Background
In the Early Carboniferous period, about 330-340 million years ago, there was widespread volcanic activity in the Glasgow area. One of the volcanoes which poured out lava was at Dumbarton Rock. Today, all that remains is the plug at the root of the long-extinct volcano.
As the magma chamber (the source of the lava from deep within the Earth) empties, the flow of lava in the vent ceases, accompanied by fracturing and subsidence. Eventually the lava cools and solidifies filling the vent to form what we call a ‘plug’. After hundreds of millions of years, the area of the volcano erodes to expose the remains of the plug and some of the underlying rocks.
Columnar Jointing
Columnar Jointing is a structure that forms in rocks that consists of columns that are separated by joints or fractures in the rock that formed when the rock contracted, most often during cooling.
From the parking space looking South West you will see an example of these columns.
In order to log this cache please send me a message with answers to the following questions:-
- Taking care to cross the road find a patch of un-weathered rock. Describe the look and the feel of the rock.
- What type of rock do you think this is.
- Can you name another place where columnar jointing can be found?
- Optional but fun would be a photograph taken at the site. Once again be mindful of traffic.