Geological Survey of Austria

Transcription

I-90 after dropping into the beautiful Kittitas Valley near the Thorp Fruit & Antique Barn. Look north at Milepost 98 across those hayfields to the white cliffs where the Yakima River has cut a canyon through some thick white rock layers. There�s plenty of sandstone in this part of Washington, but not here. These are volcanic mudflows. 10 million years old. Lahars from an extinct volcano. And you might be surprised to hear where that volcano used to stand. Notice the big Cascade rocks floating in the middle of the deposit, fragile chunks of white pumice, no sorting of the rocks by size that you�d see in a sandstone or river deposit. This thing formed quickly. No time for the big rocks to settle out. Three lahars are exposed in this cliff, but there�s a total of 15 lahars in a stack of light-colored layers here - totaling 1000 feet thick. Lahars form when cone-shaped volcanoes erupt. During the eruption, the flank of the volcano fails. Water and glacial ice in the landslide mix with rock to convert the landslide into a volcanic mudflow, like a slurry of wet concrete. Lahars follow river valleys and are deadly to those in their path. The lahars have been clocked at up to 80 miles per hour - and can be up to hundreds of feet thick stretching from valley wall to valley wall. Remember the Duwamish River entering the Puget Sound in one of our first episodes back in Seattle? The Duwamish drains an area between Seattle and Mt Rainier. 5,600 years ago, a Mt Rainier that was 1500 feet taller than today - erupted and the northeast flank of the mountain failed. The massive Osceola Mudflow - a 400 foot-thick lahar that surged 50 mph down the White River - poured into the Puget Sound 60 miles from the mountain. Fine sands from the Osceola have been redeposited as far north as where the Duwamish River enters the Sound in downtown Seattle. More than 150,000 people now live on the Osceola Lahar and Mt Rainier - an active volcano - has rebuilt itself and is poised for its next eruption. So are these volcanic mudflows at Thorp from Mt Rainier? They�re not. If you go upstream on the Yakima River from here, you don�t head towards Mt Rainier. And besides, these volcanic deposits here are heading a different direction. These mudflows extend to the southwest, beneath hay fields, beneath I-90, and continue all the way to the William O. Douglas Wilderness west of Yakima. 10 million years ago, a cone-shaped volcano erupted near White Pass and a river valley once flowed northeast from the White Pass area to here in Thorp. There�s a buried river valley underneath Interstate-90 at Milepost 98. Since 10 million years ago, a series of ridges have uplifted south of I-90. The old river is gone, the old volcano is gone. The lahars at Thorp. Striking evidence for just one of many extinct volcanoes in the Cascades.