How volcanoes shed their flanks: landslides at Lassen, Crater Lake and Hawai‘i

Landslides are not a side effect of volcanoes; they are one of the ways volcanoes change. A National Park Service article on volcanic processes says every volcanic edifice is susceptible, and that landslides are important on composite and shield volcanoes. Some, especially debris avalanches, happen during or soon after eruptions; others happen at other times.

Crater Lake National Park, Oregon: Garfield Peak and Eagle Crags above Chaski Bay, with talus resting on a massive slide block that moved down from high on the caldera wall. Image from the NPS page (USGS photo).

Why they collapse

Composite volcanoes are steep, and hydrothermal alteration has created weak areas where clay minerals have formed; most also hold large volumes of fractured rock or loosely consolidated pyroclastic deposits. Shield volcanoes also collapse, and large landslides or slumps are part of the life cycle of Hawaiian shield volcanoes such as Haleakalā, Mauna Loa and Kīlauea. Basalt weathers and erodes readily, particularly in Hawai‘i’s wet tropical climate, shield volcanoes also undergo hydrothermal alteration, and eruptions under water produce tephra and fractured rock that erode more easily.

A debris avalanche is a fast-moving landslide that happens when a sector of a volcanic edifice collapses. With enough water it can turn into a lahar. It may leave a horseshoe-shaped crater, as at Mount St. Helens.

Three parks

• Lassen Volcanic National Park, California. Chaos Jumbles is a debris-avalanche deposit from the partial collapse of one of the domes of Chaos Crags, a set of six domes emplaced about 1,050 years ago, according to the page. Three debris avalanches followed one another in quick succession, in about 1670 CE, probably through slope instability rather than because of an eruption.
• Crater Lake National Park, Oregon. After the caldera collapsed in a mega-colossal eruption, landslides and other slope movements enlarged it, accounting for almost half the width of the depression and for its scalloped walls. The largest of these caldera-widening debris avalanches, the Chaski Slide, happened about 200 years after the eruption, and its debris shows up in the bathymetry of the lake floor.
• Hawai‘i Volcanoes National Park. Large slides reach offshore on the slopes of both Kīlauea and Mauna Loa. The Hilina slump covers most of Kīlauea’s southern flank, and the Punalu‘u slump lies on Mauna Loa’s southern flank.

Chaos Crags and Chaos Jumbles. The avalanche debris of Chaos Jumbles is in the foreground (NPS photo, from the NPS page).

Image from the NPS page: a 3D view of part of the crater rim and lake floor, with the Chaski Slide (USGS).

Sources

• National Park Service, “Volcanic Processes—Landslides” (https://www.nps.gov/articles/000/volcanic-processes-landslides.htm), a page the NPS marked as under development; rewritten in hubnx's own words.