Abstract
Occurring at Porth Maen Melyn, near Fishguard is a rhyodacite lava flow, over 40 m thick, which shows a transition from massive lava through tubed and pillowed lava into isolated-pillow breccia. Evidence is presented to show that the tubes and pillows of the rhyodacite flow are primary features and were probably the result of the rapid effusion of relatively high temperature, low viscosity magma. This magma was erupted in a submarine environment, where the combined effects of water depth, gas content and magma viscosity were such that extensive vesiculation was limited. Formation of the isolated-pillow breccia was the result of desquamation of developing tubes and pillows.
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