Immediate Effects
The effects of a volcanic eruption largely depend
on how much material and what types of material
the volcano ejects. Near a volcano, lava flows can
cover the land with new rock. A much larger
area can be affected by events such as ash falls,
landslides, mudflows, pyroclastic flows, and steam
explosions.
Lava Flow
Most lava moves slowly enough that
people can move away and not be hurt. But even a
slow-moving lava flow will knock down, cover, or
burn nearly everything in its path.
Lava Flows
Near a volcanic eruption, the weight
of fallen volcanic ash can cause the roofs of
buildings to collapse. Volcanic ash is heavy because
it is made of tiny pieces of rock. Ash makes roads
slippery, and it clogs up machinery, including cars
and airplanes. Large amounts of falling ash can
suffocate plants, animals, and people.
Volcanic Ash
Mudflows are landslides that occur
when loose rocks and soil are mixed with water.
Heat from an eruption melts any ice and snow on
the volcano very quickly. Mudflows form as the
water mixes with volcanic ash and other loose
particles. Mudflows also form as ash mixes into
rivers flowing from a volcano. Fast-moving
mudflows have buried entire towns tens of
kilometers from an eruption.
Trees catch fire as a lava flow moves
through a forest in Hawaii in 1999.
Volcanic Ash
Mudflows
Large piles of volcanic ash from the 1991
eruption of Mt. Pinatubo line a street in
Olongapo, Philippines, at the start of
the cleanup effort.
As a pyroclastic flow rushes
downhill, it can knock down or burn everything in
its way. Pyroclastic flows tend to follow valleys.
However, a particularly fast-moving flow can
sweep up and over hills, then race down a neighboring valley. As a
flow passes, it can leave a thick layer of volcanic rock fragments.
Pyroclastic flows are extremely dangerous. In 1902, a pyroclastic flow
from an eruption in the West Indies completely destroyed the city of
Saint Pierre (SAYNT PEER). Almost 30,000 people were killed within a
few minutes.
Pyroclastic flows
Part of a volcano can collapse and start a landslide—
a rapid downhill movement of rock and soil. The collapse may be
caused by magma moving underground, an eruption, an earthquake,
or even heavy rainfall. A landslide can cause a tsunami if a large
amount of material falls into the ocean.
Landslides
A tsunami is a water wave
caused by an earthquake,
a volcanic eruption, or a
landslide.
Chapter 8: Mountains and Volcanoes 273