I have a confession: welded tuff isn’t my only favorite geology word. Trying to narrow down a favorite geology word is like trying to choose a favorite beer. I love too many of them to pick just one. Welded tuff is one of my favorite geology words because of what it is. It’s super hot ash, rolling down the flanks of a volcano, incinerating all in its path and fusing together into a new pyroclastic rock. It can also pick up fragments of other rocks and weld them into the ash body.
This particular piece is of such origins and comes from my petrology field trip to eastern Oregon. It’s from the Dinner Creek Tuff formation that formed in central Oregon about 15 million years ago. That puts it at about the same age as the Columbia River Basalts (1). This beauty is studded with obsidian, pumice and other pieces of rock fragment that give this consolidated piece of ash a lot of character. Of all the rocks I picked up on the trip (and there’s a lot of them) this one is my favorite because It’s a reminder of the rather violent history of Oregon’s beginnings.
1.Streck, Martin J.; Ferns, Mark L.; Ricker, Christopher; Steiner, Arron. The Dinner Creek Tuff And Other Mid-Miocene Rhyolites At The Magmatic Focal Zone Of The Columbia River Basalt Group. May 2011