USGS/Cascades Volcano Observatory, Vancouver, Washington
- North Cascades
- Geologic Background
North Cascades - Geologic Background
U.S. National Park Service Website - USGS Geology in the Parks Project, August 2000
Rocks of the North Cascades record at least 400 million years in the history of
this restless Earth -- time enough to have collected a jumble of different rocks. The
range is a geologic mosaic made up of volcanic island arcs, deep ocean sediments, basaltic
ocean floor, parts of old continents, submarine fans, and even pieces of the deep
subcrustal mantle of the earth. The disparate pieces of the North Cascade mosaic were
born far from one another but subsequently drifted together, carried along by the
ever-moving tectonic plates that make up the Earth's outer shell. Over time, the moving
plates eventually beached the various pieces of the mosaic at a place we now call western
Washington. As if this mosaic of unrelated pieces were not complex enough, some of the
assembled pieces were uplifted, eroded by streams, and then locally buried in their own
eroded debris; other pieces were forced deep into the Earth to be heated and squeezed,
almost beyond recognition, and then raised again to our view.
About 35 million years ago a volcanic arc grew across this complex
mosaic of old terranes. Volcanoes erupted to cover the older rocks with lava and ash.
Large masses of molten rock invaded the older rocks from below. The volcanic arc is still
active today, decorating the skyline with the cones of
Mount Baker and
From: North Cascades National Park Website, March 1999,
Contribution by: Patty and David Bean, Park Naturalists, November 19, 1998
The North Cascades are still rising, shifting, and forming.
Geologists believe that these mountains are a collage of terranes, distinct assemblages of rock
separated by faults. Fossil and rock magnetism studies indicate that the North Cascades Terranes were formed in other places, some many thousands of miles south of here. Attached to slowly moving plates of oceanic rock, they drifted northward, merging together about 90 million years ago. Exactly when they arrived here is still in question.
Colliding with the North American Continent, the drifting rock masses were thrust up-wards and faulted laterally into a jumbled array of mountains. The collision broke or sliced the terrane into north to south trending faults that are still evident today. Highway 20 crosses the Straight Creek fault just east of Marblemount.
Geologists believe the rocks to the west of the fault slid more than 100 miles north of the slice to the east. The rocks to the east of Straight Creek Fault are gneisses and granites, while those to the west are completely different recrystallized mudstones and sandstones. Over time, these predecessors to today's North Cascades were further faulted and eroded to a nearly level plain.
During the past 40 million years, heavier oceanic rocks thrust beneath the edge of this region. Intense heat at great depths caused them to melt. Some of the melt rose to the surface in fiery volcanic eruptions like
The rest crystallized at various depths to form vast bodies of granitic rock. The North Cascades have again pushed upward to majestic heights, exposing the roots of the ancient collision zone. Ice, water, and wind will eventually level the peaks around us, returning them bit by bit to the sea. ...
Today, the almost 700 alpine glaciers in the
northern Cascade Range are but a fraction of the size of their
ice age counterparts,
yet account for most of the glaciers in the lower 48 states. Glacial concentrations surround all major peaks.
Mount Baker's slopes support 13 glaciers.
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03/02/01, Lyn Topinka