Volcanoes:Magma Types and Eruptions

Volcanoes – different origins of magma

• The two major types of volcano have different types of magma production

• This results in different magma composition• Magma composition affects eruption style

(USGS, public domain)

(H Grobe/ AWI 2007, Wikimedia Creative Commons)

Decompression melting – divergent boundaries

• As plates move apart, pressure decreases• As pressure decreases, rock can melt at a lower

temperature• This is the same effect as water boiling at lower

temperature at high altitudes (lower air pressure)

Plates move apart due to rising of hot mantle rock

Pressure decreases as crust thins

Rock of upper mantle melts (forms mafic

magma)

Decompression melting – minerals

• Felsic minerals melt at a lower temperature/ pressure than mafic minerals

• As a result, ultramafic mantle rock partially melts to form mafic (basaltic) magma

(Colivine 2011, Wikimedia Creative Commons)

Decompression melting –viscosity and gas content

Pahoehoe toe lava from Kilauea (Hawaii Volcano Observatory / USGS 2003, public domain)

• Magma is typically 45-52% silica, so low viscosity

• There is no addition of water, so low gas content

• Result: Effusive eruptions with relatively runny lava

Mafic eruptions and lava flows

Lava fountain from Mauna Ulu. Note the lava flowing over the hill and how close observers are to the eruption. (USGS 1969, public domain)

Pahoehoe lava and Aa flows. (B Inaglory 2007, Wikimedia Creative Commons)

Flux melting – convergent boundaries

• As the oceanic plate subducts, water in hydrous minerals (mica, lawsonite, amphibole) is driven off the subducting slab, releasing water into the mantle

• Water is a flux, lowering the melting point of mantle rock.• Silica-rich rock melts in the mantle and lower crust

Pressure and heat of mantle

cause release of water from minerals in

subducting slab

Water causes silicate minerals

in overlying mantle to melt.

Less dense magma rises

Rock of lower crust melt due to heat of molten mantle rock

(forms intermediate or felsic magma)

Flux melting – minerals

• Felsic minerals melt at a lower temperature/ pressure than mafic minerals

• Low-temperature melting, due to addition of water, creates a more felsic melt

• Less dense molten mantle rock rises, causing melting of the lower crust, adding more felsic minerals to the magma

• Results in intermediate or felsic magma

Flux melting – viscosity and gas content

June 12, 1991 eruption column from Mt Pinatubo, Philippines (D Harlow/ USGS 1991, public domain)

• Magma typically contains >52% silica, so high viscosity

• Silicate chains form, trapping gases

• Typically a high watercontent

• Result: Explosive eruptions with thick, blocky lava and pyroclastic flow

Felsic eruption with pyroclastic flow

Pyroclastic flow on Mayon Volcano, Philippines. (USGS 1984, public domain)

Interactions of magma and water

• As magma rises in a volcano, it may encounter groundwater or ice (glacier, snow) and water (caldera lake) on top of the volcano

• Hot magma (up to 1,170°C) causes water to boil and flash to steam

• Explosion occurs, releasing steam, ash and more!

Water beneath the ground or on top of a volcano (ice, snow or caldera lake) is

heated by rising magma or lava

Explosion of steam, water, ash and possibly pyroclastics

(phreatic or phreatomagmatic eruption)

Phreatic eruptions – volcano type

• Any type of volcano may have a phreatic eruption

• Eruption is caused by interaction with water, NOT by magma type

Schematic diagram of a phreatomagmatic eruption. Water interacts with magma to produce ash and lava bombs. (Semhur 2007, Wikimedia Creative commons)

Phreatic and phreatomagmatic eruptions

The phreatomagmatic eruption of Eyjafjallajökull in 2010 created a large ash cloud. (R Simmon/ NASA Earth Observatory 2010, public domain)

Phreatic eruption of Mt St Helens as rising magma boils groundwater (DA Swanson/ USGS, public domain)

References

• DE Tompkins, JM Watkins (Eds) 2016. Exploring Earth and Environmental Science Year 11, Earth Science Western Australia

• USGS Volcano Hazards Program (n.d.). Glossary. Accessed 24 April 2020 https://volcanoes.usgs.gov/vsc/glossary/

https://volcanoes.usgs.gov/vsc/glossary/

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