Conversations with Professor Prestwich #2
VOLCANOES: SCALE, SETTING and PEDIGREE

Dear Geoneophytes,

As you will undoubtedly and thankfully have realized, no volcanoes are currently active in Scotland. Thus we must argue through time by analogy and look at modern examples in order to comprehend the scale of the antique wrecks of volcanoes that are found in Scotland. In our thinking it is legitimate to compare the active Stromboli, with the 340 million year old remains of the volcano that forms Arthur’s Seat in Edinburgh. Both volcanoes are of similar size (Stromboli is a little taller) and both have marine affinities. There are differences however: Stromboli is a steep-sided strato volcano, whereas Arthur’s Seat was a more gently-defined shield volcano.

Arthur’s Seat erupted on the edge of a large inland sea: when it became extinct it was gradually buried beneath thousands of feet of sediment. Subsequently the area was tilted at about 20° to the east and thus the volcano was rotated. During the present cycle of erosion the volcano was exhumed and subjected to the indignities of both fluvial and glacial erosion.

Allow me to offer a more extravagant analogy. One of the greatest mountains known to me is the great snow-covered volcano Cotopaxi, which lies about 50 miles to the south of the Equator in Peru. This peak is currently 18,876 ft. high and periodically erupts with deadly violence. I vividly remember the eruption of 1877; as in the great eruption of 1742, when 800 poor souls perished, melting snow caused great mudflows.

In general scale Cotopaxi reminds me of the great wreck of a 65 million year old volcano that some of you will travel through in the center of the Isle of Mull. My colleague Professor Judd has provided me with a map of the geology of the Island. While I find some parts of his interpretation to be a little extravagant, I do not doubt his central thesis, viz., that erosion has removed the upper part of the volcano down to the level of the chambers from which the lava flows arose.

I will leave you with the thought that, while all volcanoes differ in detail, our argument through time by analogy does allow us to reason our way to an understanding of the way in which the world works.

Cordially and magmatically yours,

Joseph Prestwich