Hummocks at Mount St. Helens, Washington

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
These same lavas are exposed in a gully within the crater of Mount St. Helens. The light-colored rock is part of a dacite lava dome erupted about 2,500 years ago. The darker rocks are basaltic lava flows erupted sometime between 2,200 and 1,700 years ago.

Hummock consisting of andesite and basalt erupted from Mount St. Helens, Washington

Photograph by L. Siebert

Photograph by E. Klimasauskas on 9 August 2001

Lavas exposed in hummock
This small hummock consists of andesite and basalt lava flows that were erupted on Mount St. Helens sometime between about 2,200 and 500 years ago. In this hummock, most of the original volcanic layers were not preserved during transport of the landslide.

Same lavas exposed in crater
The same andesite and basalt lavas are visible in the upper part of the crater walls of Mount St. Helens. These rocks are identical to those found in the hummock above. The andesite and basalt flows are younger than the white-colored dacite lava dome. The red colors are a result of iron that oxidized within the lava while still very hot.

Photograph by E. Klimasauskas on 9 June 2001

Photograph by R. Hoblitt on 11 April 1979

Hummock of young dacite rocks
This gray to pink-colored hummock is made up of dacite rocks from the Goat Rocks and summit domes, which erupted in the early 1800s and between 500 and 200 years ago, respectively. The hummock consists mainly of dacite lava and pyroclastic-flow deposits that shed from the lava domes during their growth.

Same dacite rocks on former summit of Mount St. Helens
View of the lava dome that capped the top of Mount St. Helens before the 18 May 1980 eruption. The pink to gray rocks of the summit dome seen prior to 1980 are the same rocks preserved in the hummock above. Both the Goat Rocks and summit domes were removed by the debris avalanche.

Hummock consisting of many different rock fragments from Mount St. Helens, Washington

Photograph by E. Klimasauskas in April 1998

Hummock with many lava types
This hummock consists of different types of volcanic rocks erupted from Mount St. Helens within the past 2,500 years. The original rock layers were thoroughly mixed and shattered as they swept down the volcano into the North Fork Toutle River valley as far as 22 km from the volcano.

Photo glossary of volcano terms

Texture of hummocks at Mount St. Helens