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USGS Geology in the Parks

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FIELD TRIP SIDETRIP M - Off of Ruth Creek Road; Mount Baker Highway (State Route 542)

Nooksack Cirque

Nooksack Cirque
Young nested plutons of the Chilliwack batholith in Nooksack Cirque.

Amphitheater carved from nested plutons

Hikers who reach the upper end of the North Fork of the Nooksack River and stand in the awesome amphitheater of Nooksack Cirque will experience an Ice Age scene of great grandeur. Perhaps, during the difficult hike over brushy, bouldery till and outwash that is necessary to reach the inner sanctum of the cirque, the hikers will have noticed a variety of granitic rocks in the outwash debris. The cirque, carved by numerous coalesced glaciers from east, west and south, has breached a nest of igneous plutons of the Chilliwack Batholith, mostly members of the Cascade Pass arc-root family. These nested plutons account for the variety of granitic rocks encountered in the area.

Nooksack Cirque
Geologist climbing on granitic rocks of nested plutons in Nooksack Cirque

Geologists speculate that as a glob of molten rock intrudes upward from its origin, deep in the Earth above a subduction zone, its outer margins cool, crystallize, and become rigid more quickly than the interior because they are in contact with the cool surrounding rocks that the pluton is invading.

Nooksack Cirque
Granitic rocks of nested plutons in Nooksack Cirque below cap of Hannegan volcanic rocks on Ruth Mountain

While the margins solidify, the still molten core of the pluton continues to rise. Successive surges of the core, with accompanying cooling and crystallizing of the margins, form gigantic concentric tubes of granitic rock. If the molten rock feeding the system changes composition, then each thick-walled tube will be different from its neighbor and readily recognized as such. Moreover, the the age of the tubes should decrease from the rim to the core.

The plutons so boldly exposed in the Nooksack Cirque display this pattern, albeit imperfectly. We would expect the age differences from outermost to innermost tubes to be relatively small (on the order of 3 or 4 million years), because they are all fed from a cooling magma chamber which is also crystallizing. In the case of the Nooksack nest, we know only that the pluton underlying Icy Peak, rising on the southeast side of the cirque, is less than 4 million years old, the age of the Hannegan volcanic rocks that it intrudes. Outside the nest, the Lake Ann stock, which might be an offshoot from the youngest core pluton, is 2.2 million years old (Pliocene).

On to Hannegan Pass
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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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