In one of the most famous volcanic events in recorded history, sometime in the second half of AD 79 Mount Vesuvius began a 19-hour eruption that would kill more than 10,000 people and wipe out two Roman towns. The horror of that eruption was caught in the faces of people who were entombed in volcanic debris, their bodies discovered by archaeologists centuries later.

Every year, an average of 50 volcanoes erupt around the world, mostly in a circle around the Pacific Ocean.  While some eruptions can be violent and deadly, others cause havoc in other ways.

In March 2010, for example, a volcano under the Eyjafjallajokull glacier in Iceland erupted, sending a plume of ash into the sky that disrupted air travel for millions for several weeks. Aviation authorities grounded thousands of flights, citing the hazards of flying through clouds of ash. Increased seismic activity this month around the Bardarbunga volcano, in Iceland's largest volcanic system, has geologists and business analysts wondering what impact there will be on air travel and the economy.

Here's a look at how, where and why volcanoes form.

What is a volcano?

Volcanoes are usually depicted as towering mountains missing a chunk out of the top from which steam, ash and lava spew forth.

Mount St. Helens

Mount St. Helens (named after British diplomat Baron St. Helens) erupted on May 18, 1980 when an earthquake caused the north face of the mountain to collapse and the magma within to burst forth.

The eruption ejected more than a cubic kilometre of steam, ash, lava and rock, and reduced the height of the mountain by 400 metres.

The explosion flattened every tree in an area the size of Toronto, and killed 57 people, 1,500 elk, 5,000 deer and an estimated 11 million fish. It also destroyed 200 homes, 47 bridges and 300 kilometres of highway.

In reality, a volcano is any spot on the surface of the Earth where lava flows. Magma finds its way upwards along fissures or cracks in the planet's crust and bursts out onto the surface, resulting in a volcano.

Magma that flows out of a volcano is called lava. If it comes out of the volcano in an explosive ejection — like Mount Vesuvius in AD 79 — it's called tephra.

Where are volcanoes most active?

The Earth's crust is composed of 15 tectonic plates that float on the Earth's mantle. The mantle — which makes up most of the Earth's mass — is a layer of hot, solid rock between the crust and the molten iron core.

Most volcanoes line the boundaries of the tectonic plates. They are formed either when the plates come together or when they pull apart.

One of these boundaries is referred to as Pacific Ring of Fire and extends from the west coast of the Americas to the east coast of Asia. Seventy-five per cent of the world's active volcanoes are found along this ring of fire. Volcanoes in this area are formed by tectonic plates coming together.

The Mid-Atlantic Ridge — which extends from northeast of Greenland south to the south Atlantic between South America and Africa  — is an example of an active volcano area formed by tectonic plates pulling apart. While this ridge is primarily at the bottom of the ocean, parts of it — including sections in Iceland — rise above the water's surface.

How is molten rock formed?

Tungurahua

Tungurahua volcano, south of Quito, is one of Ecuador's most active volcanoes and was active as recently as May 8, 2013.

When two continental plates meet, the denser of the two sinks beneath the lighter one. The solid crust is forced down into the extremely hot mantle, where heat and friction can turn the solid rock into magma.

Heat alone may not be enough to turn rock into magma. Increased pressure, decreased pressure or the addition of water can all play a role, depending on whether the tectonic plates are converging or diverging and whether the volcanic area is underwater.

What happens when a volcano erupts?

A volcano erupts in one of two ways: either the magma is forced up to the surface or the rising magma heats water trapped within the surface, causing an explosion of steam.

In either case, the eruption can eject rocks, volcanic ash, cinders and hot gases into the air. The rapidly cooling lava can form volcanic glass.

What is the likely aftermath of an eruption?

World's deadliest volcanic eruptions

  • April 5, 1815: Mount Tambora, Indonesia. Killed 92,000 and caused disastrous crop failures and summer snowfalls as far away as North America.
  • Aug. 26, 1883: Krakatoa, Indonesia. 36,500 killed.
  • Aug. 30, 1902: Mount Pelee, Martinique. 29,000 killed.
  • Nov. 13, 1985: Nevada del Ruiz, Colombia. 23,000 killed.
  • 1792: Unzen, Japan. 14,000 killed
  • AD 79: Mount Vesuvius, near Naples, Italy. More than 10,000 died in the eruption that obliterated the Roman towns of Pompeii and Herculaneum.

Many volcanoes produce a cloud of gas in the stratosphere containing heavy concentrations of sulphur dioxide. Scientists believe that airborne volcanic particles reflect radiation from the sun, which in turn could lower surface temperatures.

Lava can travel kilometres from the volcano over the surface of the Earth before solidifying, but the fast-moving clouds of gas, ash and rock emitted by volcanoes are at least as deadly as the lava flows. A pyroclastic flow  is hot enough to kill people and animals instantly, and nearly impossible to outrun as it can travel at speeds approaching 150 km/h.

The heat generated by the eruption of Mount Vesuvius in AD 79 was believed to have reached temperatures approaching 800 degrees Celsius. That was hot enough to literally boil the brains of people caught in the lava flow and cause their skulls to explode.

How are volcanic eruptions measured?

Until 1982, there was no official scale that measured volcanic eruptions. That year Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii devised the Volcanic Explosivity Index (VEI).

The open-ended VEI scale is logarithmic, with each succeeding interval representing a tenfold increase in the factors used to measure the eruption. Those factors include volume of ejecta and height of the plume cloud. Eruptions classified as 0 — which would be described as non-explosive — happen every day. An eruption measuring 5 on the VEI — like Mount St. Helens — is described as paroxysmal, which is between cataclysmic and colossal. Eruptions like that can be expected to occur every decade or so.

There are critics of the scale. They point out that eruptions are scored primarily on the volume of debris they eject, which does not take into account the density of the erupted material. For instance, similar amounts of magma spewing from two different volcanoes might produce great quantities of fluffy ash at one and a much smaller volume of dense volcanic rock at another. That would cause one eruption to rank higher than the other. 

Does Canada have any active volcanoes?

Yes, and at least three have erupted in the last few hundred years. The 1775 eruption of the Tseax Cone — about 60 km north of Terrace, B.C. — killed an estimated 2,000 Nisga’a First Nations people.

The most active volcanic region in Canada stretches from just north of Prince Rupert, B.C., into Yukon and to the Alaska border. It is home to about 100 active volcanoes.

Corrections

  • The original version of this FAQ page, published in 2004, contained scientific inaccuracies. It has been replaced with this updated FAQ, which contains corrections and new information. CBC thanks the readers who pointed out the inaccuracies, and apologizes for the errors.
    Jul 06, 2012 12:00 AM ET