Photograph by Kyodo, Reuters

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Rescuers investigate mountain lodges during the eruption.

Photograph by Kyodo, Reuters

Explaining Surprise Eruption of Japan Volcano Where Dozens Are Presumed Dead

Mount Ontake belongs to a class of unpredictable "stratovolcanoes."

The weekend tragedy on Japan's Mount Ontake, where more than 30 hikers are presumed dead after an eruption of toxic fumes and ash, is a deadly reminder that volcanoes—especially certain kinds—can be unpredictable.

Resting on a volcanic island where three of the Earth's crustal plates collide, Japan is often wracked by earthquakes and eruptions. The Japan Meteorological Agency currently has warnings for six other volcanoes besides Mount Ontake.

Located about 125 miles (200 kilometers) west of Tokyo, Ontake is one of many volcanoes popular with hikers in Japan—Mount Fuji is the most famous example—and is also the site of religious pilgrimage. (Related video: "Volcanoes 101.")

Although movements of magma preceding major eruptions can sometime trigger warnings, such as recent ones in Iceland, venting of steam-driven ash and fumes from an active volcano such as Mount Ontake isn't always preceded by such movements. (See also: "Icelandic Volcano Triggers Air Travel Warnings.")

That's why Saturday's eruption seems to have come without warning, sending a hail of ash and rock upon hikers enjoying autumn foliage atop the volcano.

The second highest volcano in Japan, at about 10,062 feet (3,067 meters), Mount Ontake belongs to a class called stratovolcanoes, which form where one continental plate dives beneath another and which are known for erupting at unpredictable intervals.

They are the most lovely and deadly of volcanoes, with gentle lower slopes that steepen dramatically at their narrow tips.

The ashfall that covered two miles (three kilometers) of the mountain's south slope over the weekend appears less massive than a 1979 eruption that saw superheated lava flows down the mountain, spilling 200,000 tons of ash across its slopes. A 1984 avalanche following an earthquake there traveled 8 miles (12 kilometers) and killed 15 people.

Two Types of Eruptions

What makes stratovolcanoes especially deadly is the pyroclastic eruptions that tend to come with little warning. These fast-moving eruptions of ash and rock cling to the sides of volcanoes, picking up tremendous speed—reaching perhaps 450 miles per hour (700 kilometers per hour) as they head downslope. That's the kind of eruption that occurred on Mount Ontake in 1979.

Saturday's eruption was a typically more minor kind of eruption called a phreatic eruption, which is an outburst of steam, ash, and rock unaccompanied by lava. Carbon dioxide or hydrogen sulfide fumes from these eruptions can asphyxiate people, as appears to be the case in this event. Since 1979, steam has regularly vented from Mount Ontake, which has four volcanic calderas, or cauldrons of collapsed land created by volcanic eruptions.

If they're not usually as deadly as full-scale pyroclastic eruptions, phreatic eruptions can still be very large. The 1883 phreatic eruption of Krakatau in Indonesia, is thought to have created the loudest sound in recorded history.

The eruption of the uninhabited volcanic island was heard 2,000 miles (3,200 kilometers) away in Australia. In the ensuing tsunamis, some 30,000 people were swept out to sea.

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