When thick magma
and large amounts of gas build up under the surface, eruptions can be
explosive, expelling lava, rocks and ash into the air. Less gas and more
viscous magma usually mean a less dramatic eruption, often causing streams of
lava to ooze from the vent.
The mountain-like
mounds that we associate with volcanoes are what remain after the material
spewed during eruptions has collected and hardened around the vent. This can
happen over a period of weeks or many millions of years.
A large eruption
can be extremely dangerous for people living near a volcano. Flows of searing
lava, which can reach 2,000 degrees Fahrenheit (1,250 degrees Celsius) or more,
can be released, burning everything in its path, including whole towns.
Boulders of hardening lava can rain down on villages. Mud flows from rapidly
melting snow can strip mountains and valleys bare and bury towns. Ash and toxic
gases can cause lung damage and other problems, particularly for infants and
the elderly. Scientists estimate that more than 260,000 people have died in the
past 300 years from volcanic eruptions and their aftermath.
Volcanoes tend to
exist along the edges between tectonic plates, massive rock slabs that make up
Earth's surface. About 90 percent of all volcanoes exist within the Ring of
Fire along the edges of the Pacific Ocean.
About 1,900
volcanoes on Earth are considered active, meaning they show some level of
activity and are likely to explode again. Many other volcanoes are dormant,
showing no current signs of exploding but likely to become active at some point
in the future. Others are considered extinct.
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