2 - How volcanoes work Flashcards
Where does volcanism occur?
Hot spots, divergent plate boundaries(mid ocean ridges leading to plate formation), convergent plate boundaries(subjduction leading to plate recycling and mountain range formation)
How do nuclear disntegrations in the core result in volcanism?
Nuclear disintegrations in the core release heat - mid ocean ridge/subduction volcanism occurs because plates are allowed motion & hotspot volcanism occurs because of mantle convection
Describe the composition of the Earth’s mantle.
Essentially made of peridotite - olivine(Mg/Fe silicate mineral), pyroxene, garnet
Where does melting occur?
Oceanic ridges/hotspots - decompressional melting, subduction zones - hydration(wet partial) melting
What is a geotherm?
A line that shows the rate of temperature change with respect to depth of the mantle
What is the range of upwelling speeds of the mantle at divergent plate boundaries?
10-20cm/year
How long does it take for the mantle to rise to 50km?
0.5-1 My
What occurs when the mantle rises to 50km below the surface(at a divergent plate boundary/mid ocean ridge)?
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.
Where in the Earth’s structure do hotspots orignate and how does their melting differ to other areas?
Hotspots originate from the core/mantle boundary, making their upwelling mantle temperature hotter, and therefore, melting starts deeper.
Describe how eruptions occur at subduction zones(remember wet partial melting).
Ocean crust pervaded by a deep ingression of seawater subducts at the plate boundary → profound interaction between hot igneous rock & seawater occurs with hydration of minerals before fluids eventually escape back into the ocean through hydrothermal vents → 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 → ~100km below the surface the water gets driven off by dehydration reactions due to the greatly increased pressure → hydrated minerals become anhydrous(no water) again → water rises into overlying mantle wedge from opposing plate → wet partial melting occurs(below 90km) → water forms bubbles under low pressure as magma erupts before expanding dramatically, resulting in violent volcanism
What is magma composed of?
The three phases of matter - solid crystals, liquid silicate melt & gas bubbles - there are various proportions of these as magma ascends towards the surface and these proportions control the nature of magma flow and style of eruption
Name the three types of magma and describe their characteristics.
Basaltic - 45-55% Si content/low viscosity/1000-1200 C, Andesitic - 55-65% Si content/medium viscosity/800-1000 C, Rhyolitic - 65 - 75% Si content/high viscosity/650-800 C
Describe the processes of magma storage and ascent.
Melt is less dense than surrounding crust so it rises → as it ascends, it cools, therefore increasing its density → 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 → it cools with minerals crystallising inside the melt(fractional crystallisation) → the density of the liquid falls due to crystallisation and the liquid rises through the crust again → 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
Describe an effusive eruption.
Low viscosity magma(basaltic/andesitic), gas ascent faster than magma ascent, bubbles and foams emerging from volcano
Describe an explosive eruption.
High viscosity magma(andesitic/rhyolitic), gas & magma rise coupled, explosions emerging from volcano
