Volcanoes: Types and Eruptions Volcanoes – different origins of magma

• The two major types of have different types of magma production • This results in different magma composition • Magma composition affects eruption style

(H Grobe/ AWI 2007, Wikimedia Creative (USGS, public domain) 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 Rock of upper Pressure apart due to mantle melts decreases as rising of hot (forms mafic crust thins mantle rock 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) (Colivine 2011, Wikimedia Creative Commons) magma Decompression melting – viscosity and gas content

• 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 Pahoehoe toe lava from Kilauea ( Volcano Observatory / USGS 2003, public domain) Mafic eruptions and lava flows

Lava fountain from Mauna Ulu. Note the lava flowing Pahoehoe lava and Aa flows. (B Inaglory 2007, over the hill and how close observers are to the Wikimedia Creative Commons) eruption. (USGS 1969, public domain) 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 Water causes Rock of lower crust heat of mantle silicate minerals melt due to heat of cause release of in overlying molten mantle rock water from mantle to melt. (forms minerals in Less dense intermediate or subducting slab magma rises 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

• Magma typically contains >52% silica, so high viscosity • Silicate chains form, trapping gases • Typically a high water content • Result: Explosive eruptions with thick, blocky lava and June 12, 1991 eruption column from Mt Pinatubo, (D Harlow/ USGS 1991, public domain) Felsic eruption with pyroclastic flow

Pyroclastic flow on 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 ( ) 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 Explosion of steam, water, on top of a volcano (ice, ash and possibly pyroclastics snow or caldera lake) is (phreatic or heated by rising magma or ) lava Phreatic eruptions – volcano type

• Any type of volcano may have a • 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

Phreatic eruption of Mt St Helens as rising magma boils groundwater (DA Swanson/ The phreatomagmatic eruption of USGS, public domain) Eyjafjallajökull in 2010 created a large ash cloud. (R Simmon/ NASA Earth Observatory 2010, 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/