Fall 2002
Dynamic Landscape
by Brendan McLaughlin
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Ninety-million-year-old metamorphic rock litters the glacially sculpted amphitheater of Boston Basin. |
Nov. 1, 1985 was a wet evening in the Cascade River Park. William and Alice Bower invited their friends Clair and Betty Wilson over for a game of cards in their mobile home, unaware that heavy rain had transformed the soil on the hillside above to unstable mud. The mass suddenly gave way in a mammoth slide, careening down the slope and crushing the trailer. Skagit County Sheriff’s Deputy Johnie Rose responded first to the emergency.
"I was walking through looking at the damage and we heard sounds from beneath the mud," he said. "Firemen were digging by hand to locate survivors for about nine hours."
The slide, 50 yards wide and about 6 yards deep, washed across the road and flattened the Bower home to 3 feet tall, he said. No one survived.
Geology is not the study of stagnant, unmoving rocks, but of a dynamic interplay between slow earth movements and their destructive side effects, such as the fatal 1985 mudslide. Regardless of their timeless appearance, the North Cascades did not always exist as we see them today. Geologic forces have compressed, stretched, built up and torn down this impressive landscape for millions of years. Short-term visitors often fail to recognize the profound influence geologic processes had in defining the region. The mountains create opportunities for people to vacation and explore, and for native plants and animals to flourish.
The story began 570 million years ago, when pieces of the Earth’s crust started scraping past North America, a process called lateral fault movement. Crustal plates slid past the relatively stationary North American plate, leaving portions of their rock behind from British Columbia to Wenatchee in long slabs called terranes.
Millions of years and ten terranes later, a complex collage of different rock types packed together, forming the foundation of the North Cascades. Compression, intense heat, pressure and later volcanoes, pushed the mountains upward to their current heights. As recently as 13,000 years ago, ice ages scoured and shaped the peaks. Today, the possibility of geologic hazards like mudslides makes any trek into the mountains a risky endeavor.
DISTANT ORIGINS
The terranes finished settling 90 million years ago, brought to their present location by lateral fault movement.
"A lot of the stuff that was scraped off has moved a great distance," said Jon Riedel, North Cascades National Park geomorphologist. "There’s a big controversy over how far the crustal pieces have moved."
Riedel said he thinks many of the terranes migrated to Washington from somewhere in southern California. Some, like the Nooksack terrane, contain reminders of how far they have traveled.
"The Nooksack terrane formed as a result of sediment deposited in an ocean basin off the continental margin 120 to 170 million years ago," said Scott Babcock, Western Washington University geology professor. "It includes fossilized squid, clams and shellfish. Chowder Ridge (in the Nooksack terrane) got its name from these fossils. It looked like somebody threw a bowl of chowder out over the rock."
The terranes of the North Cascades are categorized into three general domains: the Western, Metamorphic Core and Methow. Two major faults running through the area divide the domains.
During his graduate studies, Babcock became very familiar with an outcrop of rock in the Metamorphic Core Domain called Skagit gneiss. He said he spent weeks at a time crossing the high mountain ridges between Eldorado Peak and the South Pickett Range, collecting rock samples until they doubled his pack’s weight.
Geologists easily recognize rocks in this central part of the range because intense heat and pressure deep in the Earth’s crust has dramatically warped the rocks, a process known as regional metamorphism.
Ninety-million years ago, a regional metamorphic event affected almost all of the Metamorphic Core Domain, Babcock said.
"Basically, these rocks were once 80,000 to 90,000 feet (15 to 17 miles) below the surface," he said. "Very hot, high pressure metamorphosed it into the gneiss and schist seen there."
Erosion and uplift have since brought non-volcanic, metamorphic peaks like Mount Shuksan to the surface, Babcock said.
"Mount Shuksan’s summit is made of oceanic basalt that has been metamorphosed," Western professor Ned Brown said.
Brown spent 10 years helping research and draft the "Geologic Map of the Northwest Cascades, Washington" for the Geological Society of America.
"This is the general structure of most non-volcanic mountains in the North Cascades," he said. "It somehow came out of the subduction zone and was thrust up to form the top of the mountain."
VOLCANIC COMPLICATIONS
Mount St. Helens erupted in 1980, reminding Washington residents that they live in a region subject to the violent whims of the Cascade Volcanic Arc, a long string of volcanoes running through the range.
Washington’s North Cascades are located where the subducting Juan de Fuca Plate plunged deep enough under the North American plate to melt the rock, Western graduate student David Tucker said. The magma rises to the surface and creates volcanic mountains.
"North America is slowly moving to the southwest and colliding with the Juan de Fuca Plate, which is diving under it," he said.
Mount Baker is a local example of the Cascade Volcanic Arc phenomenon. Baker, a 50,000-year-old volcano, sits on the remains of a much larger volcano known as Black Butte, Tucker said.
"We tend to think of Mount Baker as this big snow cone, but there has been volcanism in the surrounding area for 1.15 million years," he said. "Baker is a young pimple on the side of a much larger volcanic field."
On a clear day, it is often possible to see a steam cloud wafting from the active crater at Mount Baker’s summit as vents around the rim release built-up heat and pressure. Water percolates far enough into the ground to reach a mass of magma beneath the mountain, instantly turning the water to steam.
Tucker assisted in a U. S. Geologic Survey mapping project of the Mount Baker area, collecting samples to characterize eruptions and their sources. With an active volcano as close as Mount Baker is to a population center the size of Bellingham, scientists should have a solid understanding of its nature, he said.
ICE SHEET AND ITS RETREAT
Just as tectonic and volcanic forces work to build the Cascades up, occasionally walls of ice descend from the north to scour and flatten them.
Ice ages periodically blanket the Northwest in a sheet of ice. The most recent episode occurred between 13,000 and 25,000 years ago, smothering the Cascades in the Cordilleran Ice Sheet and obliterating all evidence of previous ice sheets, Babcock said.
"At the end of the ice age, this was probably a very difficult place to be," Riedel said.
The Cordilleran Ice Sheet plowed through and blocked river valleys, straightening previously meandering waterways into linear U-shaped valleys, he said. Slight alterations in earth’s orbit around the sun create dramatic global temperature changes. As temperatures increased at the end of the ice age, the ice melted and water began flowing again. The only problem was the drainage patterns sometimes completely reversed, as in the case of the Skagit River, Riedel said.
Before the Cordilleran covered the land, a mountain ridge extended through the middle of the Skagit River Valley. The upper river flowed north and the lower river flowed southwest, Riedel said. The ice sheet destroyed the ridge and the Upper Skagit reversed its flow and joined the Lower Skagit, resulting in the single river seen today.
CURRENT HAZARDS
Although geologic forces appear to work slowly, catastrophes threaten to unfold at any time. Aside from his duties monitoring glaciers in the park, Riedel is responsible for geologic hazard management. Potential catastrophes include landslides, debris avalanches, slumps in sediment and rock falls, he said.
"Freeze-thaw action and steep slopes with no tree cover results in a lot of melted water that causes landslides," he said.
Aided by a faulty logging road and heavy rain, these conditions caused the 1985 mudslide, Riedel said.
Journeys into the mountains always include an element of danger, to the point that even professionals like Babcock and Brown said they fear solo expeditions. Brown said the North Cascades raw, unbridled mountains intimidated him when he moved to Washington after studying the Sierra Nevada mountain range in California.
"It’s much more wild and dangerous than the Sierras," he said.
Geologic events shape the landscape that attracts visitors to the North Cascades. The mountains’ elevation allows alpine glaciers to form, ambitious climbers to seek their fortune and endless armies of evergreens to reach astounding levels of diversity. Geologic processes created habitat for grizzlies and other native species and inspiration for hikers and poets.
After witnessing the 1985 mudslide’s wreckage, Rose said he was in awe of the amount of damage natural processes can cause.
"I was the first one there and the last one to leave," he said. "At dawn, after everyone had left, I drove up to the Cascade River Road and got out to look down on the area. I could hear rocks tumbling down. In the early morning light I could feel a calm, deathly still. It made me uncomfortable."
Perhaps a solid understanding of the history of these majestic peaks isn’t necessary in order to enjoy them. But some day, when the tireless tectonic forces turn their attention elsewhere, these mountains will be gone, laid to waste by the inevitable forces of gravity and time.
Senior Brendan McLaughlin studies environmental journalism at Huxley College. He has previously been published in The Western Front and The Planet Magazine.