Mull Day 5 by Cat

Day 5

Igneous centre 3

All of today was spent – mainly walking up and thankfully down Beinn a ‘ Ghraig. The majority of the hillside was rhyolite which had an intergrowth texture of quartz and feldspar.

A small intrusion had us puzzled for a while as it had a very complex formation history. Termed a hybrid quartz gabbro, it showed signs of magma mingling. The semi solidified granophyre intrusion was injected by a basic gabbro whose margins chilled to form basalt which was then incorporated as clasts.

We had lunch on part of the Loch Ba ring dyke (felsite-  58 Ma). We were informed we could walk along it in a big circle across the hills … but didn’t’ feel inclined to do so. Briefly; ring dykes are formed during caldera collapse (analogy is pushing a wine cork into the bottle and the wine is the magma). Nearer the top we examined vent agglomerates like on Arthurs Seat.

Loch Ba in the sunshine
Loch Ba

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