Magma

Question 1

Where does lava come from?

Answer 1

The mantle

Question 2

The lithosphere does not float on a layer of magma, the asthenosphere (mantle) consists of:

Answer 2

solid rock that can flow

Question 3

The mantle melts when:

Answer 3

1. Pressure on the rock is reduced (decompression melting)
2. Water (or another volatile) is added to the rock
3. Hot, rising magma comes in contact with other rocks and then melts them

Question 4

When rock rises from point A to point B: pressure decreases a lot, but:

Answer 4

only cools a little, so it melts

Question 5

What is assimilation?

Answer 5

A block of rock falls into magma and melts

Question 6

What is contamination?

Answer 6

partial melting of rock mixes with magma from below

Question 7

Where does magma come from?

Answer 7

1. Convergent boundary (seawater stored in the crust is released, water released from hydrous minerals is heated up, melting of the overlying mantle due to addition of volatiles)
2. Divergent boundary (hot rocks rise, pressure drops, decompression melting)
3. Hotspots

Question 8

Magma is classified into four groups based on how much silica (silicon and oxygen) is in the magma

Answer 8

1. Felsic (most silica)
2. Intermediate
3. Mafic
4. Ultramafic (least silica)

Question 9

What factors control magma composition?

Answer 9

1. Composition of source rock (rock that melted)
2. Partial melting (preferentially melts more silica-rich minerals b/c lower melting temp)
3. Crystal settling (mafic minerals form first and settle out of melt forming a cumulate, residual rock is richer in silica (more felsic), extracts mafic minerals, increasing silica content in the magma)
4. Magma mixing
5. Assimilation and contamination

Question 10

What is a discontinuous reaction?

Answer 10

When minerals become unstable with changing temp. They break down to form new minerals.

Question 11

What is a continuous reaction?

Answer 11

When the composition of plagioclase changes with changing temp

Question 12

Felsic minerals melt and crystallize at

Answer 12

low temperatures

Question 13

Mafic minerals melt and crystallize at

Answer 13

high temperatures

Question 14

How fast does molten rock cool into igneous rock?

Answer 14

Depends on:

1. Depth of the magma: magma deep in the Earth will cool more slowly than shallow magma, and shallow magma will cool more slowly than lava
2. Size and shape of magma body: The more S.A a body of magma (an intrusion) has, the faster it will lose heat. Rate of S.A to vol increases and size decreases. (basically the smaller it is, the faster it cools)
3. The presence of groundwater: Water passing through magma absorbs and carries away heat, can also create geysers.