2 - How volcanoes work

Question 1

Where does volcanism occur?

Answer 1

Hot spots

Divergent plate boundaries(mid ocean ridges leading to plate formation)

Convergent plate boundaries(subjduction leading to plate recycling and mountain range formation)

Question 2

How do nuclear disntegrations in the core result in volcanism?

Answer 2

Nuclear disintegration in Earth’s core generates heat, triggering mantle convection and driving plate tectonics and therefore, volcanism

Question 3

Describe the composition of the Earth’s mantle.

Answer 3

Primarily made up of peridotite – composed of olivine (a magnesium and iron silicate) and minor minerals like chromite

Question 4

What is adiabatic decompression melting and where does it occur?(McKenzie & Bickle, 1988)

Answer 4

Melting that occurs as mantle rises and pressure decreases without losing heat

Happens at mid-ocean ridges and hotspots

Question 5

Where does melting occur?

Answer 5

Oceanic ridges/hotspots - decompressional melting, subduction zones - hydration(wet partial) melting

Question 6

What is a geotherm?

Answer 6

A line that shows the rate of temperature change with respect to depth of the mantle

Question 7

What is the range of upwelling speeds of the mantle at divergent plate boundaries?

Answer 7

10-20cm/year

Question 8

How long does it take for the mantle to rise to 50km?

Answer 8

0.5-1 My

Question 9

What occurs when the mantle rises to 50km below the surface(at a divergent plate boundary/mid ocean ridge)?

Answer 9

Up until that point, there is minimal conductive cooling as rock is a poor conductor heat, meaning the temperature changes little. Around 50km, the rock will start to melt and droplets of melt migrate upwards to accumulate in magma reservoirs, with some of this magma eventually erupting on the seabed.

Question 10

Where in the Earth’s structure do hotspots orignate and how does their melting differ to other areas?

Answer 10

Hotspots originate from the core/mantle boundary, making their upwelling mantle temperature hotter, and therefore, melting starts deeper.

Question 11

How does hydration melting cause volcanism at subduction zones?

Answer 11

Water from hydrated oceanic crust is released under pressure (~100 km depth), lowering melting point of overlying mantle wedge, leading to wet partial melting and volcanism

Question 12

Describe how eruptions occur at subduction zones(remember wet partial melting).

Answer 12

1. Ocean crust pervaded by a deep ingression of seawater subducts at the plate boundary →
2. Profound interaction between hot igneous rock & seawater occurs with hydration of minerals before fluids eventually escape back into the ocean through hydrothermal vents
3. Water is now charged with elements from the rocks which supports a metabolic pathway for bacteria at the base of the ocean → the water with hydrated minerals is carried down as the slab sinks
4. ~100km below the surface the water gets driven off by dehydration reactions due to the greatly increased pressure
5. Hydrated minerals become anhydrous(no water) again
6. Water rises into overlying mantle wedge from opposing plate
7. Wet partial melting occurs(below 90km)
8. Water forms bubbles under low pressure as magma erupts before expanding dramatically, resulting in violent volcanism

Question 13

What phases make up magma?

Answer 13

The three phases of matter - solid crystals, liquid silicate melt & gas bubbles

Question 14

How does silica content affect magma viscosity?

Answer 14

More silica = more polymerized melt = higher viscosity

Question 15

Name the three types of magma and describe their characteristics.(Oppenheimer, 2003)

Answer 15

Basaltic - 45-55% Si content/low viscosity/1000-1200 C, effusive(runny)

Andesitic - 55-65% Si content/medium viscosity/800-1000 C

Rhyolitic - 65 - 75% Si content/high viscosity/650-800 C, explosive

Question 16

Describe the processes of magma storage and ascent.(Annen et al., 2006)

Answer 16

1. Melt is less dense than surrounding crust so it rises
2. As it ascends, it cools, therefore increasing its density
3. Once the magma density is equal to the density of the surrounding crust, it stops ascending and starts spreading laterally to form a magma reservoir
4. It cools with minerals crystallising inside the melt(fractional crystallisation)
5. The density of the liquid falls due to crystallisation and the liquid rises through the crust again
6. The process repeats and leads to either the magma cooling down and crystallising to form plutonic rock or reaching the near surface where degassing can trigger an eruption

Question 17

Describe an effusive eruption.

Answer 17

Low viscosity magma(basaltic/andesitic), gas ascent faster than magma ascent, bubbles and foams emerging from volcano

Question 18

Describe an explosive eruption.

Answer 18

High viscosity magma(andesitic/rhyolitic), gas & magma rise coupled, explosions emerging from volcano