|
Lahars
|
|
|
|
A lahar destroys a bridge
after Mt. St. Helens errupted on May 18,
1980. |
|
A
lahar is a hot or cold mixture of water and rock
fragments that flows down the slope of a volcano
or river valley. When it’s moving, a lahar
can look like a mass of wet concrete. It carries
rocks along with it as tiny as clay particles,
and as large as boulders the size of cars. A lahar
can be just a few centimeters deep and flow slowly,
or it can be hundreds of feet deep and can flow
much faster than a human being can run.
As a lahar flows downstream, it can pick up plants
in its path. It can also melt snow and ice in its
path, which can help the lahar become more than
10 times its original size. Eventually, as the
lahar flows further and further from the volcano,
the debris that it carries will get dropped off
and the size of the lahar will decrease. Lahars
are sometimes called mudflows or debris flows.
Lahars can begin when snow or ice on a volcano
melt quickly during an eruption, or when a lake
at the volcano’s crater is destroyed during
an eruption. Most often, they are caused by heavy
rain falling during or after a volcanic eruption.
Lahars almost always happen on stratovolcanoes
because these volcanoes erupt explosively, and
they are usually covered with snow or have a crater
lake. They can also happen when shield volcanoes
that are covered with huge glaciers erupt. They
can happen during eruptions, after eruptions, and
sometimes even when there is no volcanic eruption
but a sudden landslide on the slope of a volcano.
Lahars cause a lot of damage as they travel down
river valleys and spread out into low areas of
land. The rocks and other debris that they carry
can crush anything in their path, destroying bridges
and roads. Buildings and land, sometimes even whole
communities, can be buried under layers of rock
debris that are as thick as cement.
When Mount St. Helens erupted, many lahars began.
One on the east side of the mountain flowed as
fast as 100 km/hour. Another lahar swept through
a logging camp and picked up thousands of cut logs
that flowed downstream with it. It destroyed bridges
and property along its way.
 |
 |
 |
 |
| Lahars destroy by
covering |
Lahars deposit a lot
of sediment |
Lahars can block streams |
Lahars can bury buildings |
|
