Magma-Water Interactions

Question 1

What are the names of the eruptions that either; contain external water, contain no external water or are only steam + rock, no magma eruptions.

Answer 1

Hydromagmatic/phreatomagmatic
Magmatic
Phreatic

Question 2

What are the types of hydromagmatic eruptions?

Answer 2

Littoral cones - lava enters oceans, shorelines.
Rootless cones - lava flows over water saturated sediment.
Submarine - effusive, get pillow lava.
Subglacial - in meltwater.
Surtseyon - form new islands.
Phreatoplinian - Plinian but with external water
Secondary PDC deposit explosion - PDC heats water below.

Question 3

How does water impact whats produced in eruptions?

Answer 3

If water hits magma in hydromagmatic eruptions, produce tuff cones.
If water does not interact with magma, in magmatic eruptions, produce scoria cones.

Question 4

Grain size in hydromagmatic eruptions? How does water impact grain size?

Answer 4

Grain size is fine, more ash. Doesn’t change much with distance as all relatively fine to begin with.

Water forms clusters of particles, helps to form Accretionary Lapilli - clusters of spherical ash

Question 5

What are the main ash morphological features for hydromagmatic eruptions?

Answer 5

• Quenched cracks
• Stepped brittle fracture
• Blocky morphology

Higher density clasts, which contain more volatiles. This is because hydromagmatic eruptions are at deeper pre-mature fragmentation depths due to interaction with water.

Question 6

Ash morphology of magmatic eruptions?

Answer 6

More vesiculated (Bubbles).
bubble wall fragments and shards.
lower density.

Question 7

How does Molten Fuel Coolant interaction (type of hydromagmatic fragmentation) work?

Answer 7

1) Pre-mixing:
- magma-water interaction, need critical contact area
2) Trigger:
- need a trigger to bring water and magma into direct contact and get an explosive mix. Either by:
- Internal: localised vapour film collapse
- External: i.e. seismicity
3) Expansion:
- Heat transfer between magma and water
- cooling melt cant accommodate expanding water (due to heating) which results in fragmentation

Question 8

What is needed to drive magma fragmentation?

Answer 8

Sudden density change.

Causes vapour-liquid phase change, which causes water to expand and drive fragmentation.

Question 9

How does hydromagmatic fragmentation by Thermal Stress work?

Answer 9

When a melt cools when it comes into contact with water, it cools non-uniformly.
Outside cools whilst still molten interior. The interior will expand and vesiculate and cause the outside rind to quench and fracture.
Can cause explosive eruptions due to the release of stress upon fragmentation.

Question 10

Facts about Kileaua 2018 (Hydromagmatic)?

Answer 10

• Lava lake connected to magma reservoir at depth
• Lava lake dropped which caused fissure eruptions, also caused lava to flow into the ocean
• The drop also caused the summit to collapse
• It was thought that there would have been magma interaction with groundwater in response to the drop in magma and cause explosive eruptions
• But this didnt happen

Question 11

What is petrology volcanology used for?

Answer 11

To determine volcanic processes i.e. from bubbles, crystals and glass to determine magma history.

Question 12

What crystal properties can be used to determine magma history and what do they show?

Answer 12

Melt inclusions - indicate conditions crystals grew

Chemical zonation - indicate changes in conditions in which crystals grew.

Diffusion profiles - elements diffuse when crystal composition is not uniform.

P, T, X (composition) and t

Question 13

What other processes can be used to determine magma history (bar crystals)?

Answer 13

Pre-eruptive magma storage experiments:

• use composition to find P and T ranges
• can measure plag + opx + amph of actual rocks to determine storage conditions of magma in relation to experiments

Geothermometry:
- constrain temperature of magma

Volatile content of phenocrysts in melt inclusions:
- constrain pressure

Question 14

During crystallisation by cooling and decompression whats the order to crystal classes?

Answer 14

By cooling:
- pyroxene, then plagioclase then amphibole
By decompression:
- Plagioclase, then pyroxene but no amphibole

This assumes equilibrium.

Question 15

What are the different formational characteristics between phenocrysts and microlite?

Answer 15

Phenocrysts:

• form in magma storage region
• cool slowly, relatively stagnant
• means they have time to grow

Microlites:

• form by decompression as water saturated magma magma ascends
• lots of small crystals nucleated

Question 16

How does decompression rate impact crystal texture?

Answer 16

Slow, constant decompression:
- get glass with few bubbles, crystals large and chunky

Rapid decompression:

• lots of thin little crystals
• extremely fragmented

Question 17

What is diffusion?

Answer 17

Random thermal motion of particles from high to low concentration.
Creates the system to be closer to equilibrium.

Question 18

What type of diffusion is important in volcanic systems?

Answer 18

Chemical diffusion

Question 19

How does chemical diffusion work?

Answer 19

1) Growth of uniform crystal
2) Crystal rim forms of different composition to the original crystal due to changes in T, P or X.
3) Diffusive re-equilibration of original crystal and newer rim. This means diffusion is occurring making the boundary between the two fuzzy.

Question 20

How does diffusivity depend on temperature?

Answer 20

Hot - fast diffusion
Cold - slow diffusion

This means that after an eruption and cooling has occurred, diffusion will stop.
Can work out the time between growth of rim and eruption.