Re: Northwest US Cascade range geology

icycalmca_at_yahoo.com
Date: 12/13/04 

Date: 13 Dec 2004 01:43:50 -0800

Eigenvector wrote:
> As an avid hiker living in the Seattle area, I've covered quite a bit
of the
> Cascades. I'm curious as to how the range itself formed. The
volcanoes are
> obvious, but what about the surrounding peaks? From how I see it as
a
> hiker, you have a series of small broken mountains (5 to 6,000 feet
in
> height) with a couple three massive volcanoes here and there. Where
did the
> small mountains come from? Was it from the local plate getting
pushed up?
> Are they volcanes themselves? Are those mountains the remnants of
previous
> volcanoes (tummulus I think I heard it called)?

The Cascades are made up of various bits of low-density froth
(like, say, the Japanese islands, or Indonesia or) that were
carried on the denser Pacific oceanic crust as it dived under North
America. The lighter bits got scraped off and glommed onto the
continent.
As the diving Pacific plate melted, blobs of liquidy stuff burned up
through the continent, making volcanos and plutons and such.
Here's a map of part of the Cascades, showing the complex arrangement

of microplates (exotic terranes) there:

http://wrgis.wr.usgs.gov/docs/parks/noca/terranemap.html

Lots of other info to get to from there, by clicking on the map.

> Are the Olympics part of the same mountain chain, with Vancouver
Island
> thrown in there for good measure - separated from the main range body
by
> some other techtonic process?

The Olympics docked after the Cascades' parts immigrated, and
Vancouver Island (and the Queen Charlottes) seem to have arrived
later yet.

> The area is facinating to me, very rugged in a very narrow band, with
huge
> flat basaltic seas on one side and the ocean (well a strait anyway)
on the
> other.

See if you can get ahold of Ben Gadd's
Handbook of the Canadian Rockies (2nd ed. preferred).
It has an excellent layman-friendly background on the processes
involved.

A map of terranes around the North Pacific (vhat a Mess!):

http://www.mmaj.go.jp/mric_web/MMAJ_FORUM/LanceMiller/LanceMiller/LMslide2.html

Terranes in B.C. and Yukon (more in the menu at left), which have
all been slipped northward and stretched out by being squidged
between the Pacific plate and the North American plate and its
earlier agglomerations
(the Okanagan valley, e.g., is a pull-apart basin resulting from
that lateral stress):

http://gsc.nrcan.gc.ca/cordgeo/terrane_e.php

A good review of the major events in your area (with cartoons):

http://www.washington.edu/burkemuseum/geo_history_wa/The%20Challis%20Episode.htm
You want more? Just ask here.

Daryl Krupa
Living in the Past

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