Phreatomagmatic eruption

Phreatomagmatic eruptions are defined as juvenile forming eruptions as a result of interaction between water and magma. They are different from magmatic and phreatic eruptions. The products of phreatomagmatic eruptions contain juvenile clasts, unlike phreatic eruptions and are the result of interaction between magma and water unlike magmatic eruptions.Heiken, G. & Wohletz, K. 1985. Volcanic Ash. University of California Press, Berkeley] It is very common for a large explosive eruption to have magmatic and phreatomagmatic components.

Mechanisms

Several competing theories exist as to the exact mechanism of ash formation. The most common is the theory of explosive thermal contraction of particles under rapid cooling from contact with water. In many cases the water is supplied by the sea, for example Surtsey. In other cases the water may be present in a lake or Caldera-lake, for example Santorini where the phreatomagmatic component of the Minoan eruption was a result of both a lake and later the sea. There have also been examples of interaction between magma and water in an aquifer, many of the cinder cones on Tenerife are believed to be phreatomagmatic because of these circumstances.

The other competing theory is based on fuel-coolant reactions, which have been modeled for the nuclear industry. Under this theory the magma (in this case the fuel) fragments upon contact with a coolant (the sea, a lake or aquifer), the propagating stress waves and thermal contraction widening cracks and increasing the interaction surface area leading to explosively rapid cooling rates. The two mechanisms proposed are very similar and the reality is most likely a combination of both.

Deposits

Phreatomagmatic ash is formed by the same mechanisms across a wide range of compositions, basic and acidic. Blocky and equant clasts with low vesicularities are formed. The deposits of phreatomagmatic explosive eruptions are also believed to be better sorted and finer graind than the deposits of magmatic eruption. This is a result of the much higher fragmentation of phreatomagmatic eruptions.

Hyaloclastite

This is the term used to describe glass found with pillow basalts that were produced by non-explosive quenching and fracturing of basaltic glass. These are still classed as phreatomagmatic eruptions as they produce juvenile clasts from the interaction of water and magma. They can be formed at water depths of >500 m., where hydrostatic pressure is high enough to inhibit vesiculation in basaltic magma.

Hyalotuff

Hyalotuff is the term used to define rocks formed by the explosive fragmentation of glass during phreatomagmatic eruptions at shallow water depths (or within aquifers). Hyalotuffs have a layered nature that is believed to be a result of damped oscillation in discharge rate with a period of several minutes. [Starostin, A. B., Barmin, A. A. & Melnik, O.E. 2005. A transient model for explosive and phreatomagmatic eruptions. Journal of Volcanology and Geothermal Research, 143, 133-151.] The deposits are much finer grained than the deposits of magmatic eruptions, due to the much higher fragmentation of the type of eruption. The deposits appear better sorted than magmatic deposits in the field because of their fine nature, but when grain size analysis is undertaken the deposits are much more poorly sorted than their magmatic counterparts. A clast known as an accretionary lapilli is almost unique 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 particle to accrete. They have a circular structure when viewed in hand specimen and under the microscope.

A further control on the morphology and characteristics of a deposit is the water to magma ratio. It is believed that the products of phreatomagmatic eruptions are fine grained and poorly sorted where the magma/water ratio is high but when there is a lower magma/water ratio the deposits may be coarser and better sorted. [Carey, R. J., Houghton, B. F., Sable, J. E. & Wilson, C. J. N. 2007. Contrasting grain size and componentry in complex proximal deposits of the 1886 Tarawera basaltic Plinian eruption. Bulletin of Volcanology, 69, 903-926.]

urface features

There are two types of vent landforms from the explosive interaction of magma and ground or surface water; "Tuff Cones" and "Tuff Rings." Both of the landforms are associated with monogenetic volcanoes and polygenetic volcanoes. In the case of polygenetic volcanoes they are often interbedded with lavas, ignimbrites and ash- and lapilli-fall deposits.

Tuff Rings

Tuff Rings have a low profile apron of tephra surrounding a wide crater (called a "maar" crater) that is generally lower than the surrounding topography. 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.

Tuff Cones

Tuff Cones are steep sloped and cone shaped. They have wide craters and are formed of highly altered, thickly bedded tephra, they are considered to be a taller variant of a "tuff ring". [ [http://vulcan.wr.usgs.gov/Glossary/Maars/description_maars.html USGS: Maars and Tuff Cones] ]

Examples of phreatomagmatic eruptions

Minoan eruption of Santorini

Santorini is part of the Southern Aegean volcanic arc, 140 km north of Crete. The Minoan eruption of Santorini, was the latest eruption and occurred in the first half of the 17th century B.C. The eruption was of predominantly rhyodacite composition.Taddeucci, J. & Wohletz, K. 2001. Temporal evolution of the Minoan eruption (Santorini, Greece), as recorded by its Plinian fall deposit and interlayered ash flow beds. Journal of Volcanology and Geothermal Research, 109, 299-317.] The Minoan eruption had four phases. Phase 1 was a white to pink pumice fallout with dispersal axis trending ESE. The deposit has a maximum thickness of 6 m and ash flow layers are interbedded at the top. Phase 2 has ash and lapilli beds that are cross stratified with mega-ripples and dune like structures. The deposit thicknesses vary from 10 cm to 12 m. Phases 3 and 4 are pyroclastic density current deposits. Phases 1 and 3 were phreatomagmatic.

Pinatubo, 1991

Mount Pinatubo is on the Central Luzon landmass between the South China Sea and the Philippine Sea. The 1991 eruption of Pinatubo was andesite and dacite in the pre-climactic phase but only dacite in the climactic phase. The climactic phase had a volume of 3.7-5.3 km³. [Rosi, M., Peladio-Melosantos, M. L., Di Muro, A., Leoni, R. & Bacolcol, T. 2001. Fall vs flow activity during the 1991 climactic eruption of Pinatubo Volcano (Philippines). Bulletin of Volcanology, 62, 549-566.] The eruption consisted of sequentially increasing ash emissions, dome growth, 4 vertical eruptions with continued dome growth, 13 pyroclastic flows and a climactic vertical eruption with associated pyroclastic flows. [Hoblitt, R. P., Wolfe, E. W., Scott, W. E., Couchman, M. R., Pallister, J. S. & Javier, D. 1996. The preclimactic eruptions of Mount Pinatubo, June 1991. In: Newhall, C. G. & Punongbayan, R. S. (eds). Fire and Mud; eruptions and lahars of Mount Pinatubo, University of Washington press, pp 457-511.] The pre-climactic phase was phreatomagmatic.

Lake Taupo

The Hatepe eruption in 180 AD was the latest major eruption at Lake Taupo in New Zealand's Taupo Volcanic Zone. 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. The water eventually stopped the eruption though large amounts of water were still erupted from the vent. The eruption resumed with phreatomagmatic activity that deposited the Rotongaio Ash. [Wilson, C. J. N. & Walker G. P. L. 1985. The Taupo Eruption, New Zealand I. General Aspects. Philosophical Transaction of the Royal Society of London, 314, 199-228.]

ee also

*Phreatic eruption
*Types of volcanic eruptions
*Volcanic ash
*Maar
*Volcanic pipe

Further reading

*Walker, G. P. L. 1971. Grain-size characteristics of pyroclastic deposits. Journal of Geology, 79, 696-714.
*Vespa, M., Keller, J. & Gertisser, R. 2006. Interplinian explosive activity of Santorini volcano (Greece) during the past 150,000 years. Journal of Volcanology and Geothermal Research, 152, 262-286.
*Riley, C. M., Rose, W. I. & Bluth, G.J.S. 2003. Quantitive shape measurements of distal volcanic ash. Journal of Geophysical Research, 108, B10, 2504.

References