Phreatomagmatic eruption

A phreatomagmatic explosion ( ancient Greek φρέαρ phréar, fountains ', genitive φρέατος phréatos ) is a volcanic explosion that results from the direct contact of hot molten rock ( magma or lava ) or hot pyroclastic density currents with external water, for example, ground water, surface water, seawater or hydrothermal water. The water may come from a lake and glacier. The phreatomagmatic explosion is often preceded by a phreatic explosion. It will therefore also summarized along with this explosion type (and other phenomena ) under the umbrella term Hydrovulkanismus or Hydromagmatismus.


The definitions are not uniform in the literature. Originally the term phreatomagmatic explosion called a volcanic explosion when magma, thus coming into contact with groundwater ( = phreatisches water) with fresh water. Contact with sea water or surface water was not originally included in this definition. However, this narrow view of the term was not maintained. Today is neutral spoken of " external " water that comes into direct contact to magma and steam explosions triggers.

In the earlier literature is under phreatomagmatic explosion often the sequence of the phreatic explosion phreatomagmatic understood and may even to igneous explosion. Other authors refer to this type of volcanic explosion as hydro- magmatic explosion or use the assault hydrovulkanische explosion, the phreatic and phreatomagmatic explosions with includes.


A phreatomagmatic explosion can occur when external water, so do not funded with magma or lava juvenile water, comes in direct contact with magma, lava or hot pyroclastic density currents. This happens when:

  • Groundwater ( pore water ) comes into direct contact with ascending magma
  • Rising magma penetrates into water-filled rock cavities
  • Come from water ( Crater ) Lakes through breaks in direct contact with the magma
  • Pyroclastic density currents in rivers, lakes or flow into the sea
  • Lava flows at depths above 200 m
  • Sea water through breaks in contact with magma comes
  • Surface water after heavy rainfall runs into the crater and seeps into loose material

The suddenly produced steam at about 1,000 times the volume of water smashed the surrounding rock and blows a crater in the rock underground. The ejected host rock is deposited around the crater as Wall. The direct contact of magma with external water the pyroclastics are highly fragmented; However, the pyroclastics are little bubbly. Phreatomagmatic explosions can be followed by only magmatic explosions. Relatively rare later lava is promoted in this type of explosion. As a rule, end the phreatomagmatic explosions when the influx of external water dries up and / or the supply of magma comes to a halt.

Emergence of Surges

Phreatomagmatic eruptions can also high-energy surges ( surges of English or base surges, also translated as a pressure wave ) created that consist of a mixture of gases, water vapor, (old) rock particles and ashes. The term " pressure wave " for this phenomenon is not applicable; therefore the technical term surge has naturalized. Surges can be similar to the pyroclastic flows with high speed and high destructive power spread close to the ground. They have a low density between > 0.1 to about 1%) in relation to pyroclastic flows. Are more than 75 % of the particles pyroclastic, they are also referred to as surges pyroclastic. In contrast to the hot and dry " glowing clouds " which Surges phreatomagmatischer explosions are often caused by high water vapor content of " wet" and relatively cool ( 100 ° C).

Maars and their emergence

Phreatomagmatic explosions are responsible for the emergence of numerous maars. One can even say that this eruption type is the typical mechanism for formation of maars. Rarely, the sequence is also continuing to purely magmatic eruptions. Lava flows are very rare. However, the phreatomagmatic eruptions are often preceded by phreatic eruptions. Some authors therefore refer to this sequence of phreatic to phreatomagmatic eruptions as phreatomagmatic eruptions or do not distinguish between these two types of explosion.