Introduction to Rocks of the North Cascades

Virtual Tour

Physiography

Climate and Weather

Geology

Overview of
Geologic Forces

Geologic Forces
in Puget Sound

Rocks of the North Cascades

An Ideal Metamorphic Sequence

Extension of the Crust

The Basic Pattern Emerges

Strait of Georgia and San Juan Islands Geology

Hydrology

Terrestrial
Environment

Marine Environment

Species of Northern Puget Sound

Rocks of the North Cascades

Introduction1

Forbidden Peak
USGS North Cascades National Park Geology

Location of the North Cascades

The North Cascade Range in Washington State is part of the American Cordillera, a mighty mountain chain stretching more than 12,000 miles from Tierra del Fuego to the Alaskan Peninsula, and second only to the Alpine-Himalayan chain in height and grandeur. Although only a small part of the Cordillera, mile for mile, the North Cascade Range is steeper and wetter than most other ranges in the conterminous United States. In alpine scenery and geology, the range has more in common with the coast ranges of British Columbia and Alaska than it does with its Cordilleran cousins in the dry Rocky Mountains or benign Sierra Nevada. Although the peaks of the North Cascades do not reach great elevations (high peaks are generally in the 7,000 to 8,000-foot range), their overall relief, that is, the relatively uninterrupted vertical distance from valley bottom to mountain top, is commonly 4,000 to 6,000 feet, a respectable height in any world-class mountain range. Much of the range is roadless wilderness preserved from commercial exploitation by inclusion in North Cascades National Park, the Ross Lake and Lake Chelan National Recreational Areas, and several dedicated wilderness areas managed by the U.S. Forest Service.

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 Glacier Peak.

The deep canyons and sharp peaks of today's North Cascades scene are products of profound erosion. Running water has etched out the grain of the range, landslides have softened the abrupt edges, homegrown glaciers have scoured the peaks and high valleys and, during the Ice Age, the Cordilleran Ice Sheet overrode almost all the range and rearranged the courses of streams. Erosion has written and still writes its own history in the mountains, but it has also revealed the complex mosaic of the bedrock. There is much to be learned about the processes of nature in this special place.

Before setting out into the rather complex terrain of North Cascade geology, the reader had best be equipped with some basic geologic vocabulary and conceptual tools. The following sections introduce these words and tools and include briefings on minerals and rocks, geologic time, and the fundamental geologic structure of the North Cascades. Also introduced to readers is the theory of plate tectonics. Some understanding of this "unified field theory" of geology makes it possible to place the North Cascades in the big picture of geologic processes that operate on a worldwide scale.

Quartz crystals (silicon dioxide)

Minerals And Rocks2

Fundamental to understanding mountain geology is the nature of minerals and rocks, for the mountains are made out of rocks, and the rocks are made out of minerals. The simplest, most widely known and distributed mineral is quartz (silicon dioxide), which is made up of the elements silicon and oxygen. Most of the rocks in the North Cascades, and elsewhere for that matter, are made up of about seven types of silicate minerals, all of which contain a double dose of quartz (as silicon tetroxide), combined with various elements.

Rocks are classified according to the ways they are made and the proportion of various minerals that form them. Many rock names reflect their composition. All rocks belong to one of three major groups: igneous, sedimentary, or metamorphic. Many different rocks representing these three groups occur in the North Cascades.

References Cited

1. United States Geological Survey (USGS) Western Earth Surface Processes Team and the National Park Service. North Cascades Geology: World Class and Close to Home. <http://www.aqd.nps.gov/grd/usgsnps/noca/nocageol1.html>

2. United States Geolgical Survey (USGS) Western Earth Surface Processes Team and the National Park Service. North Cascades Geology: Minerals and Rocks. <http://www.aqd.nps.gov/grd/usgsnps/noca/nocageol2.html>