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North Cascades Geology

From the Fiery Furnace: Traces of the Last Volcanic Arc

Very little is left of the old Cascade Arc volcanoes fueled by the deep magma chambers now represented by the Chilliwack Batholith. Remnants of them occur at Silver Creek (west of Ross Lake), Big Bosom Buttes Caldera, Pioneer Ridge, Hannegan Pass Caldera, and Kulshan Caldera. Even so, erosion is still stripping away the remains. Only where these older volcanic deposits have been down-dropped by faulting have they been preserved from total erosion. (For more information on the ancient remnants, see Old Volcanoes.)
Remnants of the ancient Black Buttes Volcano rise up above the Deming 	Glacier.
Remnants of the ancient Black Buttes Volcano rise up above the Deming Glacier. The volcano erupted about 500,000 years ago, a younger member of the Cascade Volcanic Arc.

After Kulshan Caldera formed more than a million years ago, considerable amounts of lava erupted from various vents around what is today Mount Baker, which did not yet exist. The Black Buttes, on the west side of Mount Baker, are the remnants of an older cone that grew about 500,000 years ago. Its lavas cap nearby Heliotrope and Marmot Ridges. Flows from Black Butte volcano also make up Forest Divide on the east side of Mount Baker. Volcanologist Wes Hildreth estimates that the Black Butte volcano was roughly twice as big as the present day Mount Baker volcano. Most of it was eroded away before Mount Baker came to life.

Another prominent pile of lavas makes up Table Mountain, underlies parts of the Mount Baker Ski Area, and forms much of Ptarmigan Ridge. These lavas are about 310,000 years old, and their thickness suggests that they flowed down ancient valleys.

The Mount Baker Volcano

Mount Baker is one of the youngest members of the Cascade Volcanic Arc. It is probably less than 30,000 years old and has not been eroded enough to expose its granitic roots. The most conspicuous young lavas associated with the volcano flowed down Sulphur Creek less than 10,000 years ago, erupting from a small cinder cone near Schriebers Meadow. An explosive eruption of steam blew out of Sherman crater, near the summit of Mount Baker, in 1843, and steam still hisses into the sky from vents around the rim. We have every reason to expect future eruptions from Mount Baker.

Remnants of the ancient Black Buttes Volcano rise up above the Deming 	Glacier.
Mount Baker Volcano rises behind beds of argillite in the Nooksack Formation on Skyline Divide.

Of more than philosophical interest is the question of whether we humans are living today in is a time of arc quiescence are the growth and violent activity of the Cascade volcanoes more subdued now than they have been in much of the recent geologic past? In describing the rocks generated by the arc, we tend to be overawed by the amounts of volcanic material making up the mountainsides. Keeping in mind that all the arc rocks mentioned, rocks of both the volcanoes and their plutonic roots, were generated over a time span of 35 million years, the amount produced in any one hundred year period or even a thousand year period is not so much. The scene we see today, with only a few high Cascade volcanoes, such as Mount St Helens or Mount Baker, erupting every few hundred years or so, may not be far from the scene along the Cascade Volcanic Arc at any given time in the recent geologic past. Man’s span of attention almost fits between the catastrophic but spaced events that build a volcanic arc.

On to The Constant Levelers: Water, Ice, and Gravity
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Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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