Cascadia's Fault - Jerry Thompson [47]
Without knowing that Mount St. Helens would soon explode, Walter Sullivan had asked scientists about the eruption of another famous Cascade volcano, just to provide a frame of reference. He wrote about the explosion of Mount Mazama 7,700 years earlier, an apparently world-changing cataclysmic event. Sullivan described it so that readers could visualize what would happen again someday in the Pacific Northwest.
Roughly a hundred miles (160 km) east of the Pacific coast, Mount Mazama, like Mount St. Helens, had been created by Cascadia’s oceanic plate subducting underneath North America. This ancient stratovolcano was given its name posthumously, because it had exploded and was long gone before geologists arrived on the scene seven millennia later to piece together what happened. The upper part of the mountain had completely disappeared in a spectacular blast that caused the lower walls of the volcano to collapse inward, creating a huge, circular hole in the ground—a caldera—five miles (8 km) wide. This gradually filled with snowmelt and rainwater to form Crater Lake—with a maximum depth of 1,958 feet (597 m), the deepest lake in the United States.
Magma spilled from cracks along the shattered volcanic rim and surged downhill in avalanches that filled nearby valleys with up to three hundred feet (90 m) of hot rock, pumice, and ash. Somewhere between eleven and fourteen cubic miles (not cubic yards, cubic miles, or 46–58 km3) of magma was ejected. A towering column of ash thirty miles (48 km) high rained down for several days on eastern Oregon, Washington, Idaho, Montana, Nevada, and southwestern Canada. An ash layer half an inch (1 cm) thick was measured in Saskatchewan, 745 miles (1,200 km) from its origin.
Sullivan quoted Grant Heiken, a volcanologist at the Los Alamos Scientific Laboratory in New Mexico, who suggested that “a safe distance from which to watch such an event might be the Earth’s orbit of a space station.” In more recent times, the U.S. Geological Survey website referred to the Mazama blast as “the largest explosive eruption in the Cascades in the last one million years.” Mazama’s blast was forty-two times more powerful than Mount St. Helens’ in 1980. Only the explosion of Krakatoa (recounted by Simon Winchester in his excellent book of the same name) off the coast of Indonesia in 1883 could compare to Mazama’s magnitude and impact.
Eventually Mazama’s ash became famous in its own right. Geologists used radiocarbon dating to determine when the eruption had occurred and then, wherever researchers spotted the recognizable Mazama layer, it became an important stratigraphic marker, a distinctly visible line that appeared in bands of sediment all over western North America. Mazama provided geologists and archaeologists with a key reference point for geologic calendars and timelines. If you could find Mazama ash in a drill core sample, you could tell roughly how old the layers above and below it were and in what order major geologic events had occurred.
And that’s how Mazama became a key factor in the next phase of the investigation of Cascadia’s fault. Within eighteen months of the eruption, much of the ash had been carried away by rain, washed downstream by hundreds of creeks and dozens of rivers, and dumped in great heaps along the offshore continental shelf and in deep-sea canyons that wandered for hundreds of miles along the coast. Thousands of years later, a team of marine geologists from Oregon State University would find the distinctive Mazama line in cores of mud they gouged from the ocean bottom—cores that may have looked insignificant