Geology Reference
In-Depth Information
Flowing Rivers of Mud: Lahars
When large amounts of water are mixed with sediments, they produce a flowing river of
mud called a
lahar.
Lahars often occur with other hazards covered in this chapter, in-
cluding jokulhlaups and volcanoes. In the case of lahars from volcanic activity, the vol-
canic eruption of heated rock and lava can melt glaciers and snow, resulting in lahars.
Jokulhlaups, similarly, may result in lahars when the melted glacial water mixes with
loose, muddy sediments.
As lahars move through towns, they flow like water but with much greater force. When
they stop moving, everything they've inundated becomes cemented in the drying mud.
Watching the Poles: Geomagnetism
The earth's magnetic field may not seem like much of a hazard. On the contrary, it is
quite useful if you are lost in the woods with a good compass. Occasionally, however,
the strength of the earth's magnetic field changes, growing stronger or more active.
These periods are called
magnetic storms,
and during magnetic storms the earth's mag-
netic field may interfere with technological systems that humans depend on. GPS sys-
tems and other satellites can be disrupted. The changes in magnetic activity can also
cause blackouts when they affect electrical grids. To prepare for the potential hazards of
geomagnetism, the U.S. Geological Survey (USGS) works with NASA and the U.S. Depart-
ment of Defense to monitor the earth's magnetic field activity.
