Volcanic Hummock
Volcano Terms and Definition

Hummocks

are rounded or conical mounds within a volcanic landslide or debris avalanche deposit. Hummocks contain a wide range of rock debris, reflecting the variation of deposits that previously formed the flanks of the volcano. Some hummocks contain huge intact blocks tens to hundreds of meters in diameter. Some of the original layering of lava flows and other deposits can be seen in these large hummocks, but most of the large rock fragments are thoroughly shattered. In other hummocks the rock debris is thoroughly mixed as if the material had been in a blender and thoroughly mixed together.

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.

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.

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.