Phreatomagmatic eruptions occur when rising magma interacts with external surface water such as a shallow lake, sea, or groundwater. Some authors call them hydromagmatic eruptions.
This interaction flashes water to steam, causing an explosion that will produce liquid water droplets, steam, ash, lapilli, blocks, or volcanic bombs.
We collectively called the ejected debris pyroclasts or tephra. However, some authors don’t consider these two terms to be synonyms.
Usually, phreatomagmatic eruptions will produce mostly juvenile pyroclasts, often over 95%. Juvenile means they come from erupting magma.
However, those involving underground water will have more accidental clasts. Accidental clasts are fragments derived from country rock.
Note that phreatomagmatic eruptions are not the same as phreatic eruptions. The latter occurs when magma, lava, new hot volcanic deposits, or hot rock heats underground or surface water, causing an explosion.
Therefore, phreatic eruptions will have mostly accidental clasts with minimal or no juvenile or magmatic clasts. This happens because it doesn’t involve direct magma-water interaction.
Lastly, the word phreatomagmatic comes from the Greek word phreat or phrear, meaning “water well,” and magmatic from magma. It implies that the eruption occurs from water-magma interaction.
Composition and characteristics
Phreatomagmatic eruptions can occur in any magma type, from felsic or acidic to intermediate to mafic and ultramafic.
Also, it is not limited to certain eruption types. It can occur anywhere water interacts with magma. For instance, a Plinian eruption can turn phreatomagmatic if it encounters water.
However, magma properties like viscosity and temperature may affect the extent of the explosion. Also, it will influence how well fragmentation happens.
One characteristic of phreatomagmatic eruption is that the pyroclasts formed will have finer grains than magmatic eruptions. Also, they will have regular shapes and are better sorted.
Furthermore, vitric shards or juvenile clasts may be blocky, moss-like, platy, droplet-like, or fusiform shapes. This shows the different ways in which magma interacts with water. Also, it is evidence that this external water and magma interaction drives the eruption, not volatiles.
That is not all. Phreatomagmatic eruptions show evidence of external water cooling. These include numerous accretionary lapilli, ash beds with vesicles, layers of ash plastered on objects, and rain-splash micro bedding.
Note that phreatomagmatic eruptions don’t include submarine eruptions. However, some have. An example is the highly gas-rich, like Surtsey in Iceland, which formed a new island.
How do phreatomagmatic eruptions occur?
Phreatomagmatic eruptions happen when hot, rising magma directly interacts with underground or shallow surface water. Water will flash into steam, expand, and cause an explosion.
This explosion will eject water droplets, steam, ash, lapilli, blocks, or bombs into the air. This debris may include pumice, scoria, glassy shards, or accretionary lapilli.
Also, it may cause lateral surges and, in some cases, pyroclastic flows.
Pyroclastic flows are dangerous and are fast-flowing, ground-hugging hot gas and volcanic matter. Lateral surges are a smaller, weaker version.
For instance, the Kuchinoerabujima volcano in Japan on 29 May 2015 caused a pyroclastic flow.
Apart from water, snow and ice can generate phreatomagmatic explosions. For instance, the Eyjafjallajökull eruption in 2010 was phreatomagmatic. It resulted from magma erupting under ice.
Usually, phreatomagmatic eruptions are Surtseyan- or Taalian-style. However, they can occur in any volcanism where magma interacts with water.
Also, note that exsolving magma volatiles can contribute to magma’s expansion and fragmentation in phreatomagmatic eruptions. However, their impact is less compared to eruptions like Plinian.
Let us dive deeper into this eruption. We discuss the water source, how it happens, fragmentation, and the factors influencing efficiency.
1. Water source
Water comes from the sea, like in Surtsey eruptions, from caldera-like lakes, or both, such as seen at Santorini in Greece and Minoan eruptions in Crete.
Also, magma may interact with water from aquifers.
2. How it occurs
Phreatomagmatic eruptions often involve hundreds or thousands of pulses of discrete or episodic explosions. These close-timed explosions will each eject water droplets, gas, and pyroclasts.
Furthermore, eruptions may form a continuous column or several short-lived, i.e., pulsating ones, and can go on for days to months.
The depth of the explosion will affect the way materials are ejected. Also, it may influence the composition of pyroclasts.
What about deposition? Pyroclast deposition is by fall out for those ejected into the air.
Also, small pyroclastic flows or lateral surges may help deposit formed pyroclasts. We present surges that will dominate subaerial deposits.
Sometimes, these eruptions form localized lahars. These are destructive mudflows formed on volcano slopes.
3. How fragmentation occurs
Magma fragmentation happens from thermal contraction due to water’s rapid cooling of magma. Another way is via steam expansion and the explosion at the magma-water interface.
Also, the transformation of thermal energy to mechanical energy will cause fragmentation. This happens when steam explodes after magma heats water. This explosion creates a fragmentation wave that travels into the surface of rising magma.
The fragmentation wave and contraction widened cracks, increasing the water-magma contact surface area. This results in explosively quick chilling.
3. Fragmentation efficiency
Fragmentation efficiency and resulting energy depend on how well thermal energy is transformed into mechanical and kinetic energies.
Higher eruption efficiency means more mechanical energy is released. This increases fragmentation. Thus, grain sizes will be finer, and pyroclasts will be distributed more.
Therefore, clasts’ sizes and their distribution can reflect how efficiently fragmentation happened and to what degree.
Factors influencing fragmentation efficiency include.
- The water-magma ratio is influenced by discharge rates and the amount of water available.
- Efficiency of water-magma mixing
- Magma viscosity
- Magma temperature or temperature difference
- Confining pressure or depth
- Mode of magma and water contact
Of these factors, the water-magma ratio and mixing efficiency control explosive behavior. These depend on the magma mass meeting with water, turbulence, and availability of external water.
On water-to-magma ratios, too little, like 0.1, will result in less volumetric expansions. This, in turn, reduces fragmentation.
On the other hand, too much, like 50, will overwhelm the thermal energy needed to drive phreatomagmatic fragmentation. Therefore, you will have pillow lava.
An intermediate water-to-magma of about 0.3 will result in the highest fragmentation efficiency.
Also, there is a critical pressure or depth that prevents phreatomagmatic fragmentation. When exceeded, pillow lava will form again. Also, vesiculations will be inhibited.
Occurrence and examples
Phreatomagmatic explosions are common since they occur from any eruption style, including effusive if they encounter water.
Examples of phreatomagmatic eruptions include:
- Fukutoku-Oka-no-Ba volcano, Ogasawara 2021
- Hatepe eruption in 232 ± 12 AD New Zealand
- Second phase eruption of Eyjafjallajökull in Iceland in 2010
- Hunga Tonga-Hunga Ha’apai (HTHH) volcano in Tonga on 15 January 2022
- La Palma or at the Caldera del Rey vent system in Tenerife in 1949 and
- Caldera Blanca tuff ring Canary Island
- Grímsvötn eruptions in Iceland beneath Vatnajökull ice cap in Iceland
- 1991 eruption of Mount Pinatubo
- Minoan eruption of Santorini
Phreatomagmatic eruption landforms
These eruptions will form maars, tuff cones, and tuff rings. These hydrovolcanic landforms are the most common on Earth after scoria cones.
They are abundant because they can form from anywhere and involve any eruption style so long as water is present. For instance, an effusive Hawaiian or Strombolian will become explosive if magma or lava encounters water.
Also, phreatomagmatic eruptions may form littoral cones. These rootless cones form when flowing lava encounters water or wet sediments.
Hazards
Phreatomagmatic eruptions are very hazardous. They can generate pyroclastic current or nuées ardentes, lateral surges and jets or plumes laden with tephra.
Although relatively small, they are dangerous because they can happen without warning.
Also, what was to be an effusive eruption can turn explosive upon magma or lava encountering water. Any volcanism can turn explosive if it interacts with water.
Areas like Auckland, New Zealand, set on a monogenetic field, have had many phreatomagmatic eruptions and may have more in the future.
Lastly, most of these eruptions precede larger, more hazardous eruptions.
References
- Brand, B. D. & Brož, P. (2015). Hydrovolcanic feature. In Hargitai, H. & Kereszturi, Á. (ed) Encyclopedia of planetary landforms (2nd ed. pp. 946-951). Springer.
- Zimanowski, B Büttner, R, Dellino, P. White, J. D. L & Wohletz, K. H (2015). Magma-water interaction and phreatomagmatic fragmentation, In Sigurdsson, H. (ed.) The encyclopedia of volcanoes (2nd ed. pp 473-487). Elsevier Science Publishing Co. Inc.
- Brown, R. J. & Calder, E. S. (2005). Pyroclastics. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5, pp. 386-397). Elsevier Academic
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg