Sunday, August 9, 2009

Where in the West: Broken Top

As you may remember from about a week ago, I left you at an unknown, nearly covered-in-smoke mountain for the August WITH. The rocky, broken-looking mountain is Broken Top, a volcano near Sisters, Oregon. Broken Top was correctly identified by Lockwood, who blogs at Outside the Interzone.

The Cascades Volcano Observatory's (CVO) main Broken Top Volcano page has a lot links to information, maps, and graphics about Broken Top, including this description. Essentially, Broken Top is a stratovolcano, part of the Three Sisters-Broken Top volcanic area. Broken Top is similar to other Cascade stratovolcanoes - including Mt. Ranier, Mt. St. Helens, and the South and Middle Sisters - but its original stratovolcano shape has been strongly modified by glacial erosion.

The age of the Broken Top eruptions is not precisely known, but it is thought to have been inactive for about 100,000 years. It's younger than the 0.3 Ma Shevlin Park Tuff, which erupted from a now buried vent, and is older than the South Sister.

Geologists once thought that Broken Top's craggy appearance was due to volcanic explosions that caused it to "blow its top," but that idea turned out to be erroneous. Consequently, my question about whether Broken Top should be classified as an erosional, depositional, or tectonic landform is a little tricky. As Lockwood pointed out, tectonic activity (namely subduction along the west edge of the North American plate) led to the volcanic eruptions that formed the intitial Broken Top volcano, and Pleistocene glaciation then caused the shape that can be seen today. Properly speaking, Broken Top is an erosional landform, namely that of a deeply eroded volcano.

Some References:
Conrey, R. M., Donnelly-Nolan, J., Taylor, E. M., Champion, D., and Bullen, T., 2001, The Shevlin Park Tuff, Central Oregon Cascade Range: Magmatic Processes Recorded in an Arc-Related Ash-Flow Tuff: American Geophysical Union, Fall Meeting 2001, Abstracts.

Scott, W. E., and Gardner, C. A., 1990, Field trip guide to the central Oregon High Cascades, Part 1: Mount Bachelor-South Sister area: Oregon Geology, September 1990, v.42, n.5: Part 2: Bend Area: Oregon Geology, September 1990, v.42, n.6

Taylor, E. M., 1987, Field Geology of the Northwest Quarter of the Broken Top 15' Quadrangle, Deschutes County, Oregon: Oregon Department of Geology and Mineral Industries, Special Paper 21, 20 p. and map.


Lockwood said...

Here's a few more resources:
an overview map of the area; an indispensible guide to volcanism in the PNW; from that guide, a general overview of central Oregon Cascades geology by Ed Taylor, who happened to be one of my favorite profs here at OSU, and the detailed road guide to the area. Note that it does not go by Broken Top, but it does give a good idea of the areas volcanism. The best place to see it, as Silver Fox pointed out, is from the Cascade Lakes highway, which I think is AKA Century Drive. I backpacked in to Park Meadow, directly north of the peak, in the late 80's. The hike to that meadow was pretty easy. We camped there and did day hikes around the area- the lakes and streams coming off that mountain are gorgeous! With Three Sisters just to the west, and Broken Top to the south, I think that was the most beautiful camp site I've ever stayed at.

Silver Fox said...

Cool! Thanks for the great refs.