A supervolcano must erupt more than 1, cubic km cubic miles of material, compared with 1. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in Not surprisingly, supervolcanoes are the most dangerous type of volcano. Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated, however, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion.

This creates a huge hole or caldera into which the surface collapses. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.

Yellowstone has produced many smaller but still enormous eruptions more recently. Long Valley had an extremely hot and explosive rhyolite explosion about , years ago.

An earthquake swarm in alerted geologists to the possibility of a future eruption, but the quakes have since calmed down. A supervolcano could change life on Earth as we know it.

Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. No one knows when the next super eruption will be. Skip to main content. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings.

Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world. Schematic representation of the internal structure of a typical cinder cone. Explosive eruptions caused by gas rapidly expanding and escaping from molten lava formed cinders that fell back around the vent, building up the cone to a height of 1, feet. The last explosive eruption left a funnel-shaped crater at the top of the cone. After the excess gases had largely dissipated, the molten rock quietly poured out on the surrounding surface of the cone and moved downslope as lava flows.

This order of events--eruption, formation of cone and crater, lava flow--is a common sequence in the formation of cinder cones. S ome of the Earth's grandest mountains are composite volcanoes--sometimes called stratovolcanoes.

They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8, feet above their bases. Helens and Mount Rainier in Washington. M ost composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents.

Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. T he essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface.

The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc. Schematic representation of the internal structue of a typical composite volcano. W hen a composite volcano becomes dormant, erosion begins to destroy the cone.

The products from the successive eruptions over thousands of years build the cones. Caldera Volcanoes Caldera volcanoes, such as Taupo and Okataina which includes Mt Tarawera , have a history of infrequent but moderate-large eruptions.

The caldera forming eruptions create super craters km in diameter and deposit cubic kilometres of ash and pumice. The style of eruption depends on a number of factors, including the magma chemistry and content, temperature, viscosity how runny the magma is , volume and how much water and gas is in it, the presence of groundwater, and the plumbing of the volcano.

For information on volcanic hazards which can be produced by our volcanoes, click here. Hydrothermal eruption An eruption driven by the heat in a hydrothermal systems. Hydrothermal eruptions pulverise surrounding rocks and can produce ash, but do not include magma. These are typically very small eruptions.

Sometimes the craters have been filled in by lava flows or lava domes, sometimes they are filled with glacial ice, and less commonly they are filled with water. They show an internal layered structure due to varying intensities of the explosions that deposit different sizes of pyroclastics. If lava flows are emitted from tephra cones, they are usually emitted from vents on the flank or near the base of the cone during the later stages of eruption. Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.

This volcano was born in a farmers corn field in and erupted for the next 9 years. Lava flows erupted from the base of the cone eventually covered two towns. In stratovolcanoes the collapse and formation of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive eruptions that form extensive fall deposits and pyroclastic flows.

On shield volcanoes, like in Hawaii, the evacuation of the magma chamber is a slow drawn out processes, wherein magma is withdrawn to erupt on from the rift zones on the flanks. In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks.

As they rise they may encounter a depth or pressure where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase i.

When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution. In other words, the gas bubbles begin to expand. If the liquid part of the magma has a low viscosity, then the gas can expand relatively easily.

When the magma reaches the Earth's surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a effusive or non-explosive eruption will occur, usually as a lava flow. If the liquid part of the magma has a high viscosity, then the gas will not be able to expand very easily, and thus, pressure will build up inside of the gas bubble s. When this magma reaches the surface, the gas bubbles will have a high pressure inside, which will cause them to burst explosively on reaching atmospheric pressure.

This will cause an explosive volcanic eruption and the production of pyroclastic material. Effusive or Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas.

The explosive bursting of bubbles fragments the magma into clots of liquid that cool as they fall through the air. These solid particles become pyroclasts or volcanic ash. Clouds of gas and tephra that rise above a volcano produce an eruption column that can rise up to 45 km into the atmosphere.

Eventually the tephra in the eruption column will be picked up by the wind, carried for some distance, and then fall back to the surface as a tephra fall or ash fall. This type of eruption is called a Plinian eruption.

If the eruption column collapses a pyroclastic flow will occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption.

The deposits that are produced are called ignimbrites if they contain pumice or pyroclastic flow deposits if they contain non-vesicular blocks. A Plinian eruption and pyroclastic flow from Vesuvius volcano killed about 20, people in Pompeii in 79 CE.

If the gas pressure inside the magma is directed outward instead of upward, a lateral blast can occur. Directed blasts often result from sudden exposure of the magma by a landslide or collapse of a lava dome. This happened at Mt. Lahars Volcanic Mudflows. A volcanic eruption usually leaves lots of loose unconsolidated fragmental debris. When this loose material mixes with water from rainfall, melting of snow or ice, or draining of a crater lake, a mudflow results. Volcanic mudflows are called lahars.

These can occur accompanying an eruption or occur long after an eruption. Lahars may be hot or cold and move at high velocity as they fill stream valleys that drain the volcano. At the base of the volcano, they spread out and cover wide areas. In general, they dev estate anything in their path, carrying away homes, buildings, bridges, and destroying roads, and killing livestock and people.

In a lahar produced by a mild eruption of Nevado de Ruiz volcano in Colombia wiped out the village of Armero, about 60 km away from the volcano and killed about 23, people.

Although the predominant gas erupted from volcanoes is H 2 O vapor, other gases are erupted can have disastrous effects on life. The Chlorine, Sulfur. In an CO 2 gas emission from Lake Nyos in Cameroon killed more than people and cattle.

The gases can also have an effect on the atmosphere and climate. Much of the water on the surface of the earth was produced by volcanoes throughout earth history. The Eruption of Mount. Helens, Prior to , Mount St. Helens last erupted in On March 21, a 4. Small eruptions took place through mid April and the summit of the mountain developed a new crater due to the explosions. By the end of April surveys showed that the north face of the mountain had begun to bulge upwards and outwards at rates up to 1 m per day.

By May 12, the bulge had displaced parts of the northern part of the volcano a distance of about m. Geologists now recognized that this bulge could soon develop into a landslide. At AM on May 18, a magnitude 5. This led to a violent eruption that took place over about the next minute. The earthquake triggered a large landslide that began to slide out to the north, initially as three large blocks. In all, 62 people lost their lives, either by being buried by the debris avalanche deposit, or suffocating by breathing the hot gases and dust of the blast.

Over the next several days melted snow combined with the new ash to produce lahars that roared down the North and South Forks of the Toutle River and drainages to the south of the volcano.

In general, the eruption had been much larger than most anticipated, but the fact that a hazards study had been carried out, that public officials were quick to act and evacuate the danger zone, and that the volcano was under constant monitoring, resulted in the minimization of loss of life to only 62 instead of a much larger number that could have been killed had not these efforts been in place.

Since the eruption, several volcanic domes have been emplaced in the crater and some have been blasted out. In the future, it is expected that new domes will continue to form, eventually building the volcano back to a form that will look more like it did prior to the eruption. Predicting Volcanic Eruptions. Before discussing how we can predict volcanic eruptions, its important to get some terminology straight by defining some commonly used terms.

Active Volcano - An active volcano to volcanologists is a volcano that has shown eruptive activity within recorded history. Thus an active volcano need not be in eruption to be considered active. Extinct Volcano - An extinct volcano is a volcano that has not shown any historic activity, is usually deeply eroded, and shows no signs of recent activity.

How old must a volcano be to be considered extinct depends to a large degree on past activity. Dormant Volcano - A dormant volcano sleeping volcano is somewhere between active and extinct. A dormant volcano is one that has not shown eruptive activity within recorded history, but shows geologic evidence of activity within the geologic recent past. Because the lifetime of a volcano may be on the order of a million years, dormant volcanoes can become active volcanoes all of sudden.

These are perhaps the most dangerous volcanoes because people living in the vicinity of a dormant volcano may not understand the concept of geologic time, and there is no written record of activity.

These people are sometimes difficult to convince when a dormant volcano shows signs of renewed activity. Such hazards maps delineate zones of danger expected from the hazards discussed above: lava flows, pyroclastic flows, tephra falls, lahars, floods, etc.

Short - term prediction of volcanic eruptions involves monitoring the volcano to determine when magma is approaching the surface and monitoring for precursor events that often signal a forthcoming eruption.

Earthquakes - As magma moves toward the surface it usually deforms and fractures rock to generate earthquakes. Thus an increase in earthquake activity immediately below the volcano is usually a sign that an eruption will occur. Ground Deformation - As magma moves into a volcano, the structure may inflate. This will cause deformation of the ground which can be monitored. Instruments like tilt meters measure changes in the angle of the Earth's surface.