Phreatomagmatic eruption

The propagating stress waves and thermal contraction widen cracks and increase the interaction surface area, leading to explosively rapid cooling rates.

[citation needed] Phreatomagmatic ash is formed by the same mechanisms across a wide range of compositions, basic and acidic.

They can be formed at water depths of >500 m,[1] where hydrostatic pressure is high enough to inhibit vesiculation in basaltic magma.

Hyalotuff is a type of rock formed by the explosive fragmentation of glass during phreatomagmatic eruptions at shallow water depths (or within aquifers).

Hyalotuffs have a layered nature that is considered to be a result of dampened oscillation in discharge rate, with a period of several minutes.

A clast known as an accretionary lapilli is distinctive to phreatomagmatic deposits, and is a major factor for identification in the field.

Accretionary lapilli form as a result of the cohesive properties of wet ash, causing the particles to bind.

The tephra is often unaltered and thinly bedded, and is generally considered to be an ignimbrite, or the product of a pyroclastic density current.

They are built around a volcanic vent located in a lake, coastal zone, marsh or an area of abundant groundwater.

There was minor initial phreatomagmatic activity followed by the dry venting of 6 km3 of rhyolite forming the Hatepe Plinian Pumice.

The vent was then infiltrated by large amounts of water causing the phreatomagmatic eruption that deposited the 2.5 km3 Hatepe Ash.

For a typical sub-glacial eruption, overlying glacial ice is melted by the heat of the volcano below, and the subsequent introduction of meltwater to the volcanic system results in a phreatomagmatic explosion.

[12] The melting of the overlying Vatnajökull ice cap also forms sub-glacial lakes which, when conditions are right, can burst forth as catastrophic glacial outburst floods known as jökulhlaup.