Geoscience Reference
In-Depth Information
Lahars—Hot or cold mixture of water and rock fragments flowing down the
slopes of a volcano. These vary in speed from a few feet per second to tens of
meters per second. They can also vary in size. The size and consistency of a
lahar can change as it moves down the slope of a volcano. At its origination
point, it can include water from snow and ice melt, vegetation, and rocks.
This collection of materials as it moves down the slope can increase the size
and flow of the lahar to many times of what it was when it started. As the
lahar moves further away from the volcano, it begins losing its heavier loads,
and will slow down.
Volcanic gases—Numerous gases are emitted from a volcano's soil, magma,
fumaroles (vents from which volcanic gas escapes into the atmosphere), and
hydrothermic systems. All can induce serious health problems, which include
respiratory issues, skin irritations, mucous membrane irritations, and death.
Most gases are of such concentrations that they can be fatal to both humans
and animals.
Lava Flows—Lava flows can be a result of an explosive or nonexplosive release
from the volcano. Typically, lava flows are slow enough to enable people to
get out of their way. However, some flows can move more than 20 miles per
hour. Lava will incinerate, surround, or bury anything in its path, and there
is no way of stopping it. Lava flows also bury agricultural land under several
inches to several feet of black rock.
Pyrolclastic Flows—High-density mixtures of hot, dry rock fragments and
hot gases that move at high speeds away from the vent where they originated.
These flows will destroy almost everything in their path. They can move at
speeds of more than 50 miles per hour.
One historic volcano that has erupted in the United States is Mount St. Helens,
which last erupted on May 18, 1980 (Figure 5.9a and b). Scientists had been moni-
toring this volcano (and still do) after a series of hundreds of earthquakes occurred.
(a)
(b)
USGS
USGS
Figure 5.9 Mount St. Helens (a) one day before eruption in 1980 and (b) shortly
after eruption. (United States Geological Survey.)
