November 16-17, 2009 Storm

November 17, 2009

Here is a precipitation map of this last storm (with reported landslides):

I suspect we have a landslide or two up in the area north of Highway 90, but so far, nothing has been reported.


Landslides can be destructive, destroying houses, infrastructure, and kill or injure people. However, we don’t usually think about landslides being bad for your health.

Washington State has a complex geology. Much of the western Cascades is made up of accreted terrains , composed of both oceanic and continental rocks. Parts of these terrains contain asbestos (which occurs naturally, despite a relatively large number of people believing it artificial). Asbestos is a fairly blanket term for a wide variety of minerals, some harmless, some very dangerous. The most well known example of the dangers of asbestos can be seen in Libby, Montana, where vermiculite mining with occurrences of fibrous tremolite asbestos caused widespread health problems and death for many of the residences and workers.

In Washington State, asbestos outcrops across the state. Most of the outcrops are small, uneconomical to mine or develop and probably pose little danger with limited exposure. However, some larger deposits occur in Snohomish, Skagit, Whatcom, Kittitas, and Klickitat Counties. These deposits can cause weakness within rocks and are sometimes associated with weak, friable material, places where we would expect landslides to occur. The prime landslide that contains asbestos in Washington State is the Swift Creek Landslide in Whatcom County. The landslide material is composed mostly of serpentinite, a friable, weak rock in terms of stability with high amounts of chrysotile. Its origin was probably an uplifted oceanic plate that was probably composed of ultramafic material, such as dunite that was then metamorphosed and transformed into serpentinite. The landslide has produced a significant amount of material which has been transported downhill into the valley below, depositing chrysotile laden sediments. These sediments, especially during flood events, deposit in places where people can come into long-term exposure, which can result in long-term health problems.

Swift Creek might be the most well known landslide to contain asbestos in Washington State, but since asbestos occurs throughout Washington State, many other landslides have the potential to contain asbestos. This map represents deep-seated landslides that have the potential to contain asbestos within them.

Washington State Asbestos-DSLS Map

Washington State Asbestos-DSLS Map

This map is not a perfect representation, as available data is scarce. The map was created by overlaying identified asbestos occurrences found in Bulletin No. 37, Inventory of Washington Minerals (Valentine and Huntting, 1960) with the 100k geologic units (with slight modification on unit selection). The units that were identified with asbestos occurrences were then intersected with deep-seated landslides from DGER Washington’s Statewide Landslide Database (the database is located within the menu). These deep-seated landslides are of all ages, from relict to active. Points were then selected at the centroid of the polygons to create a point file of landslides that potentially contain asbestos materials. It isn’t a perfect method by any means, but it at least gives us an idea that more of these landslides probably exists throughout Washington State. I am in the process of intersecting the landslide layer with ultramafic units known to contain serpentinite, which will help expand and potentially more accurately capture landslides potentially containing asbestos.

Valentine, Grant M.; Huntting, Marshall T., reviser, 1960, Inventory of Washington minerals; Part I–Nonmetallic minerals; 2nd edition: Washington Division of Mines and Geology Bulletin 37, Part I, 2nd ed., 2 v.