Magma

Question 1

Adiabatic process

Answer 1

A thermodynamic process in which no heat enters or leaves the system during expansion or compression

Question 2

Adiabatic cooling

Answer 2

It occurs when crust or mantle rises, undergoes expansion and the temperature falls, with no loss or gain of thermal energy

Question 3

Adiabatic heating

Answer 3

It occurs when crust or mantle material descends and temperature rises as it contracts, with no loss or gain in thermal energy

Question 4

Divergent plate margin

Answer 4

They occur where two plates are moving apart and magma is rising up between them

Question 5

Partial melting

Answer 5

It occurs where some of the minerals in a rock melt to form a magma

Question 6

Hot spots

Answer 6

an area of high heat flow above a mantle plume

Question 7

Convergent plate margins

Answer 7

Are where two plates are colliding and magma is formed above a subduction zone or deep in the crust

Question 8

Decompression melting

Answer 8

Decompression of the ultramafic peridotite causes partial melting and produces mafic magmas

Question 9

How is magma produced at a convergent plate boundary?

Answer 9

Two plates collide, one is forced down, the geothermal gradient increases and partial melting results

Question 10

Describe the process of flux melting

Answer 10

An oceanic plate is subducted, water in the rocks is carried down into the hot mantle. As the descending plate heats up, the water is released into the mantle rock above the plate. The water lowers the melting point of the minerals in the mantle rock and partial melting produces magma.

Question 11

How is basaltic magma produced at an oceanic-oceanic crust?

Answer 11

The magma is mafic to intermediate in composition, initially the magma has only thin oceanic crust to rise through. If the magma rises quickly at shallow depths then it will be basalt.

Question 12

How is intermediate to silicic magma produced at a continental- oceanic plate?

Answer 12

Magma rises through thicker silicic crust and this may partially melt due to increasing temps. Rising mafic magma has temp of 1000+ and silicic material melts at just 800. The resulting mixing gives intermediate to silicic magma.

Question 13

How is silicic magma produced at a continental-continental plate margin?

Answer 13

Neither plate is subducted. High pressures and the mass of sediment (formed fold mountains) combine to force the base crust down. Partial melting at base produces silicic magma.

Question 14

Magma

Answer 14

Molten rock which is beneath the earth’s surface

Question 15

What determines the density of magma?

Answer 15

composition, pressure and temperature

Question 16

What is the effect of Composition on the density of magma?

Answer 16

Mafic magma is rich in iron compared to silicic magma and this makes it more dense

Question 17

What is the effect of Pressure on density of magma?

Answer 17

An increase in pressure causes the magma to compress, which increase its density making it less buoyant

Question 18

What is the effect of Temperature on the density of magma?

Answer 18

An increase in temperature cause the magma to expand, which decreases its density making it more buoyant

Question 19

What is the viscosity of magma determined by?

Answer 19

Composition- mafic magma low viscosity as silica content is low vies versa
Temperature- increase in temp reduces viscosity

Question 20

How can magma recharge be detected?

Answer 20

Microgravity- sensors detect local changes in subsurface density from magma movement
Seismometer- tracks migration of small earthquakes from depth towards surface
Ground water heated- increase in pressure can cause crustal deformation and release of gases can also be measured

Question 21

What factors can prevent magma mixing?

Answer 21

density contrast
viscosity contrast
thermal barrier

Question 22

How are geysers produced?

Answer 22

Groundwater is heated at depths. Convection causes the gas-rich, super heated water to rise. As it rises, the pressure decreases and ‘flash’ boiling into steam result. The column of cooler water above it is thrust up explosively. The water then drains back into the ground

Question 23

How can gases exsolve from magma?

Answer 23

As magma rises to the surface, the reduction in pressure allows dissolved gases to exsolve. This happens because there is less magma above it pressing down. Bubbles can also form in a magma due to recharge and mixing.

Question 24

What can provide the earliest indication of magma moving?

Answer 24

Earthquakes

Question 25

What types of earthquakes indicate magma movement?

Answer 25

Short period- caused by the fracturing of brittle rock as magma forces its way upwards
Long period- indicate increased gas pressure in the magma chamber
Harmonic tremors- sustained vibrations, 0.5-5 Hz the result of magma vibrating in the vent as it moves upwards

Question 26

What does ground movement indicate and how is it measured?

Answer 26

Magma is moving up under the volcano. Rate of swelling is measured using tiltmeters and GPS accurate to 1 millimetre and can measure vertical and horizontal movement.

Question 27

What do gas emissions indicate?

Answer 27

As magma nears the surface and the confining pressure decreases, gas escapes. Changes in its compositions e.g. increases SiO2 levels can indicate the magma is nearing the surface.

Question 28

What can changes in ground water indicate?

Answer 28

Increased gas pressure will cause water levels to rise and suddenly drop before an eruption and the increased heat flow can reduce flow in aquifers. (borehole and well measurements)

Question 29

Extrusion

Answer 29

The extrusion of magma onto the earth’s surface where it forms a lava flow