Many small but violent volcanic eruptions occur when magma is withdrawing from the surface, rather than during initial advance.
Because volcanic eruptions necessarily involve transfer of magma from Earth's interior to its surface, volcanologists typically focus attention on processes taking place during magma rise and decompression.
Some eruptions "fail", however, with magma advancing toward but not reaching the surface. Others erupt only small volumes, while still others alternate vigorous eruption with periods of lava drainback.
Loss of gas from static or slowly rising bubbly magma, or transfer of magma into laterally propagating cracks (dikes or sills), causes withdrawal of the leading magma front and changes conditions at which magma encounters wallrock and groundwater; the changes can facilitate processes that can trigger explosive magma-water interaction.
Shallow magma withdrawal can promote so-called phreatic eruptions if during initial storage sufficient heat is transferred to form ephemeral hydrothermal systems, while deep withdrawal drives excavation of deep debris-filled structures called diatremes that are associated with small-volume surface deposits.
|Date||Wednesday, 8 August 2018|
|Time||1:00pm - 2:00pm|
|Location||Benson Common Room (Gn9), Geology Building, 360 Leith Street, Dunedin|