Cascadia's Fault - Jerry Thompson [52]
Once the Columbia has forced its way over the bar and out to sea, it veers north, pushed by the Davidson Current and huge Pacific storms. Along the southwestern shore of Washington State, the river dumps more of its load, giving birth to long spits of sand and narrow barrier islands parallel to the mainland with miles and miles of beautifully isolated beaches.
In satellite photos you can see a muddy plume swirling across the edge of the continental shelf before apparently disappearing into the ocean depths. Losing momentum, the Columbia finally drops what’s left of that pulverized mountain debris onto the sea floor, piling it up precariously at the head of a steep canyon. The finer grains of silt keep moving farther out to sea.
Reading two 1970 papers by LaVerne Kulm and Gary Griggs (an OSU graduate student when this story began), I learned why those canyons and channels cutting across the flat surface of the ocean floor looked so much like a big river and its tributaries meandering across the prairies. It’s because that’s exactly what they are. This spidery web of channels is basically an extension of the Columbia River system across the sea floor—through the mud on top of the Juan de Fuca plate.
When massive glaciers filled the mountain valleys and covered most of the Pacific Northwest more than ten thousand years ago, the level of the sea was roughly 425 feet (130 m) lower than it is now. With so much water trapped in glacial ice on land, the ocean shrank and the continental shelf was exposed as dry land cliffs. In those days the Columbia had to flow much farther west to reach the sea. It ran across the drying continental shelf, cutting a groove all the way to the outer edge. Then it sliced a canyon down the steep slope to the flat oceanic plate below.
As ice sheets came and went over millions of years and the level of the sea rose and fell, so the Columbia periodically changed course. When ice was thick in the mountains and the sea level was low, the river’s main current ran directly west across the exposed shelf and down to the sea through what is now known as Astoria Canyon. When the glaciers started melting again, gradually raising sea level and redrowning the canyon, the river’s current swung northwest, pushed by strong ocean currents and storms, and meandered across the reflooded continental shelf until it reached the edge again farther north, where it cut another groove down the continental slope at what is now called Willapa Canyon, off the coast of Washington.
During several of these big melting cycles, broken slabs of ice plugged a narrow valley in the upper Columbia basin near the Idaho–Montana border, creating an ice dam 2,500 feet (760 m) high that backed up an inland sea called Glacial Lake Missoula, as big as Lake Erie and Lake Ontario combined. Eventually the dam broke—twelve thousand years ago—releasing a catastrophic torrent of water and debris that roared down the channel cut by the river, spilling out to sea across the continental shelf, then gushing down Astoria and Willapa Canyons before spreading like thick mud soup across the Juan de Fuca plate.
That’s why the Cascadia basin—the spreading section of ocean floor otherwise known as the Juan de Fuca plate—came to look so much like an underwater prairie. It got buried repeatedly by glacial outwash and periodic floods until the ocean floor was 1.9 to 2.5 miles (3–4 km) deep in silt, sand, and debris.
When I read about this, I recalled the “no trench” part of Walter Sullivan’s story in the New York Times. Evidently the subduction zone was so full of Columbia sediment and Missoula mud that the first wave of geologists and oceanographers to explore the Cascadia basin with echo sounders could not see much of a trench where they thought one should have been. The early technology used for mapping the sea floor could not penetrate the mud to see the real basement: the boundary zone where the Juan de Fuca plate was dipping down and thrusting its way underneath North America.
With no trench obvious, Cascadia became “a special case,