Volcanics
The closing of the Iapetus Ocean pushed the Earth’s plates into each other creating a ring of fire. There are many volcanic features in this part of Mayo, and on Mweelrea mountain where the igneous intrusions broke through the old Ordovician rocks there are shards of volcanic glass, known as ignimbrites. These shards were formed by the rapid cooling of ejected materials. Volcanic rocks occur around Bohaun and Sraheen in the Erriff Valley where there are what are known as pillow lavas. Volcanic rocks by Lough Nafooey are thought to have been produced in an oceanic island arc, and wave worn rocks show that some volcanoes were under water. In this chart we see what this part of the globe was like during the mid Ordovician period, about 470 million years ago. The northern end of Ireland was an ocean away from the south. As Laurentia made its way towards Avolania, the heavy ocean floor was pushed under the lighter land mass causing massive disruption. This process, known as subduction (see Science Spin 46) causes intensely hot magma to rise and break through the disrupted crust.
The Reek
Croagh Patrick is so well known that it is conical shape has almost become an icon for County Mayo. Unlike the Ordovician rocks nearby, the Reek began as a heap of slightly younger Silurian sand, that became solidified into extremely tough quartzite. This Silurian belt of sandstones, siltstones, shallow water limestones and lots of quartzite, stretches along the southern Clew Bay coast until it meets the Carboniferous rocks to the east laid down about 100 million later as the sea level began to rise again to cover the Devonian deserts. Another belt of Silurian rocks extends from Roonagh Quay, the departure point for Clare Island, to Louisburg Old Head. Between the two belts is another Silurian feature known as the Emlagh Thrust, consisting of baked, folded and faulted sandstones, and siltstones, with interbeds of red mudstones. Some of these mudstones have drying cracks, which shows that they must have been exposed to the sun, and above these fresh-water fishes and dry land plant fossils begin to appear. Because of the similarity of rocks and fossils, it is thought that this was a continuation of the Silurian Midland Valley in Scotland, which is just one indicator of how the Earth’s landscape never stays the same.
IRELAND NW IRELAND SE
TK, Source Archive.