12 September 2011


Posted by Callan Bentley

Have you ever seen anything like this?

You’re looking at an outcrop of very interesting rock. The bright red stuff is officially called “clinker,” though many of the Wyoming locals where it outcrops incorrectly call it “scoria.”

What is clinker? It’s sedimentary rock that’s been baked by a burning coal seam beneath. The idea is that in places where sedimentary strata including coal outcrop on the surface, the coal seams can be set alight by prairie wildfires or lightning strikes, and then they burn back into the stratum, away from the surface outcrop (though at slower rates where there is less oxygen immediately available). The heat released by the combustion contact metamorphoses the sediments adjacent to the burning coal. This heat scorches the layers immediately below the coal to some extent, (as a lava flow will do) but most of the heat travels up, and thoroughly transforms the overlying rock. In some places, the heat may be so intense as to actually go beyond contact metamorphism, and melt the overlying sediments. When this happens, it produces a slag-like material called “paralava.” This paralava may mobilize through small vent systems that cut across the stratification of the host rock.

Here’s a more removed photo of the same outcrop, seen near Buffalo, Wyoming, on the western edge of the Powder River Basin. The strata in question are all Eocene in age, part of the Wasatch Formation. An annotated version follows, and both of these images may be enlarged with a click:

A closer look at the paralava vent (dark, sub-vertical feature) follows, with Sheridan College’s geologist Tom Johannesmeyer for scale. I think you can get a sense of the crazy wind we were experiencing as this photo was taken – strong enough that small pebbles were being picked up by the wind and slammed into us:

We were driven off the outcrop by this sudden squall: it blew up in a matter of minutes, blasting our faces with aeolian grit and then freezing rain. But not before we had collected some nice samples. One of them is now in my lab, where next semester’s Environmental Geology students will get a chance to examine it, and ponder its implications.

‘Where’s there’s clinker, there’s fire…’ or at least there was fire.