Wednesday, September 15, 2010

Glacier Peak Debris Flows

The Washington Casade Range has five strato volcanoes: Mount Saint Helens, Mount Adams, Mount Rainier, Glacier Peak and Mount Baker. Mount Rainier has been described as the most dangerous volcano in the United States due to the fact that significant population centers have been built on areas underlain by mud flows from the volcano. Mount Saint Helens is of course well known having erupted 30 years ago. Mount Adams is not so well known in western Washington, but is a significant icon above the Yakima Valley and south central Washington. Mount Baker provides a scenic back drop for the lower Skagit Valley and San Juan Islands and is very apparent even from Seattle.
I often think of Glacier Peak as being the sneakiest at it lies tucked back into the core of the North Cascade Range where its summit and flanks blend in with the high peaks around it. Hence, it is easily the least known of Washington's five strato volcanoes. Although not well known, it has been an active volcano with at least three major mud flows since the end of the last glacial period 13,000 years ago and Glacier Peak dacite fragments show up in preglacial period sediments throughout Puget Sound. A major mud flow from Glacier Peak would have a significant impact to residents in Skagit Valley and might create some transportation challenges for people like me in the Bellingham area wanting to head south.

I have encountered the soils derived from the mud flow colored orange on the map above. The deposit is just beneath the surface in areas east of Sedro Wooley along Highway 20 and is reflected in extensive wetlands in that area. Poorly drained soils west of the Mount Vernon area are also underlain by Glacier Peak mud flows. 
Based on a comment by Dave Tucker via email and posted below, the above map perhaps overstates the travel distance of mud flows from Glacier Peak. Based on his observations and others the orange area on the map may in part be the result of reworked Glacier Peak material transported down valley. Not an unreasonable suggestion as I would assume a bunch of mud and ash up valley would then be transported down valley. And based on that, the catastrophic risk may be substantially less. 

1 comment:

Dan McShane said...

Via Dave Tucker email to me regarding Glacier Peak:

I think the jury is still out whether there have really been Glacier Peak lahars all the way to the lower Skagit. Kevin Scott and I made an extensive search down the river by raft and never found a lahar diamict of any kind [there is an old GP lahar deposit in the Sauk above Rockport]. There is distinctive GP h'blende bearing clasts in alluvium well down the valley, but I'm unconvinced that any lahars from any volcano have ever reached as far as, say, Concrete or lower on the river. Not even Baker.
I’m giving a [free] Baker talk at the Whatcom Museum Thursday, October 14th, 7:30 PM. Will focus on eruptive history and hazards.
Dave Tucker