Basalt dyke, Isle of Man

The dark-coloured basalt dyke was intruded as molten magma that rose almost vertically through the surrounding horizontal layers of Carboniferous limestone. This contains fossils showing that the lime sediments were originally laid down in shallow tropical seas around 350 million years ago.

Basalt dykes like this one show that, around 50 million years ago, northwest Britain was a major centre of volcanic activity, as the North Atlantic Ocean began to open up. Fractures caused by stretching of the crust, allowed basalt magma to feed up to volcanoes above.

Some forms of basalt, like this one, weather quite quickly and are easily removed by wave erosion to form trenches.