Phreatomagmatic eruptions are Volcano eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain Ejecta (magmatic) .[Heiken, G. & Wohletz, K. 1985. Volcanic Ash. University of California Press, Berkeley] It is 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, such as in the
Surtsey eruption. In other cases the water may be present in a lake or
caldera-lake, as at
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
on
Tenerife are considered to be phreatomagmatic because of these circumstances.
The other competing theory is based on fuel-coolant reactions, which have been modeled for nuclear reactors. Under this theory, the fuel (in this case, the magma) fragments upon contact with a coolant (the sea, a lake or aquifer). The propagating stress waves and thermal contraction widen cracks and increase the interaction surface area, leading to explosively rapid cooling rates.
Deposits
Phreatomagmatic ash is formed by the same mechanisms across a wide range of compositions, basic and acidic. Blocky and equant
Clastic rock with low vesicle content are formed.
The deposits of phreatomagmatic explosive eruptions are also considered to be better sorted and finer grained than the deposits of magmatic eruption. This is a result of the much higher fragmentation of phreatomagmatic eruptions.
Hyaloclastite
Hyaloclastite is 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
Hyalo
tuff 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.
[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, 133–51.] 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 grain size analysis reveals that the deposits are much more poorly sorted than their magmatic counterparts. 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. They have a circular structure when specimens are viewed in hand and under the
microscope.
A further control on the Geomorphology and characteristics of a deposit is the water to magma ratio. It is considered 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, 903–26.]
Surface features
There are two types of vent
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,
and ash- and
lapilli-fall deposits. It is expected that tuff rings and tuff cones might be present on the surface of
Mars.
[Keszthelyi, L. P., W. L. Jaeger, C. M. Dundas, S. Martínez-Alonso, A. S. McEwen, and M. P. Milazzo, 2010, Hydrovolcanic features on Mars: Preliminary observations from the first Mars year of HiRISE imaging, Icarus, 205, 211–29, [1] .][Brož P., and E. Hauber, 2013, JGR-Planets, Volume 118, 8, 1656–75, " Hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on Mars" .]
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 rock density current. They are built around a
volcanic vent located in a
lake, coastal zone,
marsh or an area of abundant
groundwater.
Tuff cones
Volcanic cone 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, formed by less powerful eruptions. Tuff cones are usually small in height.
Koko Crater is 1,208 feet.
[ USGS: Maars and Tuff Cones]
Examples
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 BC. 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-
Ripple marks 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.
1991 eruption of Mount Pinatubo
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 km
3.
[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, 549–66.] The eruption consisted of sequentially increasing ash emissions, dome growth, 4 vertical eruptions with continued dome growth, 13
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 climactic 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.
Hatepe eruption
The
Hatepe eruption in 232 ± 12 AD was the latest major eruption at Lake Taupō in
New Zealand's Taupō Volcanic Zone. There was minor initial phreatomagmatic activity followed by the dry venting of 6 km
3 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 km
3 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 Transactions of the Royal Society of London, 314, 199–228. ]
Grímsvötn eruptions
The Grímsvötn volcano in Iceland is a sub-glacial volcano, located beneath the Vatnajökull ice cap. 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.
Grímsvötn is host to an active geothermal system and is prone to phreatomagmatic eruptions.
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.
See also
Further reading
-
Walker, G. P. L. 1971. Grain-size characteristics of pyroclastic deposits. Journal of Geology, 79, 696–714.
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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, 262–86.
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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.
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