This morning Callan Bentley at Mountain Beltway published a blog post that included the photo of a beautiful sample of a Kilbourne Hole xenolith cut in half to reveal the mantle peridotite core surrounded by a thin selvage of basaltic rock. These sorts of basalt covered xenoliths are not so uncommon as they might at first seem – indeed, Siim Sepp at Sandatlas proceeded to produce his own sample from Lanzarote in the Canary Islands. In fact, I’ve collected samples like this at Kilbourne Hole in New Mexico and Dish Hill in California’s Mojave Desert, and I’ve seen similar samples described from the cinder cones on Mauna Kea in Hawaii. In all of these localities the xenolithic core of the sample was ejected pyroclasticly from cinder cones that produced broadly alkaline basaltic lava compositions (though Kilbourne Hole was more of a dry heave, if you ask me). I’ve also collected mantle xenoliths with a similar basalt selvage from Haulalai Volcano in Hawaii, however in that case the xenoliths were left behind as a lag deposit in a basaltic lava channel.
What I want to push back on here, though, is the term that Callan coined for these samples, “xenobomb”. Though the samples of this type from cinder cones could arguably be called volcanic bombs, the samples cited by Callan and Siim lacked the streamlined features of a true lava bomb. In fact, I’ve seen a spectacular sample of a xenolith-cored lava bomb, complete with a beautiful aerodynamic basaltic bomb coating in the petrology lab collection of the University of Wisconsin-Madison (sadly I don’t have a photo). What really irks me about “xenobomb” is not the shape of the particular samples that Callan and Siim used to define it, but the etymology of the word. “Xenobomb” suggests a “foreign” bomb. That’s just not what these samples are. Perhaps in their new lab environments they’re xenobombs, of a sort, but on the volcanoes where they were collected “xenobomb” suggests bomb pieces that are foreign to the eruption from which they originated, and that’s just plain wrong. Plus, I’d hate to think of the cinder cones that formed them as xenobombers – can you imagine the hatred that would be heaped on them by American politicians? Before long there’d be campaigns to rid America of the xenobombers and innocent cinder cones like Wizard Island and Sunset Crater could be swept up in the hysteria. Words have meaning – use them cautiously.