The next day, our explorers headed to Cowiche Canyon in Yakima. Cowche Canyon is a natural area managed by the Cowiche Canyon Conservancy. It has miles of well maintained trails that wander through a gorgeous canyon close to the city. The main trail in the canyon bottom follows Cowiche Creek along an old train grade (the elevated flat-topped train bed). Other trails criss-cross the hills. The habitat here is similar to Umtanum Creek. The hills ad uplands are shrub-steppe, and the canyon bottoms are mostly riparian (next to a river) with shrubs and small trees. There are a few bigger cottonwoods, paper birch, and ponderosa pines. Since it is fall, the autumn colors are AMAZING! On our hike we would be looking for interesting plants and animals, but the focus would be on geology, the rocks.
Cowiche Canyon is a great place to see lava flows up close. There are many interesting layers of lava and the two sides of the canyon have two very different stories. We asked the explorers to look for differences in the size, shapes, texture, color, and place of different rocks. What observations would they see? On the south side of the canyon, the explorers noticed the rocks looked like talk columns. In fact, many of them looked like they had a regular shape. Some places had crack, and folds, and places where the rock was broken into thin plates, but most of it was big tall columns. As we got closer, we could see many holes in the broken rocks. One of our explorers observed, “They looked like bubbles!”. And that’s exactly what they are and lava is the kind of rock usually filled with bubbles.
This lava is part of a huge area of Washington and other western states covered with Columbia basalt. Basalt is lava that cools very quickly and has lots of magnesium and iron in it The Columbia basalt erupted from a hot spot about 15 million years ago, probably the same hotpot that is now under Yellowstone National Park! As we followed the trail, one of our explorers pointed out some of the rocks below were splintered into plates but the ones above were not. What could have caused that? We did some brainstorming. One explorer asked, “Were the rocks like that before or after they got here?”. “Well, they were lava, they flowed here like a gooey liquid, imagine a hot fudge cake batter”, I suggested. “So, they weren’t broken when they got here”, another explorer chimed in. “What if as the first gooey hot fudge batter was cooling, we poured another gooey hot fudge batter on top, what would happen?”. “It would smash down the one on the bottom, just like those rocks!” a very smart explorer replied.
So, we had also figured out that the rocks at the bottom of the canyon are the oldest, and those at the top are the youngest. Our eagle-eyed explorers scanned the cliffs of the canyon, and eventually noticed some weird shapes up high. These weird shapes are one of the special features of Cowiche Canyon. High up, only on the north side of the canyon, are several large cliff faces, with weird smooth rocks, that look like they were squeezed out of a tube. We played a guessing game, but these were super hard to figure out. This lava is called Tieton andesite and is about 1.6 million years old. It’s the youngest lava here, so it’s on top. Andesite is a very thick lava-rock. As this lava was flowing, it ran into the glaciers that used to be here, creating these strange flower-like shapes where the lava cooled quickly.
As we went further up the canyon, it narrowed, and we walked right next to tall cliffs of lava. In some places, huge lava boulders almost covered the trail. Looking up at the side of the cliff, we could see the exact spot they had fallen from. What made these boulders and rocks fall off the cliff? We examined some of the rocks, noticed lots of cracks, and decided on two ideas:
- freeze-thaw weathering: water gets in the cracks, freezes, breaks the rocks
- biological weathering: plant roots grow in the cracks and slowly make the crack bigger, until the rock breaks
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