2.16 Evidence for magma moving

Question 1

What are the main sources of evidence for current magma movement?

Answer 1

• earthquakes (seismic survey)
• ground movement (using tiltmeters and GPS)
• gas emissions
• changes in ground water

Question 2

What do short-period earthquakes indicate about magma movement?

Answer 2

microseismic events (short-period earthquakes) occur because of the fracturing of brittle rock as magma forces its way upwards

Question 3

What do long-period earthquakes indicate about magma movement?

Answer 3

long-period earthquakes indicate increased gas pressure in the magma chamber of a volcano

Question 4

What do harmonic tremors indicate about magma movement?

Answer 4

sustained vibrations (known as harmonic tremors) in the range of 0.5-5 Hz are the result of magma vibrating in the vent as it moves upwards

Question 5

What do harmonic tremors indicate about magma movement?

Answer 5

sustained vibrations (known as harmonic tremors) in the range of 0.5-5 Hz are the result of magma vibrating in the vent as it moves upwards

Question 6

What are the characteristics of a magma chamber?

Answer 6

magma chambers are areas where the crust or mantle is partially melted
(the magma chamber beneath Yellowstone, on average, is about 90% solid rock with 10% liquid in its pores)

Question 7

How can seismic waves be used to locate a magma chamber?

Answer 7

• when seismic waves encounter molten material they either slow down (P waves) or are stopped (S waves)
• if material is partially melted then both P and S waves will slow down
• the magma chamber creates a shadow zone allowing its size and depth to be calculated

Question 8

How can monitoring ground movement with GPS and tiltmeters give information about magma movement?

Answer 8

swelling of a volcano suggests that magma is moving up under the volcano

Question 9

How can gas emissions give information about magma movement?

Answer 9

• as magma nears the surface and the confining pressure decreases, gases escape
• sulphur dioxide is one of the main components of volcanic gases and increasing amounts being released suggests that magma is nearing the surface
• there may be changes in the ratio of CO₂/SO₂
• changes in gas composition may indicate variation in magma supply rate or a change in magma type

Question 10

How can changes in ground water give indications of magma movement?

Answer 10

borehole and well measurements can be used to monitor changes in a volcano’s subsurface gas pressure and thermal regime - increased gas pressure will cause water levels to rise and suddenly drop right before an eruption, and the increased local heat flow can reduce flow in aquifers