18 May 2016

Pillow basalt exposures in the Columbia River basalts

Posted by Callan Bentley

On Monday, I had a great field trip in the Columbia River flood basalts (+Channeled Scablands), and was delighted to see so much exquisite columnar jointing. These lava sheets are immense: they cover more than 163,000 km2 (>63,000 square miles!) of eastern Washington and adjacent states: it’s a great example of a Large Igneous Province, or LIP. This area is best appreciated by spending a day driving across it and realizing you’re seeing only a tiny fraction of the place. I was surprised at the number of pillow basalt exposures we encountered – because the region is relatively dry today, I had erroneously assumed that when the basalt flows erupted during the Miocene (around 17-14 Ma) that the climate would have been similar, but from the many pillows I saw, it appears that there was plenty of water around. Pillows form when lava encounters a cool liquid and chills quickly along its edges, producing a breadcrust-like rounded exterior. When cross-sectioned, the pillows show a radial fracture set, and may have a “tail” pointing downward, a shape like Casper the Friendly Ghost.

Here are a few examples from near Hawk Creek Campground, in the Lake Roosevelt National Recreation Area:



There is often plenty of orangey palagonite between the pillows:


Obsidian crust (black, shiny) visible where the weathering rind has broken off:





Three-dimensional exposure of a pillow’s exterior:


Here are some really big pillows (several meters across), seen near Trinidad, Washington, adjacent to the Columbia River Gorge:


Sorry about the lack of scale in that shot; here’s one with me in it so you can get a sense of how large the pillows are:

SONY DSC Photo by Bill Richards


Finally, here’s a GigaPan showing an exposure with both pillows and a basalt flow that came in from the right side of the image, and has a fan-shaped array of columns on its nose. The pillows are above that prominent “warbonnet” feature, and to the left. See if you can find both 3D exterior examples, as well as cross-sections with the radial jointing: