Types of volcanic eruptions facts for kids

Some of the amazing things that come out of volcanoes when they erupt: a huge Plinian eruption column, Hawaiian pahoehoe flows (like ropes), and a fiery arc of lava from a Strombolian eruption.

Volcanoes erupt in many different ways, sending out hot material from a hole or crack in the ground. Scientists who study volcanoes, called volcanologists, have given names to these different types of eruptions. Often, these names come from famous volcanoes where that type of eruption was first seen. Some volcanoes might only show one type of eruption, while others can have many different types during a single period of activity.

There are three main ways volcanoes erupt:

• Magmatic eruptions are the most common type. They happen when gas inside the magma expands and pushes the magma out.
• Phreatic eruptions occur when water gets superheated by nearby magma, turning into steam. This steam then explodes, breaking apart existing rocks without releasing any new magma.
• Phreatomagmatic eruptions happen when magma and water mix directly. This is different from phreatic eruptions, where no new magma comes to the surface.

Within these main types, there are several smaller categories. The gentlest eruptions are Hawaiian and submarine ones. Next are Strombolian eruptions, followed by Vulcanian and Surtseyan eruptions. The more powerful types are Pelean eruptions and then Plinian eruptions. The strongest eruptions are called Ultra-Plinian. Subglacial and phreatic eruptions are named for how they happen, and their strength can vary. Scientists use the Volcanic Explosivity Index (VEI) to measure how strong an eruption is. This scale goes from 0 to 8, and each step up means the eruption is ten times bigger.

How Do Volcanoes Erupt?

This diagram shows how the VEI scale relates to the total amount of material a volcano throws out.

Volcanic eruptions happen because of three main reasons:

• Gases trapped in the magma expand and push the magma out. This causes magmatic eruptions.
• Steam explosions throw out bits of rock. This leads to phreatic eruptions.
• Hot magma cools very quickly when it touches water, causing it to break apart explosively. This creates phreatomagmatic eruptions.

Volcanic activity can be either explosive or effusive. Explosive eruptions are like big blasts that shoot magma and rock fragments into the air. Effusive eruptions, on the other hand, are calmer, with lava flowing out without big explosions.

The Impact of Volcanic Eruptions

Volcanic eruptions can be very different in how strong they are. On one side, you have gentle Hawaiian eruptions. These have lava fountains and runny lava flows that are usually not very dangerous. On the other side, Plinian eruptions are huge, violent, and very dangerous explosions. A single volcano can have many different eruption styles, from calm to explosive, even during one period of activity. Volcanoes don't always erupt straight up from a single hole at their top either. Some volcanoes have eruptions from their sides or from long cracks in the ground. For example, many Hawaiian eruptions start from long cracks called rift zones.

Scientists used to think that magma would mix deep inside the magma chamber for thousands of years before an eruption. However, volcanologists from Columbia University found that the 1963 eruption of Costa Rica's Irazú Volcano was likely caused by magma that traveled directly from the Earth's mantle in just a few months!

Understanding Eruption Strength: The VEI

The Volcanic Explosivity Index (VEI) is a scale from 0 to 8 that measures how strong an eruption is. It's used by the Smithsonian Institution's Global Volcanism Program to understand the impact of past and present lava flows. The VEI works like the Richter scale for earthquakes. Each number on the scale means the eruption is ten times bigger than the one before it. Most volcanic eruptions are between VEI 0 and 2.

Magmatic Eruptions: When Magma Explodes

Magmatic eruptions happen when gas inside the magma expands and causes explosions. These can range from small lava fountains in Hawaii to huge Ultra-Plinian eruption columns that reach over 30 km (19 mi) high. That's even bigger than the eruption of Mount Vesuvius in 79 AD that buried the ancient city of Pompeii!

Hawaiian Eruptions: Gentle Lava Flows

A diagram showing a Hawaiian eruption. (key: 1. Ash plume 2. Lava fountain 3. Crater 4. Lava lake 5. Fumaroles 6. Lava flow 7. Layers of lava and ash 8. Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12. Dike) .

Hawaiian eruptions are named after the volcanoes in Hawaii, like Mauna Loa, where this type of eruption is very common. They are the calmest kind of volcanic event. They involve very runny, basalt-type lava with not much gas. Hawaiian eruptions produce less material than other types. The steady flow of small amounts of lava builds up the wide, gently sloping shape of a shield volcano. These eruptions don't just happen at the main top of the volcano. They often come from vents around the summit and from long cracks called fissure vents that spread out from the center.

Hawaiian eruptions often start as a line of vents along a fissure, creating a "curtain of fire." As time goes on, the lava usually starts to come out from just a few of these vents. Eruptions from the main vent often look like large lava fountains, which can shoot hundreds of meters high. The bits of lava from these fountains usually cool in the air and fall as cindery scoria fragments. If there are a lot of hot bits in the air, they might not cool fast enough and can form spatter cones when they land. If a lot of lava comes out quickly, it can even form lava flows fed by the spatter. Hawaiian eruptions can last a very long time. For example, Puʻu ʻŌʻō on Kilauea erupted continuously for over 35 years! Another cool feature of Hawaiian volcanoes is the formation of active lava lakes, which are pools of hot lava with a thin crust on top.

Ropey pahoehoe lava from Kilauea, Hawaiʻi.

The lava from Hawaiian eruptions is basaltic and comes in two main types. Pahoehoe lava is smooth and can look like ropes or billows. It can move as a sheet, with "toes" pushing forward, or as a snaking column. A'a lava flows are thicker and move slower than pahoehoe. They can be 2 to 20 m (7 to 66 ft) thick. A'a flows are so thick that their outer layers cool into a rough, rocky mass. This outer layer acts like insulation, keeping the inside hot. A'a lava moves in a unique way: the front of the flow gets steep from pressure behind it until it breaks off, and then the rest of the lava moves forward. Pahoehoe lava can sometimes turn into A'a lava if it gets thicker or moves faster, but A'a lava never turns back into pahoehoe.

Hawaiian eruptions create some unique volcanic features. Small bits of volcanic material are carried by the wind and cool quickly into teardrop-shaped glassy fragments called Pele's tears (named after Pele, the Hawaiian volcano goddess). When the wind is very strong, these bits can stretch into long, thin strands known as Pele's hair. Sometimes, basalt lava becomes reticulite, which is the lightest rock type on Earth.

Even though they're named after Hawaii, Hawaiian eruptions can happen elsewhere. The tallest lava fountain ever recorded was during an eruption of Mount Etna in Italy on November 23, 2013. It reached a steady height of about 2,500 m (8,200 ft) for 18 minutes, and briefly peaked at 3,400 m (11,000 ft)!

Volcanoes known for Hawaiian activity include:

• Puʻu ʻŌʻō, a smaller cone on Kilauea in Hawaiʻi. It erupted continuously from 1983 to 2018. The eruptions started with a 6 km (4 mi)-long "curtain of fire" on January 3, 1983. Later, the eruptions focused on one spot, building up the cone.
• Mount Etna, Italy.
• Mount Mihara in 1986.

Strombolian Eruptions: Popping Gas Bubbles

A diagram showing a Strombolian eruption. (key: 1. Ash plume 2. Lapilli 3. Volcanic ash rain 4. Lava fountain 5. Volcanic bomb 6. Lava flow 7. Layers of lava and ash 8. Stratum 9. Dike 10. Magma conduit 11. Magma chamber 12. Sill) .

Strombolian eruptions are named after the volcano Stromboli, which has been erupting almost non-stop for hundreds of years. These eruptions happen when gas bubbles inside the magma burst. These gas bubbles join together to form larger bubbles, called gas slugs. These slugs get big enough to rise through the lava. When they reach the surface, the change in air pressure makes the bubble pop loudly, throwing magma into the air like a soap bubble. Because the lava has a lot of gas pressure, these eruptions keep happening in short, explosive bursts with loud sounds, often every few minutes.

The word "Strombolian" has been used for many different types of eruptions, from small blasts to large columns of material. But true Strombolian eruptions are short, explosive bursts of moderately thick lava, often thrown high into the air. These columns can reach hundreds of meters tall. The lava from Strombolian eruptions is a type of thick basaltic lava, and it mostly forms scoria (bubbly, dark rock). Because Strombolian eruptions are relatively gentle and don't damage the volcano much, they can continue for thousands of years, making them one of the least dangerous eruption types.

An example of the fiery lava arcs created during Strombolian activity. This picture is of Stromboli itself.

Strombolian eruptions throw out volcanic bombs (large pieces of lava) and lapilli (smaller fragments) that fly in curved paths before landing around the vent. The steady build-up of these small fragments creates cinder cones, which are made entirely of basaltic pyroclasts (rock fragments). This process usually forms neat rings of tephra (volcanic debris).

Strombolian eruptions are similar to Hawaiian eruptions, but they have differences. Strombolian eruptions are louder, don't create long-lasting eruption columns, and don't produce things like Pele's tears or Pele's hair. They also produce fewer molten lava flows, though the erupted material can form small streams.

Volcanoes known for Strombolian activity include:

• Parícutin, Mexico. This volcano erupted from a crack in a cornfield in 1943. After two years, the explosive activity slowed down, and lava started flowing from its base. Eruptions stopped in 1952, and the volcano reached a height of 424 m (1,391 ft). This was the first time scientists could watch a volcano's entire life cycle.
• Mount Etna, Italy, which has shown Strombolian activity in recent eruptions, like in 1981, 1999, 2002–2003, and 2009.
• Mount Erebus in Antarctica, the southernmost active volcano in the world, which has been erupting since 1972 with frequent Strombolian activity.
• Stromboli itself. This volcano, which gives its name to this type of eruption, has been active throughout history. It has had almost continuous Strombolian eruptions, sometimes with lava flows, for over a thousand years.

Vulcanian Eruptions: Explosive Blasts

A diagram showing a Vulcanian eruption. (key: 1. Ash plume 2. Lapilli 3. Lava fountain 4. Volcanic ash rain 5. Volcanic bomb 6. Lava flow 7. Layers of lava and ash 8. Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12. Dike)

Vulcanian eruptions are named after the volcano Vulcano in Italy, after scientist Giuseppe Mercalli observed its eruptions from 1888–1890. In Vulcanian eruptions, the magma is thick, making it hard for trapped gases to escape. Like Strombolian eruptions, this causes high gas pressure to build up. Eventually, this pressure breaks the "cap" holding the magma down, leading to an explosive eruption. Unlike Strombolian eruptions, the lava fragments thrown out are not smooth or aerodynamic. This is because Vulcanian magma is thicker and contains more broken rock from the old cap. These eruptions are also more explosive than Strombolian ones, with eruptive columns often reaching between 5 and 10 km (3 and 6 mi) high. Also, the material from Vulcanian eruptions is usually andesitic to dacitic, not basaltic.

Vulcanian activity usually starts with a series of short, powerful explosions, lasting from a few minutes to a few hours. These blasts throw out volcanic bombs and large blocks. These eruptions break down the lava dome that was holding the magma in. Once the dome breaks apart, the eruptions can become quieter and more continuous. So, a growing lava dome can be a sign that a Vulcanian eruption is coming. When the dome collapses, it sends a lot of pyroclastic material down the volcano's slopes.

Tavurvur volcano in Papua New Guinea erupting.

Close to the vent, you'll find large volcanic blocks and bombs. "Bread-crust bombs" are very common. These deeply cracked volcanic chunks form when the outside of the ejected lava cools quickly into a glassy or fine-grained shell, but the inside keeps cooling and expanding with gas. The expanding center cracks the outside. Most of the material from Vulcanian eruptions is fine ash. This ash doesn't spread very far, and its large amount shows that the magma broke into many small pieces due to high gas content. Sometimes, this is caused by water mixing with the magma, suggesting that some Vulcanian eruptions are partly hydrovolcanic (involving water).

Volcanoes that have shown Vulcanian activity include:

• Sakurajima, Japan, which has had almost continuous Vulcanian activity since 1955.
• Tavurvur, Papua New Guinea, one of several volcanoes in the Rabaul Caldera.
• Irazú Volcano in Costa Rica had Vulcanian activity during its 1963–1965 eruption.
• Anak Krakatoa, Indonesia, has had repeated Vulcanian activities since it formed in 1930.

Vulcanian eruptions are thought to make up at least half of all known eruptions in the last 10,000 years.

Peléan Eruptions: Deadly Pyroclastic Flows

A diagram showing a Peléan eruption. (key: 1. Ash plume 2. Volcanic ash rain 3. Lava dome 4. Volcanic bomb 5. Pyroclastic flow 6. Layers of lava and ash 7. Stratum 8. Magma conduit 9. Magma chamber 10. Dike) .

Peléan eruptions are named after Mount Pelée in Martinique. In 1902, a Peléan eruption there caused one of the worst natural disasters in history. In these eruptions, a huge amount of gas, dust, ash, and lava fragments are blasted out of the volcano's main crater. This is often caused by the collapse of thick lava domes made of rhyolite, dacite, and andesite, which can create large eruptive columns. A sign that a Peléan eruption is coming is the growth of a "Peléan spine" or "lava spine," which is a bulge at the volcano's top before it completely collapses. The material from the eruption then falls down, forming a fast-moving pyroclastic flow (a mix of rock blocks and ash). These flows can rush down the mountain at incredible speeds, often over 150 km (93 mi) per hour. These fast-moving landslides make Peléan eruptions some of the most dangerous in the world. They can destroy towns and cause many deaths. The 1902 eruption of Mount Pelée caused terrible destruction, killing over 30,000 people and completely destroying the town of St. Pierre. It was the worst volcanic event in the 20th century.

Peléan eruptions are best known for the glowing hot pyroclastic flows they create. The way a Peléan eruption works is very similar to a Vulcanian eruption. However, in Peléan eruptions, the volcano's structure can hold more pressure. This means the eruption happens as one very large explosion instead of several smaller ones.

Volcanoes known for Peléan activity include:

• Mount Pelée, Martinique. The 1902 eruption destroyed the island, leaving only 3 survivors in St. Pierre. Lava dome growth happened right before the eruption.
• Mayon Volcano, the Philippines' most active volcano. It has had many different types of eruptions, including Peléan ones. About 40 ravines spread out from its top, providing paths for frequent pyroclastic flows and mudflows to the lowlands below. Mayon's most violent eruption in 1814 killed over 1200 people.
• The 1951 eruption of Mount Lamington. Before this eruption, people didn't even know it was a volcano. Over 3,000 people died, and it has become a key example for studying large Peléan eruptions.
• Mount Sinabung, Indonesia. Since 2013, this volcano has been emitting pyroclastic flows with frequent collapses of its lava domes.

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  Pyroclastic flows at Mayon Volcano, Philippines, 1984.

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  The lava spine that grew after the 1902 eruption of Mount Pelée.

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  Mount Lamington after the terrible 1951 eruption.

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  The 2016 eruption of Mount Sinabung.

Plinian Eruptions: Giant Ash Columns

A diagram showing a Plinian eruption. (key: 1. Ash plume 2. Magma conduit 3. Volcanic ash rain 4. Layers of lava and ash 5. Stratum 6. Magma chamber) .

Plinian eruptions are named after the famous eruption of Mount Vesuvius in 79 AD, which buried the Roman towns of Pompeii and Herculaneum. They are specifically named after Pliny the Younger, who wrote about the event. Plinian eruptions start deep in the magma chamber. Gases dissolved in the magma form bubbles as they rise through the magma conduit. These bubbles join together, and once they fill about 75% of the conduit, they explode! The narrow space of the conduit forces the gases and magma upwards, forming a huge eruptive column. The speed of the eruption is controlled by how much gas is in the column. Sometimes, the rocks at the surface crack under the pressure, creating a wider opening that pushes the gases out even faster.

These massive columns are the main feature of a Plinian eruption. They can reach 2 to 45 km (1 to 28 mi) high into the atmosphere. The thickest part of the plume, right above the volcano, is pushed up by the expanding gas. As it gets higher, the plume spreads out and becomes less dense. Heat and expanding volcanic ash push it even further up into the stratosphere. At the very top, strong winds can carry the plume away from the volcano.

The eruption column from Redoubt Volcano on April 21, 1990, seen from the Kenai Peninsula.

These very explosive eruptions usually involve gas-rich dacitic to rhyolitic lavas and happen most often at stratovolcanoes. Eruptions can last from hours to days, with longer eruptions happening at volcanoes with more felsic (silica-rich) magma. Even though they are usually linked to felsic magma, Plinian eruptions can happen at basaltic volcanoes if the magma chamber has layers with more silica at the top, or if magma rises very quickly.

Plinian eruptions are similar to both Vulcanian and Strombolian eruptions, but instead of separate explosions, Plinian eruptions create continuous eruption columns. They are also like Hawaiian lava fountains because both types produce steady eruption columns.

Areas affected by Plinian eruptions get covered in a thick layer of pumice (light, porous volcanic rock) and ash. This can cover an area from 0.5 to 50 km³ (0 to 12 cu mi) in size. The material in the ash plume eventually falls back to the ground, covering the landscape in many cubic kilometers of ash.

Lahar flows from the 1985 eruption of Nevado del Ruiz completely destroyed the town of Armero in Colombia.

The most dangerous part of Plinian eruptions are the pyroclastic flows. These are created when the eruption column collapses. They rush down the side of the mountain at extreme speeds, up to 700 km (435 mi) per hour, and can travel hundreds of kilometers. The hot material from the volcano's top melts snow and ice, which mixes with tephra to form lahars. Lahars are fast-moving mudflows, like wet concrete, that move as fast as a river rapid.

Major Plinian eruptions include:

• The AD 79 eruption of Mount Vesuvius buried the Roman towns of Pompeii and Herculaneum under ash and tephra. It's the classic example of a Plinian eruption. Mount Vesuvius has erupted many times since then. Its last eruption was in 1944. The report by Pliny the Younger is why scientists call these "Plinian" eruptions.
• The 1980 eruption of Mount St. Helens in Washington, which tore apart the volcano's summit, was a VEI 5 Plinian eruption.
• The strongest eruptions, with a VEI of 8, are called "Ultra-Plinian" eruptions. An example is the eruption at Lake Toba 74,000 years ago, which released 2800 times more material than Mount St. Helens in 1980.
• Hekla in Iceland had a basaltic Plinian eruption in 1947–48. For the past 800 years, it has followed a pattern of violent initial eruptions of pumice followed by long flows of basaltic lava from the lower part of the volcano.
• Pinatubo in the Philippines on June 15, 1991. It produced 5 km³ (1 cu mi) of dacitic magma, a 40 km (25 mi) high eruption column, and released 17 megatons of sulfur dioxide.
• Kelud, Indonesia, erupted in 2014 and ejected about 120,000,000 to 160,000,000 cubic metres (4.2×10⁹ to 5.7×10⁹ cu ft) of volcanic ash, causing economic problems across Java.

Phreatomagmatic Eruptions: Water Meets Magma

Phreatomagmatic eruptions happen when water and magma interact. They are caused by the magma shrinking very quickly when it touches water. This is different from magmatic eruptions, which are caused by gas expansion. The big temperature difference between the water and magma causes violent interactions that lead to the eruption. The material from phreatomagmatic eruptions is thought to be more uniform in shape and finer-grained than material from magmatic eruptions, because of the different ways they erupt.

Scientists are still debating the exact details of phreatomagmatic eruptions. Some believe that "fuel-coolant reactions" might be more important for the explosive nature than just quick shrinking. These reactions might break the volcanic material into tiny pieces by sending out shock waves, making cracks wider and increasing the surface area. This leads to very fast cooling and explosive eruptions driven by shrinking.

Surtseyan Eruptions: Explosions in Shallow Water

A diagram showing a Surtseyan eruption. (key: 1. Water vapor cloud 2. Compressed ash 3. Crater 4. Water 5. Layers of lava and ash 6. Stratum 7. Magma conduit 8. Magma chamber 9. Dike) .

A Surtseyan eruption is a type of volcanic eruption that happens when water and lava interact in shallow water. It's named after its most famous example, the eruption and formation of the island of Surtsey off the coast of Iceland in 1963. Surtseyan eruptions are like the "wet" version of land-based Strombolian eruptions, but because they happen in water, they are much more explosive. As water is heated by lava, it quickly turns into steam and expands violently. This breaks the magma it touches into fine ash. Surtseyan eruptions are common for volcanoes that form oceanic islands in shallow water, but they can also happen on land. This occurs when rising magma comes into contact with an aquifer (a rock layer holding water) close to the surface under the volcano. The material from Surtseyan eruptions is usually oxidized palagonite basalts (though andesitic eruptions happen rarely). Like Strombolian eruptions, Surtseyan eruptions are generally continuous or happen in regular bursts.

A key feature of a Surtseyan eruption is the formation of a pyroclastic surge (also called a base surge). This is a cloud that spreads out along the ground, along with the eruption column. Base surges are caused by the eruption column, which is full of vapor, collapsing under its own weight. This column is denser than a regular volcanic column. The densest part of the cloud is closest to the vent, making it wedge-shaped. Along with these sideways-moving rings, you'll find dune-shaped deposits of rock left behind by the movement. These deposits are sometimes disturbed by "bomb sags," which are dents made by rocks that were thrown out by the explosion and fell back to the ground. Piles of wet, round ash called accretionary lapilli are another common sign of a surge.

Over time, Surtseyan eruptions tend to form maars, which are wide, low volcanic craters dug into the ground, and tuff rings, which are circular structures made of quickly cooled lava. These structures are formed by eruptions from a single vent. If eruptions happen along fracture zones, long cracks might be formed. Such eruptions tend to be more violent than those that form tuff rings or maars, like the 1886 eruption of Mount Tarawera. Littoral cones are another feature caused by water and volcanoes. They are created by explosive deposits of basaltic tephra, though they aren't true volcanic vents. They form when lava builds up in cracks, superheats, and explodes in a steam explosion, breaking the rock apart and depositing it on the volcano's side. Repeated explosions like this eventually build the cone.

Volcanoes known for Surtseyan activity include:

• Surtsey, Iceland. This volcano grew from the ocean floor and appeared above the Atlantic Ocean off the coast of Iceland in 1963. The first eruptions involving water were very explosive. But as the volcano grew, the rising lava interacted less with water and more with air. Eventually, the Surtseyan activity decreased and became more like Strombolian eruptions.
• Ukinrek maars in Alaska, 1977, and Capelinhos in the Azores, 1957, are both examples of Surtseyan activity happening above water.
• Mount Tarawera in New Zealand erupted along a rift zone in 1886, killing 150 people.
• Ferdinandea, an underwater mountain in the Mediterranean Sea, rose above sea level in July 1831. It caused a dispute over who owned it between Italy, France, and Great Britain. The volcano didn't build strong enough tuff cones to resist erosion and soon disappeared back below the waves.
• The underwater volcano Hunga Tonga in Tonga rose above sea level in 2009. Both of its vents showed Surtseyan activity for much of the time. It also had an earlier eruption in May 1988.

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  Surtsey, erupting 13 days after appearing above the water. A tuff ring surrounds the vent.

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  The crack formed by the 1886 eruption of Mount Tarawera, an example of a fracture zone eruption.

Submarine Eruptions: Volcanoes Under the Sea

A diagram showing a Submarine eruption. (key: 1. Water vapor cloud 2. Water 3. Stratum 4. Lava flow 5. Magma conduit 6. Magma chamber 7. Dike 8. Pillow lava) .

Submarine eruptions happen underwater. It's estimated that 75% of all volcanic material comes from submarine eruptions, especially near mid ocean ridges. It was hard to detect deep-sea volcanic eruptions, so their details were mostly unknown until new technology in the 1990s made it possible to observe them.

Submarine eruptions can create seamounts (underwater mountains), which might eventually break the surface and form volcanic islands.

Underwater volcanism is caused by different processes. Volcanoes near plate boundaries and mid-ocean ridges are built when mantle rock rises and melts as pressure decreases. Eruptions linked to subducting zones (where one plate slides under another) happen when the sinking plate adds gases to the rising plate, lowering its melting point. Each process creates different types of rock. Mid-ocean ridge volcanoes mainly produce basalt, while subduction zone flows are mostly calc-alkaline, which are more explosive and thicker.

The speed at which mid-ocean ridges spread varies a lot. It can be 2 cm (0.8 in) per year at the Mid-Atlantic Ridge or up to 16 cm (6 in) along the East Pacific Rise. Faster spreading rates likely lead to more volcanism. The technology to study seamount eruptions didn't exist until hydrophones (underwater microphones) made it possible to "listen" to sound waves, called T-waves. These waves are released by submarine earthquakes linked to underwater volcanic eruptions. Land-based seismometers can't detect sea-based earthquakes smaller than a magnitude of 4, but sound waves travel well in water over long distances. A system in the North Pacific, used by the United States Navy to detect submarines, has found an event every 2 to 3 years on average.

The most common type of underwater lava flow is pillow lava, which is rounded and named for its unusual shape. Less common are glassy, flat sheet flows, which show that larger amounts of lava are flowing. Volcanic sedimentary rocks are common in shallow-water areas. As plate movement carries volcanoes away from where they erupted, the eruption rates slow down, and water erosion wears the volcano down. The final stages of eruption cover the seamount in alkalic flows. There are about 100,000 deepwater volcanoes in the world, but most are no longer active. Some examples of seamounts are Kamaʻehuakanaloa (formerly Loihi), Bowie Seamount, Davidson Seamount, and Axial Seamount.

Subglacial Eruptions: Volcanoes Under Ice

A diagram of a Subglacial eruption. (key: 1. Water vapor cloud 2. Crater lake 3. Ice 4. Layers of lava and ash 5. Stratum 6. Pillow lava 7. Magma conduit 8. Magma chamber 9. Dike) .

Subglacial eruptions happen when lava and ice interact, often under a glacier. This type of volcanism occurs in very cold, high places. It's thought that subglacial volcanoes that aren't actively erupting often release heat into the ice above them, creating meltwater. This mix of meltwater means that subglacial eruptions often cause dangerous jökulhlaups (floods) and lahars (mudflows).

Studying volcanoes under ice is a relatively new field. Early reports described unusual flat-topped, steep-sided volcanoes called tuyas in Iceland. It was suggested that these formed from eruptions under ice. The first English paper on this topic was published in 1947 by William Henry Mathews. He described the Tuya Butte field in northwest British Columbia, Canada. The process that builds these structures, as first guessed in the paper, starts with the volcano growing under the glacier. At first, the eruptions are like those in the deep sea, forming piles of pillow lava at the base of the volcano. Some of the lava shatters when it touches the cold ice, forming a glassy, broken rock called hyaloclastite. After a while, the ice finally melts into a lake, and the more explosive Surtseyan activity begins, building up sides made mostly of hyaloclastite. Eventually, the lake boils away from the continuous volcanism, and the lava flows become more effusive and thicker as the lava cools much slower, often forming columnar jointing (rock columns). Well-preserved tuyas show all these stages, like Hjorleifshofdi in Iceland.

The results of volcano-ice interactions create various structures. Their shape depends on complex interactions between the eruption and the environment. Glacial volcanism is a good way to figure out where ice used to be, making it an important climate marker. Since these structures are embedded in ice, there are worries that as glaciers worldwide melt, tuyas and other structures might become unstable, leading to huge landslides. Evidence of volcanic-glacial interactions is clear in Iceland and parts of British Columbia. It's even possible that they play a role in the melting of ice during ice ages.

Herðubreið, a tuya volcano in Iceland.

Volcanic products from under ice have been found in Iceland, British Columbia (Canada), the U.S. states of Hawaii and Alaska, the Cascade Range in western North America, South America, and even on the planet Mars. Volcanoes known for subglacial activity include:

• Mauna Kea in tropical Hawaii. There's evidence of past subglacial eruptions on this volcano's summit, in the form of a deposit from about 10,000 years ago, during the last ice age, when Mauna Kea's summit was covered in ice.
• In 2008, the British Antarctic Survey reported a volcanic eruption under the Antarctica ice sheet 2,200 years ago. This is believed to be the biggest eruption in Antarctica in the last 10,000 years. Volcanic ash from this volcano was found using an airborne radar survey, buried under later snowfalls in the Hudson Mountains, near Pine Island Glacier.
• Iceland, known for both glaciers and volcanoes, often has subglacial eruptions. An example is an eruption under the Vatnajökull ice cap in 1996, which happened under an estimated 2,500 ft (762 m) of ice.
• As part of the search for life on Mars, scientists have suggested that there might be subglacial volcanoes on the red planet. Several possible sites for such volcanism have been studied and compared to similar features in Iceland. Scientists believe that microbes could live in deep, hot groundwater under Mars's polar regions, where subglacial volcanism might have occurred.

Phreatic Eruptions: Steam Blasts

A diagram showing a phreatic eruption. (key: 1. Water vapor cloud 2. Magma conduit 3. Layers of lava and ash 4. Stratum 5. Water table 6. Explosion 7. Magma chamber)

Phreatic eruptions (or steam-blast eruptions) are caused by expanding steam. When cold ground or surface water touches hot rock or magma, it gets superheated and explodes. This breaks the surrounding rock and shoots out a mix of steam, water, ash, volcanic bombs, and volcanic blocks. The key thing about phreatic explosions is that they only blast out pieces of existing solid rock from the volcano's opening; no new magma comes out. Because they are caused by rock cracking under pressure, phreatic activity doesn't always lead to a full eruption. If the rock is strong enough, an eruption might not happen, but cracks will likely form, making future eruptions more likely.

Phreatic eruptions are often a sign that more volcanic activity is coming. They are usually weak, but there have been exceptions. Some phreatic events can be triggered by earthquakes, another sign of volcanic activity. They can also happen along dike lines (cracks filled with magma). Phreatic eruptions can create base surges, lahars, avalanches, and a "rain" of volcanic blocks. They can also release deadly toxic gas that can suffocate anyone nearby.

Volcanoes known to show phreatic activity include:

• Mount St. Helens, which had phreatic activity just before its huge 1980 eruption (which was a Plinian eruption).
• Taal Volcano, Philippines, in 1965 and 2020.
• La Soufrière of Guadeloupe (Lesser Antilles), activity in 1975–1976.
• Soufrière Hills volcano on Montserrat, West Indies, from 1995–2012.
• Poás Volcano, which has frequent geyser-like phreatic eruptions from its crater lake.
• Mount Bulusan, known for its sudden phreatic eruptions.
• Mount Ontake, all historical eruptions of this volcano have been phreatic, including the deadly 2014 eruption.
• Mount Kerinci, Indonesia, which has almost annual phreatic eruptions.

See also

In Spanish: Erupción volcánica para niños

• List of volcanic eruptions in the 21st century
• List of Quaternary volcanic eruptions
• Prediction of volcanic activity
• Timeline of volcanism on Earth