Mind the Gap

Among the many geological wonders spread across Washington State lies a region whose morphology played a particularly significant role during one of the planets most cataclysmic floods; the Wallula Gap. Heading southeast through the Pasco Basin leads to this scenic gorge whose cliffs of exposed Miocene basalt tell a story of this planet’s immense power.

The looming cliffs belong to the Columbia River Basalt Group, the youngest and best-preserved continental flood basalt province on Earth. The CRBG consists of seven major flood basalt groups that spread over 210,000 km^2 across eastern Washington, eastern Oregon, and western Idaho (Camp et.al, 2017). Wallula Gap is comprised of mostly Wanapum basalt, the flows that signaled the waning stage of the main lava pulses around 14.5 million years ago and Saddle Mountain basalt whose late-stage eruptions occurred roughly 6 million years ago (figure 1) (Camp et.al, 2017). Wallula Gap sits just outside the southern portion of the Columbia Basin, where the accumulation of basalt are deepest at nearly 4 km. (Camp et.al, 2017). These immense floods of molten basalt filled in the original topography of a vast portion of the Pacific Northwest, creating a new bed of thick rock. However, this is only the first of two floods to shape the landscape of the Gap. The first flood exhibited the power of earth’s internal processes in adding new crust to the lithosphere. The second flood displayed the amazing ability of earth’s external processes to weather and erode.

Roughly 4 million years after the last basalt eruption, the last Ice Age began and a new stage of land formation began its course. From the north, the Cordilleran Ice sheet advanced south from Canada, in doing so its Purcell Lobe obstructed the Clark Fork River in modern-day northern Idaho. The damming of Clark Fork River flooded back into Missoula Montana where nearly 800 cubic kilometers accumulated creating what is now known as Glacial Lake Missoula (Baker et.al, 2016). Then, only a short 12 to 15 thousand years ago, the glacial dam broke and the floods of Glacial Lake Missoula tore through northern Idaho, down throughout eastern Washington and Oregon creating the Channeled Scablands almost overnight before emptying into the Pacific Ocean.

With max discharge rates of up to 10 to 20 million m^3/sec, the floodwaters began the process of erosion at a staggering rate (Denlinger, R.P., O’Connell, D.H.R., 2010). The violent currents took advantage of the natural occurring joints in the underlying geology, plucking columns of basalt out of the bedrock (Baker et.al, 2016). Scoured bedrock tens of meters deep can be seen all throughout eastern Washington, including regions subjected to prolonged erosion where vast coulees were formed, hundreds of meters deep (Denlinger, R.P., O’Connell, D.H.R., 2010).
When the floodwaters reached Wallula Gap (figure 2) the erosion of the underlying basalt created a constriction that caused a hydraulic dam. As water struggled to pass through the Gap the Pasco Basin flooded 336 m deep creating a short-lived Lake Lewis (Denlinger, R.P., O’Connell, D.H.R., 2010). Markers indicating the water elevation of Lake Lewis can be found on top of Badger Mountain in Richland Washington, whose peak barely rose above water level. Turbulent waters continued to dam behind Wallula Gap and in the process ripped up the bedrock creating two basalt buttes known as the Twin Sisters. All of this occurred within the first 23 hours of the dam break and took only another 200 plus hours for the Pasco Basin to drain (Denlinger, R.P., O’Connell, D.H.R., 2010). The entire process took little over a week to forever change the setting of a large swath of the Pacific Northwest.

Standing at the mouth of the Gap today, it is awe-inspiring to look through the calm gorge and recognize the powerful events which shaped the beautiful landscape that it is. In the study of geology, we recognize that some of the most serene and stunning landscapes are products of millions or billions of years of powerful and sometimes violent processes. Though it is because of these processes in the past that we have such a dynamic and diverse planet to enjoy as our playground in the present.

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