Sunday, August 31, 2014

Geo 730: August 31, Day 608: Cinder Pit

As indicated by the topo map I posted yesterday, this was an active gravel pit until fairly shortly before I started visiting the area in the early 80's. It was only getting started being recolonized by vegetation the first few times I stopped here, and it was easy to spot from the road. But on moist to wet ground, such as we have here on the north side of the cinder cone, alders move in fast, and can get big surprisingly quickly. As you're driving the road up toward the historical town site, this is easy to miss if you're not looking for it.

I don't have a photo of them, but several cinders I've picked up here appear to have pumice-like inclusions of melted rock of a different composition than the surrounding basalt. Next time I get back here, I need to look around and see if similar samples are fairly easy to find. My preliminary guess is that they are xenoliths (foreign rock inclusions) from the underlying hydrothermally altered rock, which, being heavily hydrated, "popped" when they were heated by the magma, and depressurized by being lifted to the surface. The unofficial name for these sorts of fragments is "xenopumice."

Photo unaltered. August 5, 2012. FlashEarth Location.

No comments: