Some years ago, I was in a group walking near Buxton and Miller's Dale, in Derbyshire. From Miller's Dale we followed the "Limestone Trail" but then cut across country to return to Buxton. We descended into a hollow where the limestone gave way to black igneous rocks. As it was a hot summer day and the hollow was sheltered from the wind, it was rather close.

I asked if anyone was warm. Several girls agreed. I told them this was unsurprising, as we were standing in a volcanic vent. There were some screams and several people looked anxiously around as if expecting an imminent eruption.

I pointed out that this particular vent had last done anything interesting about 250 million years ago, during the Carboniferous era. That didn't entirely reassure everyone.


Note: the Lower Carboniferous is referred to as the Mississippian, in US usage. Do I get marks for correct spelling?