28 Aug A Geological Paradise
Eastern Washington – there are few places in the Pacific Northwest, or within the US for that matter, where such a wide variety of geologic features are concentrated into a small area for all to see and explore. Take the image below of the White Bluffs, within the Hanford Reach National Monument, which illustrates at least SIX different geologic processes and events that have occurred over the last 15 million years. Some of these events lasted for millions of years – others for only a few decades or less. Can you identify them in this north-facing photo I captured in May of 2017?
The six notable geologic events visible in this photo, from oldest to youngest, are:
- Repeated flows of volcanic Columbia River basalt. Starting about 17 million years ago, one lava flow after another spread out across the Columbia Plateau. Altogether as many as 300 basalt flows erupted until about 6 million years ago, reaching almost three miles thick beneath Rattlesnake Mountain! The youngest flow along the Saddle Mountains (ridge along top of image below) erupted about 10 million years ago.
- Warping of the basalt flows into a series of parallel ridges and valleys, known as the Yakima Folds. Tectonic forces within the Earth’s crust squeezed the lava flows from the north and south, wrinkling the basalt surface into a series of east-west folds across a large part of eastern Washington. This process started even before the last basalt flows erupted – demonstrated by the fact that younger flows are less deformed than older flows. As folding continued the ridges grew taller while basins deepened. The last half-dozen or so lava flows are visible along the uplifted Saddle Mountains at the top of the image.
- Deposition of the Ringold Formation within the down-warped valleys of the Yakima Folds – followed by rapid downcutting of the Columbia River. Toward the end of basalt volcanism, ancient deposits of the Columbia River (i.e., Ringold Formation) accumulated within the Pasco Basin. As folding continued these sediments piled up to 1,000 ft thick between ~3 to 10 million years ago. Basalt flows, exposed along the Saddle Mountains, lie deeply buried beneath the cover of younger sediments in the foreground. Starting about three million years ago the Columbia River suddenly began to erode, removing hundreds of feet of the Ringold deposits along the center of the basin. The Ringold Formation immediately underlies the area affected by land sliding along the White Bluffs across most of the lower left half of the attached image.
- Erosion and deposition by repeated Ice Age floods. Starting ~1 to 2 million years ago, Ice Age megafloods further eroded Ringold deposits from the center of the basin where the power of the floods was greatest. Around the margins of the basin, however, where floodwaters moved more slowly, the floods sometimes deposited slackwater “rhythmites,” represented by the banded sediment layers visible in the lower right portion of the image. Up to a dozen rhythmites exposed along the White Bluffs suggests as many separate megafloods occurred here during the last Ice Age that ended about 15,000 years ago. To view a growing collection of eye-opening videos of Ice Age Floodscapes go to: https://www.youtube.com/channel/UCNS9qfD-DQWvRrjKsIIUzvw/videos.
- Irrigation-Induced Landslides. A massive landslide near Locke Island is represented in the lower-left half of the image. This landslide, along with many other recent large landslides along the White Bluffs, are caused by excess irrigation water finding its way to the Columbia River. In the case of the Locke Island slide the irrigation water most likely came from a man-made pond 2 miles to the NE. This irrigation water easily drained through Ice- Age flood deposits, but then flowed laterally toward the White Bluffs upon encountering the impermeable Ringold Formation. Even though irrigation water was cut off to the pond in the mid-1990’s, it continues to visibly sap out and cause land sliding atop the Ringold Formation today.
- Recent sand dunes. A band of white, sand dunes along the edge of the bluffs (photo upper-left) are the result of the Locke Island Landslide. The landslide debris consists of disturbed and loosened sediment that is actively transported up and over the bluffs by strong SW winds blowing directly onto the bluffs. Based on an analysis of historical aerial photos, the band of active sand dunes did not appear until the 1970’s – coincident with the start of land sliding.