Phreatomagmatic eruptions are

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plat ...

eruptions resulting from interaction between

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts.Heiken, G. & Wohletz, K. 1985. Volcanic Ash. University of California Press, Berkeley It is common for a large

explosive eruption In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma su ...

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 with

Surtsey Surtsey (" Surtr's island" in Icelandic, ) is a volcanic island located in the Vestmannaeyjar archipelago off the southern coast of Iceland. At Surtsey is the southernmost point of Iceland. It was formed in a volcanic eruption which began ...

. In other cases the water may be present in a lake or

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

-lake, for example

Santorini Santorini ( el, Σαντορίνη, ), officially Thira ( Greek: Θήρα ) and classical Greek Thera (English pronunciation ), is an island in the southern Aegean Sea, about 200 km (120 mi) southeast from the Greek mainland. It is t ...

, 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 cone A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruption ...

s on

Tenerife Tenerife (; ; formerly spelled ''Teneriffe'') is the largest and most populous island of the Canary Islands. It is home to 43% of the total population of the Archipelago, archipelago. With a land area of and a population of 978,100 inhabitant ...

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. 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 BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...

and acidic. Blocky and equant clasts with low

vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...

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 Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low- viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More tha ...

that were produced by non-explosive

quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ...

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 Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

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

aquifers An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

). Hyalotuffs have a layered nature that is considered to be a result of dampened oscillation in discharge rate, with a period of several minutes. 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 Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range ...

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 microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...

. A further control on the morphology 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

landform A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, ...

s from the explosive interaction of magma and ground or surface water; ''

cones'' and ''tuff rings.'' Both of the landforms are associated with monogenetic volcanoes and

polygenetic volcanoes Polygenesis can refer to: * Polygenesis (linguistics), a theory of language origin * Polygenism, a theory of human origin * Gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through w ...

. In the case of polygenetic volcanoes they are often interbedded with lavas,

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surro ...

s and ash- and

-fall deposits. It is expected that tuff rings and tuff cones might be present on the surface of

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

Tuff rings

Tuff rings have a low profile apron of

tephra Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they r ...

surrounding a wide crater (called a ''

maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallo ...

'' crater) that is generally lower than the surrounding topography. The

is often unaltered and thinly bedded, and is generally considered to be an

, or the product of a

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

density current. They are built around a volcanic vent located in a

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

, coastal zone,

marsh A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found ...

or an area of abundant

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...

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, formed by less powerful eruptions. Tuff cones are usually small in height.

Koko Crater Koko Crater ( Hawaiian: ''Kohelepelepe'' or ''Puʻu Mai'') is an extinct tuff cone located on the Hawaiian island of Oahu near Hawaii Kai. It is northeast of Hanauma Bay and south of the Koʻolau Range. Geology Koko Crater is a part of the ...

is 1,208 feet.

Examples of phreatomagmatic eruptions

Minoan eruption of Santorini

is part of the Southern Aegean volcanic arc, 140 km north of

Crete Crete ( el, Κρήτη, translit=, Modern: , Ancient: ) is the largest and most populous of the Greek islands, the 88th largest island in the world and the fifth largest island in the Mediterranean Sea, after Sicily, Sardinia, Cyprus, ...

. The

Minoan eruption The Minoan eruption was a catastrophic volcanic eruption that devastated the Aegean island of Thera (also called Santorini) circa 1600 BCE. It destroyed the Minoan settlement at Akrotiri, as well as communities and agricultural areas on ne ...

of Santorini, was the latest eruption and occurred in the first half of the 17th century BC. The eruption was of predominantly

rhyodacite Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from ra ...

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

beds that are cross stratified with mega- ripples and

dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...

-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 The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Ph ...

and the

Philippine Sea The Philippine Sea is a marginal sea of the Western Pacific Ocean east of the Philippine archipelago (hence the name), the largest in the world, occupying an estimated surface area of . The Philippine Sea Plate forms the floor of the sea. I ...

. The 1991 eruption of Pinatubo was

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

and

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained ( aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyo ...

in the pre-climactic phase but only dacite in the climactic phase. The climactic phase had a volume of 3.7–5.3 km³. The eruption consisted of sequentially increasing ash emissions, dome growth, 4 vertical eruptions with continued dome growth, 13

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

s and a climactic vertical eruption with associated pyroclastic flows. The pre-climactic phase was phreatomagmatic.

Hatepe eruption

The

Hatepe eruption The Hatepe eruption, named for the Hatepe Plinian pumice tephra layer, sometimes referred to as the Taupō eruption or Horomatangi Reef Unit Y eruption, is dated to 232 CE ± 10 and was Taupō Volcano's most recent major eruption. It is tho ...

in 232 ± 12 AD was the latest major eruption at Lake Taupo in

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...

's Taupo Volcanic Zone. There was minor initial phreatomagmatic activity followed by the dry venting of 6 km³ of

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...

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 Transactions of the Royal Society of London, 314, 199–228.