere?

Answer 1

30°C per km

Question 2

Why does mantle not melt at 40km depth, despite the increased temperatures at this depth?

Answer 2

the increase in pressure at this depth raises the melting point above the geotherm

Question 3

Why does hot mantle rock not melt as it rises (and pressure reduces)?

Answer 3

as the pressure decreases, the volume increases causing a decrease in temperature (this is known as adiabatic cooling)

Question 4

In which layer is the mantle closest to its melting point?

Answer 4

the asthenosphere / low velocity zone

Question 5

Why does the asthenosphere occur closer to the surface (around 5km) at divergent plate margins?

Answer 5

the lithosphere is being pulled apart and therefore thins, bringing the asthenosphere closer to the surface

Question 6

What is decompression melting?

Answer 6

partial melting that occurs as a result of a reduction in pressure in the mantle

Question 7

What type of magma is produced by the partial melting of the mantle?

Answer 7

mafic (45-52% silica)

Question 8

What rock type is produced at the surface (extrusive) at divergent boundaries?

Answer 8

basalt (often basalt pillow lavas at mid-ocean ridges)

Question 9

What type of magma is generated by partial melting at hot spots (where mantle plumes rise)?

Answer 9

mafic (45-52% silica)

Question 10

At what tectonic setting do flood basalts form?

Answer 10

hot spots

Question 11

What is a flood basalt?

Answer 11

a very large volume of lava erupted over very short time scales (less than 1 million years)

Question 12

What process(es) cause melting at convergent plate margins?

Answer 12

• subduction of oceanic lithosphere creates an area of low pressure and decompression melting
• volatiles from hydrated minerals (clays) in the oceanic lithosphere reduce the melting point of the mantle (flux melting)

Question 13

What is flux melting?

Answer 13

adding water to mantle rock reduces the melting point

Question 14

What type of magma is produced at oceanic-oceanic convergent plate margins?

Answer 14

• partial melting of ultramafic mantle creates mafic magma
• where the overriding oceanic lithosphere is older and thicker, the magma produced may become intermediate due to partial melting of the lithosphere

Question 15

What type of magma is produced at oceanic-continental convergent plate margins?

Answer 15

• partial melting of ultramafic magma produces mafic magma
• mafic magma passes through the thicker silicic crust and partial melting adds silica to the magma creating intermediate or silicic magma

Question 16

What is the typical temperature of mafic magma?

Answer 16

1000°C

Question 17

What is the typical melting point of silicic material/rocks?

Answer 17

800°C

Question 18

Why do silicic and mafic magmas not tend to mix well?

Answer 18

they have different viscosities

Question 19

What type of magma is generated at a continental-continental plate margin?

Answer 19

• as there is no subduction or continental thinning, there is no decompression melting or flux melting
• melting will only occur where crustal thickening pushes the continental crust sufficiently deep to reach the 800°C isotherm where the silicic rocks will partially melt