One of the most dramatic geological features found around our planet is columnar jointing formed from the cooling of molten rock. Many national parks and monuments as well as countless waterfalls and even one or two sal-called “alien landing sites” are based on this amazing feature.
The very first place I saw these columns was on a geology field trip to Bombo Quarry near Kiama, NSW, Australia – but I have now seen them almost on every continent.
What rocks form columnar jointing?
While many rooks show columns formed in basalt, this style of jointing can be found in the cooled lava flows of basalt through to rhyolite in composition. They can also be found in intrusions like dikes, sills, and plugs. For example, the columns making up the Devils Tower in Wyoming USA are formed in an intermediate extrusive rock called phonolite that was extruded as a plug-like feature.
Similar looking features – but not formed from cooling – can be found in muddy sediments and permafrosted materials. But these are shallow or thin features, whereas true colling columns can be up to 100’s of meters long.
How many sides?
Many people think that these columns have a fixed number of sides – like 5 or 6. But in reality, they can have 4,5,6,7,8,9 sides. For this reason, geologists refer to them as polygonal columns or polygonal jointing (poly=many gonal=sides).
In basalt, researchers have found that two distinctive layers can be identified. The first and lower layer, which is the one most of us recognize, is the long straight columns that geologists call the “colonnade faces”. The second and upper layer is made up of much thinner columns that are often curved and not as regular. Geologists call this the “entablature faces”.
In some rarer cases, the colonnade faces occur again at the top.
Why does columnar jointing form?
It is now thought that these joints form from the even cooling of a thick layer of molten material and as that material cools it shrinks and the joints form. The cooling tends to form from the top downwards in the flow as the molten rock ‘hardens’ beyond its liquid phase. So the joint could be forming at the top of the flow, but the rock could still be molten below it. As the lower material cools past its molten state, the joint continues to form.
One feature that you can often find on the columns is horizontal striations, sometimes called chisel marks, around the columns. It is thought that these striations form as the joint ‘grows’ downwards as the material turns from molten to solid, and so by measuring the distance between the striations we may be able to work out the rate the flow cooled.
There are so many examples of where you can find columnar jointing around the world. Here are just a few more examples.
And if you would like to visit some of the most iconic basalt column sites around the globe, like Iceland, then consider joining us on one of our field adventures. Below are some photos of columns form our trip to Iceland.