Types of Volcanic Eruptions

Volcanic Eruptions

The most common type of volcanic eruption occurs when magma (the term for lava when it is below the Earth’s surface) is released from a volcanic vent. Eruptions can be effusive, where lava flows like a thick, sticky liquid, or explosive, where fragmented lava explodes out of a vent. In explosive eruptions, the fragmented rock may be accompanied by ash and gases; in effusive eruptions, degassing is common but ash is usually not.

Volcanologists classify eruptions into several different types. Some are named for particular volcanoes where the type of eruption is common; others concern the resulting shape of the eruptive products or the place where the eruptions occur. Here are some of the most common types of eruptions:

Hawaiian Eruption

In a Hawaiian eruption, fluid basaltic lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit or on the flank of a volcano. The jets can last for hours or even days, a phenomenon known as fire fountaining. The spatter created by bits of hot lava falling out of the fountain can melt together and form lava flows, or build hills called spatter cones. Lava flows may also come from vents at the same time as fountaining occurs, or during periods where fountaining has paused. Because these flows are very fluid, they can travel miles from their source before they cool and harden.

Hawaiian eruptions get their names from the Kilauea volcano on the Big Island of Hawaii, which is famous for producing spectacular fire fountains. Two excellent examples of these are the 1969-1974 Mauna Ulu eruption on the volcano’s flank, and the 1959 eruption of the Kilauea Iki Crater at the summit of Kilauea. In both of these eruptions, lava fountains reached heights of well over a thousand feet.

Strombolian Eruption

Strombolian eruptions are distinct bursts of fluid lava (usually basalt or basaltic andesite) from the mouth of a magma-filled summit conduit. The explosions usually occur every few minutes at regular or irregular intervals. The explosions of lava, which can reach heights of hundreds of meters, are caused by the bursting of large bubbles of gas, which travel upward in the magma-filled conduit until they reach the open air.

This kind of eruption can create a variety of forms of eruptive products: spatter, or hardened globs of glassy lava; scoria, which are hardened chunks of bubbly lava; lava bombs, or chunks of lava a few cm to a few m in size; ash; and small lava flows (which form when hot spatter melts together and flows downslope). Products of an explosive eruption are often collectively called tephra.

Strombolian eruptions are often associated with small lava lakes, which can build up in the conduits of volcanoes. They are one of the least violent of the explosive eruptions, although they can still be very dangerous if bombs or lava flows reach inhabited areas. Strombolian eruptions are named for the volcano that makes up the Italian island of Stromboli, which has several erupting summit vents. These eruptions are particularly spectacular at night, when the lava glows brightly.

Vulcanian Eruption

A Vulcanian eruption is a short, violent, relatively small explosion of viscous magma (usually andesite, dacite, or rhyolite). This type of eruption results from the fragmentation and explosion of a plug of lava in a volcanic conduit, or from the rupture of a lava dome (viscous lava that piles up over a vent). Vulcanian eruptions create powerful explosions in which material can travel faster than 350 meters per second (800 mph) and rise several kilometers into the air. They produce tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas and rock that flow almost like fluids).

Vulcanian eruptions may be repetitive and go on for days, months, or years, or they may precede even larger explosive eruptions. They are named for the Italian island of Vulcano, where a small volcano that experienced this type of explosive eruption was thought to be the vent above the forge of the Roman smith god Vulcan.

Plinian Eruption

The largest and most violent of all the types of volcanic eruptions are Plinian eruptions. They are caused by the fragmentation of gassy magma, and are usually associated with very viscous magmas (dacite and rhyolite). They release enormous amounts of energy and create eruption columns of gas and ash that can rise up to 50 km (35 miles) high at speeds of hundreds of meters per second. Ash from an eruption column can drift or be blown hundreds or thousands of miles away from the volcano. The eruption columns are usually shaped like a mushroom (similar to a nuclear explosion) or an Italian pine tree; Pliny the Younger, a Roman historian, made the comparison while viewing the 79 AD eruption of Mount Vesuvius, and Plinian eruptions are named for him.

Plinian eruptions are extremely destructive, and can even obliterate the entire top of a mountain, as occurred at Mount St. Helens in 1980. They can produce falls of ash, scoria and lava bombs miles from the volcano, and pyroclastic density currents that raze forests, strip soil from bedrock and obliterate anything in their paths. These eruptions are often climactic, and a volcano with a magma chamber emptied by a large Plinian eruption may subsequently enter a period of inactivity.

Lava Domes

Lava domes form when very viscous, rubbly lava (usually andesite, dacite or rhyolite) is squeezed out of a vent without exploding. The lava piles up into a dome, which may grow by inflating from the inside or by squeezing out lobes of lava (something like toothpaste coming out of a tube). These lava lobes can be short and blobby, long and thin, or even form spikes that rise tens of meters into the air before they fall over. Lava domes may be rounded, pancake-shaped, or irregular piles of rock, depending on the type of lava they form from.

Lava domes are not just passive piles of rock; they can sometimes collapse and form pyroclastic density currents, extrude lava flows, or experience small and large explosive eruptions (which may even destroy the domes!) A dome-building eruption may go on for months or years, but they are usually repetitive (meaning that a volcano will build and destroy several domes before the eruption ceases). Redoubt volcano in Alaska and Chaiten in Chile are currently active examples of this type of eruption, and Mount St. Helens in the state of Washington spent several years building several lava domes.

Surtseyan Eruption

Surtseyan eruptions are a kind of hydromagmatic eruption, where magma or lava interacts explosively with water. In most cases, Surtseyan eruptions occur when an undersea volcano has finally grown large enough to break the water’s surface; because water expands when it turns to steam, water that comes into contact with hot lava explodes and creates plumes of ash, steam and scoria. Lavas created by a Surtseyan eruption tend to be basalt, since most oceanic volcanoes are basaltic.

The classic example of a Surtseyan eruption was the volcanic island of Surtsey, which erupted off the south coast of Iceland between 1963 and 1965. Hydromagmatic activity built up several square kilometers of tephra over the first several months of the eruption; eventually, seawater could no longer reach the vent, and the eruption transitioned to Hawaiian and Strombolian styles. More recently, in March 2009, several vents of the volcanic island of Hunga Ha'apai near Tonga began to erupt. The onshore and offshore explosions created plumes of ash and steam that rose to more than 8 km (5 miles) altitude, and threw plumes of tephra hundreds of meters from the vents.

The causes of tsunami

What causes a tsunami?... A tsunami is a large ocean wave that is caused by sudden motion on the ocean floor. This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide. The impact of a large meteorite could also cause a tsunami. Tsunamis travel across the open ocean at great speeds and build into large deadly waves in the shallow water of a shoreline.

Subduction Zones are Potential Tsunami Locations

Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".

Image by USGS

Accumulated Seismic Energy

As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.

Image by USGS

Earthquake Causes Tsunami

Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered.

Image by USGS

Tsunami Races Away From the Epicenter

The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline.

Image by USGS

Tsunamis Travel Rapidly Across Ocean Basis

Tsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours.

Image by USGS

Tsunami "Wave Train"

Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.

Image by USGS

The material above describes how tsunamis are generated and how they travel rapidly across an ocean basin. For more detailed information on this topic the following websites are recommended.

Volcanic Ash

What is Volcanic Ash?

Volcanic ash consists of powder-size to sand-size particles of igneous rock material that have been blown into the air by an erupting volcano (see image at right). The term is used for the material while it is in the air, after it falls to the ground and sometimes after it has been lithified into rock. The terms "volcanic dust" and "volcanic ash" are both used for the same material, however "volcanic dust" is more appropriately used for powder-size material.

Tephra / Pyroclastic Terminology
Particle Name	Particle Size
Blocks / Bombs	over 64 mm (2.5 inches)
Lapilli	under 64 mm (2.5 inches)
Volcanic Ash	under 2 mm (.079 inches)
Volcanic Dust (Fine Volcanic Ash)	under 0.063 mm (0.0025 inches)
"Tephra" and "pyroclastics" are general terms used in reference to particles of igneous rock material of various sizes that have been ejected from volcanoes. They are classified by size. The terms "ash" and "dust" communicate a specific size of tephra or pyroclastic particles. These are summarized in the table above.

Properties of Volcanic Ash

At first glance, volcanic ash looks like a soft, harmless powder (see image at right). Instead, volcanic ash is a rock material with a hardness of about 5+ on the Mohs Hardness Scale. It is composed of irregularly-shaped particles with sharp, jagged edges (see image at right). Combine the high hardness with the irregular particle shape and volcanic ash can be an abrasive material. This gives these tiny particles the ability to damage aircraft windows, be an eye irritant, cause unusual wear on moving parts of equipment that they come in contact with and cause many other problems discussed below in the "Impact of Volcanic Ash" section.

Volcanic ash particles are very small in size and have a vesicular structure with numerous cavities (see image at right). This gives them a relatively low density for a rock material. This low density, combined with the very small particle size allows volcanic ash to be carried high into the atmosphere by an eruption and carried long distances by the wind. Volcanic ash can cause problems a long distance from the erupting volcano.

Volcanic ash particles are insoluble in water. When they become wet they form a slurry or a mud that can make highways and runways slick. Wet volcanic ash can dry into a solid, concrete-like mass. This enables it to plug storm sewers and stick in the fur of animals that are in the open when ash falls at the same time as rain.

Ash Eruptions and Ash Columns

Some magmas contain enormous amounts of dissolved gas under very high pressures. When an eruption occurs the confining pressure on these gases is suddenly released and they expand rapidly, rushing from the volcanic vent and carrying small bits of magma with them. Ground water near a magma chamber can be flashed into steam with the same result. These are the source of ash particles for some eruptions. The enormous quantity of hot, escaping, expanding gas rushing from the vent can drive an eruption column of ash and hot gases high into the air.

The image at right shows a portion of the ash column produced by the May, 1980 eruption of Mount St. Helens. In that eruption, the explosive release of hot volcanic gases into the atmosphere produced a column of rising tephra, volcanic gases and entrained air that rose to an altitude of 22 kilometers in less than ten minutes. Then, strong prevailing winds carried the ash to the east at about 100 kilometers per hour. In less than four hours, ash was falling on the city of Spokane about 400 kilometers away from the vent. Two weeks later dust from the eruption had been carried around the Earth.

The Mount St. Helens eruption was exceptional in its size and intensity. A more typical ash release is shown in the image at the top right of this page. In that image, Cleveland Volcano, located on Chuginadak Island in the Aleutian Island Chain of Alaska, releases a small ash plume that within minutes detatches from the volcano and is carried away by the wind.

Ash Plumes, Ashfalls and Ash Fields:

Once ash is released into the air by a volcano, the wind has an opportunity to move it. This movement, along with air turbulence, work to distribute the suspended ash over a broad area. These clouds of ash being moved by the wind are known as ash plumes. An image at below right shows an ash plume produced by the eruption of Chaitén Volcano in southern Chile on May 3, 2008. This plume begins in Chile, crosses Argentina and extends hundreds of kilometers out over the Atlantic Ocean, spreading out as it travels.

As an ash plume moves away from the volcanic vent it no longer has the rush of escaping gases to support it. The unsupported ash particles begin to fall out. The largest ash particles fall out first and the smaller particles remain suspended longer. This can produce an ashfall deposit on the ground below the ash plume. These ashfall deposits are generally thickest near the vent and thin with distance. A map showing the ash distribution from the May 18, 1980 eruption of Mount St. Helens is shown at right.

Ashfall deposits are generally thick and coarse in particle size near the volcano. However, at distance the deposit gets thinner and finer.

An ash field is a geographic area where the ground has been blanketed by the fallout of an ash plume. An image at below right shows an ash field east of Chaitén Volcano in southern Chile from May, 2008. The white groundcover of ash can clearly be seen.

The Impact of Volcanic Ash:

Volcanic ash presents numerous hazards to people, property, machinery, communities and the environment. Several of these are detailed below.

Impact on Human Health :

People exposed to falling ash or living in the dusty environment after an ash fall can suffer a number of problems. Respiratory problems include nose and throat irritation, coughing, bronchitis-like illness and discomfort while breathing. These can be reduced with the use of high-efficiency dust masks but exposure to the ash should be avoided if possible.

Long term problems might include the development of a disease known as "silicosis" if the ash has a significant silica content. The U.S. National Institute of Occupational Safety and Health recommends specific types of masks for those exposed to volcanic ash. Anyone who already suffers from problems such as bronchitis, emphysema, or asthma should avoid exposure.

Dry volcanic ash can stick to a moist human eye and the tiny ash particles quickly cause eye irritation. This problem is most severe among people who wear contact lenses. Some skin irritation is reported by people in ashfall areas, however, the number of cases and their severity are low.

Impact on Agriculture:

Livestock suffer the same eye and respiratory problems that were described above for humans. Animals that feed by grazing could become unable to eat if the ash covers their food source. Those who eat from an ash-covered food source often suffer from a number of illnesses. Farmers in ashfall areas may need to provide supplementary feed to their animals, evacuate them or send them to early slaughter.

An ashfall of just a few millimeters usually does not cause severe damage to pastures and crops. However, thicker ash accumulations can damage or kill plants and pasture. Thick accumulations can damage the soil by killing microphytes and blocking the entry of oxygen and water. This can result in a sterile soil condition.

Impact on Buildings:

Dry ash weighs about ten times the density of fresh snow. A thick ashfall on the roof of a building can overload it and cause it to collapse (see image at below right). Most buildings are not designed to support this additional weight.

Immediately after a heavy ashfall one of the priority jobs is clearing the ash from the roofs of buildings. If rain falls before the ash is removed it can be absorbed by the ash and increase the weight. Wet ash can have a density of twenty times that of fresh snow.

Volcanic ash can fill the gutters on a building and clog the downspouts. The ash alone can be very heavy and if it becomes wet from rain the weight will often pull gutters from houses. Ash in combination with water can be corrosive to metal roofing materials. Wet ash is also a conductor and when accumulated around the external electrical elements of a building it can lead to serious injury or damage.

Air conditioners and air-handling systems can fail or be damaged if their filters are clogged or their vents are covered by volcanic ash. Moving parts on equipment can be worn rapidly if abrasive ash gets between them.

Impact on Appliances:

Fine ash and dust can infiltrate into buildings and cause problems with appliances. The abrasive ash can produce unusual wear on the moving parts within electric motors. Vacuum cleaners, furnaces and computer systems are especially vulnerable because they process lots of air.

Impact on Communications:

Volcanic ash can have an electrical charge that interferes with radio waves and other broadcasts transmitted througth the air. Radio, telephone and GPS equipment may not be able to send or receive signals with an erupting volcano nearby. The ash can also damage physical facilities such as the wires, towers, buildings and equipment needed to support communications.

Impact on Power Generating Facilities:

Volcanic ash can cause a shutdown of power generating facilities. These facilities are sometimes turned off to avoid damage from the ash. They can remain down until the ash has been removed. This protects essential equipment from failure but disrupts power service for millions of people.

Impact on Ground Transportation:

The initial impact upon transportation is a limit on visibility. The ash fills the air and blocks sunlight. It can be as dark as night in the middle of the day. The ash also covers road markings. Just one millimeter of ash can obscure the center and baselines of a highway.

Another impact is on cars. They process enormous amounts of air which will contain volcanic dust and ash. This initially gets captured by the air filter but it can quickly be overwhelmed. Then abrasive dust goes into the engine to damage carefully machined parts and clog tiny openings.

Volcanic ash accumulates on the windshields of cars, creating a need to use the wipers. If the wipers are used the abrasive ash between the windshield and the wipers can scratch the window, sometimes producing a frosted surface that is impossible to see through.

Volcanic dust and ash covering the roads can result in a loss of traction. If the roads get wet the dry ash turns into a very slippery mud. Roads and streets must be shoveled as if a snow that does not melt has fallen.

Impact on Air Transportation:

Modern jet engines process enormous amounts of air. They pull air into the front of the engine and exhaust it out the back. If volcanic ash is pulled into a jet engine it can be heated to temperatures that are higher than the melting temperature of the ash. The ash can melt in the engine and the soft sticky product can adhere to the inside of the engine. This restricts airflow through the engine and adds weight to the plane.

Volcanic ash has led to engine failure on a few planes. Fortunately the pilots were able to land safely with their remaining engines. Today, volcanoes are monitored for signs of eruption and planes are routed around areas that might contain airborne ash.

Volcanic ash suspended in the air can have an abrasive effect on planes flying through it at hundreds of kilometers per hour. At these speeds, ash particles impacting the windshield can sandblast the surface into a frosted finish that obscures the pilot's view. The sandblasting can also remove paint and pit metal on the nose and on the leading edges of wings and navigation equipment.

At airports the same problems are encountered with runways as are seen on roads. The markings on runways can be covered with ash. Planes can lose traction upon landing and take-off. And, the ash must be removed before operations return to normal.

The International Civil Aviation Organization recognized the need to keep pilots and air traffic controlers informed of volcanic hazards. To do that they worked with government agencies to establish several Volcanic Ash Advisory Centers. These centers monitor volcanic activity and report on ash plumes within their monitoring area.

Impact on Water Supply Systems:

Water supply systems can be impacted by ashfalls. Where a community utilizes an open water supply such as a river, reservoir or lake, the fallen ash will become a suspended material in the water supply which must be filtered out before use. Processing water with suspended abrasive ash can be damaging to pumps and filtration equipment.

The ash can also cause temporary changes in the chemistry of the water. Ash in contact with water can lower the pH and increase the concentration of ions leached from the ash material. These include: Cl, SO₄, Na, Ca, K, Mg, F and many others.

Impact on Waste Water Systems:

Ash falling on city streets will immediately enter the storm sewer system. If ash-laden sewer water is processed the suspended ash can overload equipment and filters and cause damage to pumps and valves. It also becomes a disposal problem. Mud or slurry of ash can harden into a material similar to concrete.

Planning for Volcanic Ash

Communities located near or downwind of volcanoes with a potential of producing ash eruptions should consider the potential impact of volcanic ash and plan for ways to deal with it and minimize its impact. It is much easier to become educated about a problem and take action in advance than it is to face an enormous problem without warning.