This now hot water mixes with volcanic ash to produce exceptionally hazardous flows that
move as fast as 60 km/hour. In Columbia, the eruption of Nevado del Ruiz in 1985 produced
a lahar that killed more than 23,000 people as it swept over villages and flattened everything
in its way. In 1991, a typhoon arrived just after Mt. Pinatubo in the Philippines erupted.
The rains soaked the volcanic ash and dust that blanketed the entire region and produced
lahars that killed 1,500 people and displaced thousands more from their homes.
Figure 10.53: A lahar is a mudflow that forms from volcanic ash and debris. ( 9 )Slump and Creep
Fortunately not all types of erosion by gravity cause so many problems. Some less dramatic
types of movement, correctly calledslumpandcreep,move earth materials slowly down a
hillside. When materials slump down a hillside, they tend to move as a large block along a
curved surface. This type of earth movement often happens when a slope is undercut, leaving
little or no support for the overlying materials. It can also happen when too much weight
is added to an unstable slope. It is very unfortunate when that extra weight comes from
building someone’s home on a slippery slope. When earth materials slump down a hillside, a
crescent shaped scar marks the place they moved from (Figure10.54). A wise homeowner
will look for these crescent shaped scars along surrounding hillsides when considering buying
a new home. If they are present, it is a good possibility that earth materials have slipped
before.
The term creep is used to describe the very gradual movement of soil downhill, because
it just barely creeps along. Creep is such a slow way that earth materials move that no
human would likely notice. One way to tell that earth materials are slowly moving downhill
is to look at the growth of trees. Have you ever seen a tree whose trunk bends almost
horizontally to the ground and then grows upwards? If there are many trees growing this