[californiadisasters] Long Runout Landslides-The Real Significance of 5/18/80

In all the hubbub surrounding the May 18, 1980, eruption of Mount St. Helens was missed by most people what happened immediately preceding the eruption and that to me was the most significant aspect of the entire episode as the eruption itself was fairly typical for Cascade volcanoes.

Indeed what happened is what allowed the eruption to happen at all on that memorable Sunday morning.

I'm not referring to the M5.1 earthquake right before the eruption but what the earthquake triggered, to wit, a long runout landslide, the first ever in history to be photographed.

What is a long runout landslide? Well, for a useful frame of reference, typical landslides function like frictional masses, they fall out laterally about as far as they fall down vertically.

Long runout landslides are oddballs that don't conform to this principle but typically fall out laterally much further than they fall vertically and in some cases even "fall" uphill for some distance in the case of reaching the opposite side of a valley for example.

On land they can run out up to 20 times or more further than they dropped in elevation and in underwater regimes they can even runout up to 100 times further such as in the long runout landslide debris fields around the Hawaiian Islands.

There even appear to be long runout landslides on Mars by all appearances.

How these mysterious beasts function is not 100% clear although there are some very solid theories although each one does not seem applicable to the circumstances of all the known long runout landslides that have been investigated.

An entire flank of Mount Saint Helens, to wit, "The Bulge" that scientists for weeks before the main eruption had been monitoring as it ominously pushed out from the north side of the once symmetrical mountain fell off the side of the mountain as a result of the earthquake.

It sped along to the northeast well in front of the surge from the lateral blast and thus was not being propelled by it but by its own mechanism.

This is significant to California Disaster members because the two main Cascade volcanoes in California have generated their own prehistoric long runout landslides.

The Chaos Jumbles of Mt. Lassen National Park are the debris field from a long runout landslide that fell off the Chaos Crags just north of Lassen Peak.

Those low-slung hills just east of Interstate-5 out in the middle of Shasta Valley between Weed and Yreka are actually large rubble heaps from a monstrous long runout landslide that tore off the side of a once larger and more top-heavy and structurally unstable prehistoric Mount Shasta.

Unrelated to volcanism in California are three known long runout landslides in Southern California, one in Death Valley National Park in Eureka Valley and one on the backside of the San Bernardino Mountains known as the Blackhawk Slide and one in Southeast California in the Shadow Valley basin.

There were surely other long runout landslides in prehistoric California and what has happened before will happen again.

Californians in volcanic zones need to be aware of these hazards but so do Californians living in other geological settings.

Kim Patrick Noyes
Paso Robles, CA

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