Hummocks at Mount St. Helens, Washington
Texture of hummocks at Mount St. Helens
|Many hummocks in the 1980 debris-avalanche deposit at
Mount St. Helens contain intact blocks of the pre-1980 cone of
the volcano. Some of the same rock layers found in the hummocks can be
matched up with rock layers visible in the crater walls, much like pieces
of a giant jigsaw puzzle. Other hummocks are thoroughly mixed, consisting
of many different rocks exposed in the crater walls.
Photograph by E. Klimasauskas on 29 April 1999
Photograph by E. Klimasauskas 12 July 2001
Lavas exposed in hummock|
This hummock contains mostly dacite erupted from Mount St. Helens about 2,500 years ago. The dark rocks capping the hummock are from lava flows erupted between 2,200 and 1,700 years ago. The same layering of the two types of lavas is found in the crater of Mount St. Helens, located about 7 km away (see next photograph).
Same lavas exposed in crater