Imperial College Rock Library

Glossary: Phreatomagmatic Eruption

Synonyms: phreatomagmatic activity
    Phreatomagmatic eruptions are volcanic eruptions driven by the direct interaction between magma and external bodies of water such as the sea, lakes or groundwater. They differ from magmatic eruptions which are driven by volatiles dissolved within magma, and phreatic eruptions in which involve indirect interaction (i.e. heat transfer) between magma and water.
The products of phreatomagmatic eruptions are pyroclastic materials containing juvenile (magma-derived) clasts and experience high degrees of fragmentation. Phreatomagmatic pyroclastic deposits, therefore, tend to have finer grain-size and better size sorting than equivalent magmatic eruptions. The mechanism of fragmentation in phreatomagmatic eruptions is thought to involve rapid contraction of magma due to cooling by water, but may also include fracturing due to gas pressure and due to collapse of steam envelopes.
Typical products of phreatomagmatic eruptions include hyaloclastite breccia, peperite and hyalotuff, which contain chilled, glassy juvenile clasts. Concentrically zoned accretionary lapilli are also a distinctive product of phreatomagmatic activity.
Phreatomagmatic activity can be associated with any magma type and occur at the same volcanic centres as any other activity types. Eruptions into crater lakes or summit glaciers can, for example, occur at stratovolcanoes, shield volcanoes and calderas. Sustained phreatomagmatic eruptions are often submarine or subglacial and include Surtseyan, Vulcanian and Phreatoplinian activity.

Related Terms

accretionary lapilli, caldera, hyaloclastite breccia, peperite, phreatic eruption, phreatoplinian eruption, pyroclastic rock, stratovolcano, surtseyan eruption