Idaho is a volcanologist’s paradise. The Columbia River basalts, the Snake River Plain, the Challis Group, and the comparatively minor events from basin and range faulting; you can’t walk very far in Idaho without stumbling over an igneous rock of some kind.
Along the South Fork of the Boise River, lava flows have repeatedly dammed the river, creating impoundment lakes until the dammed up waters overtop the lava dam, and the river slowly cuts through the fresh lava.
You don’t have to be a geologist to see that the basalt layer here originally extended all the way across the canyon. The layers match up, the levels are exactly the same on each side of the canyon and the inferred dam is very clear. Chermical and radiological testing says it is all the same lava flow, laid down at the same time. This was probably the Smith Prairie eruption, the latest of perhaps as many as six separate damming events on the South Fork across some 2 million years. The Smith Prairie dam, shown here, occurred about 185,000-200,000 years ago. In the time since, the river has cut more than 150 meters down through the basalt.
A lava flow on this scale isn’t a single event. Some of the later lava ran down into the newly created lake. When lava encounters large volumes of water, it forms a characteristic shape, called “pillow basalts.” You can see examples of pillow basalts – evidence parts of the Smith Prairie lava flow ran into the impounded river – in the lower right foreground of the photo. The lumpy, rounded shapes with the gassy, rough surface are the signature of molten rock meeting water. Lava makes no distinction between salt water and fresh water. Here’s some video footage from Hawai’i, where lava from the eruption of the Kūpaʻianahā vent of the Kīlauea volcano is running into the Pacific Ocean today.
The Soouth Fork of the Boise River has cut though about 150 meters of the lava dam here. Over 185,000 years, that’s an average of about .08 centimeters a year. Rock is hard. WC suspects most of the erosion occurred in the first 10,000 years or so when the gradient – waterfall, possibly – was much steeper and the erosional power of the water was much higher.
Geology is the least popular subject here at Wickersham’s Conscience, but WC thinks it is pretty cool. And that it wasn’t that long ago that it was pretty hot. By the way, there’s no reason why it can’t happen again tomorrow.