Igneous Rocks

The rocks at Scarlett, near Castletown, are around 340 million years old. The layered rocks in the background are Carboniferous limestones that formed in shallow tropical seas. In contrast to the gentle conditions under which the limestones formed, the rougher rocks in the foreground are basalts and agglomerates formed by the eruption of a volcano.



This wall of rock which is seen curving off to the left is a dolerite dyke. It is an igneous rock which, while still molten, forced its way from deep in the earth up a fissure towards the surface. The dyke is probably of a similar age (around 340 million years old) to the surrounding basalt and agglomerate rocks into which it intruded.





These Carboniferous agglomerates and debris flows formed through volcanic activity. As the volcano erupted, probably under water, it would throw out molten rock and solidified fragments of lava which then fell back down to the sea floor in a chaotic mass of lava and volcanic blocks.





The rounded structures in the photo are called pillow lavas. They are 340 million years old basalts that were erupted from a volcanic fissure under water. As a result of being erupted under water, they cooled very rapidly which caused them to form these distinctive pillow shapes as they ‘blobbed’ out of the fissure, rather like toothpaste being squeezed out of a tube.




The layered rocks are Carboniferous limestones and the more structureless rocks are volcanic rocks. These volcanic rocks flowed down a sloping sea bed. As they flowed along the sea bed they were often injected beneath the underlying beds of limestone, lifting off whole blocks. The photo shows volcanic rock solidified in the process of dislodging a block of limestone.






This igneous dyke intruded into approximately 340 million year old Carboniferous limestone around 65 million years ago at the start of the Tertiary period. As such, it is among the youngest rocks on the Isle of Man.

A 65 million year old Tertiary dyke intruded into Carboniferous limestones.