A scientist stands on one of the many small hills called
hummocks that form the chaotic surface of a massive landslide
deposit in the upper North Fork Toutle River valley below
Mount St. Helens volcano (10 km in distance). Before the landslide
and eruption on May 18, 1980, a forest grew on this part of
the valley floor, and a highway followed the meandering river
to Spirit Lake, a popular recreation area.
The landslide deposit extends about 22 km from the volcano
and buries the river valley to an average depth of
about 45 m. In places, the deposit is nearly 200 m thick! The
landslide covers an area of about 60 km2.
An exceptionally large landslide deposit was discovered
at Mount Shasta shortly after the eruption of Mount St. Helens.
This landslide has a volume of about 45 km3--nearly
20 times larger than the one that buries the North Fork Toutle
River valley (above)--and it covers an area of 675 km2.
|