November 18, 2012
def. Columnar Jointing:
A structure that forms in rocks (most commonly in basalt) that consists of columns (mostly commonly hexagonal in shape) that are separated by joints or fractures in the rock that formed when the rock contracted, most often during cooling.
Columnar jointing is always a joy to observe in rocks in the field. Stumbling upon perfectly geometric columns of rock can only be described as magical. Even the most austere scientist might find herself (or himself) gaping in awe at the flawless shapes and wondering if men or Gods carved those immaculate columns. However, that majestic columnar jointing can easily be explained with a little bit of physics.
Most commonly, columnar jointing is observed in basalt. Let me try to explain how columnar jointing forms in basalt.The diagram below will be helpful for the explanation.
Basalt is an igneous, volcanic rock. For those of you who need a little Geology 101 refresher, “igneous” means that the rock formed from a melt and “volcanic” means that the melt erupted at the Earth’s surface as lava before it cooled to form the rock. After lava is erupted onto Earth’s surface, it cools. However, lava may take awhile to cool, and as it cools there may be a temperature gradient. Most commonly, the top of the lava flow will be cooler than the bottom of the lava flow.
When the lava cools, it contracts. This is because hot things generally take up more space than cool things. Think about hot steam, for instance. When you open the lid of a simmering pot or a tea kettle, that hot steam wants to escape and expand into the air. Or think about those balloons from your last birthday celebration. Have you ever notice how balloons tend to droop overnight? Partly, that may be because the helium in the balloons is escaping, but it’s also often because the gas inside the balloons cools down and contracts with the cooler nighttime temperatures. Sometimes, if you prop those drooping birthday balloons in the sun the next morning, they’ll pop back up again as the gas inside them warms up and expands.
When objects contract, they often crack or fracture. When contraction occurs at centers which are equally spaced (see the above diagram), then a hexagonal fracture pattern will develop. If the contraction is not evenly spaced, then other geometries of fractures, such as 5-sided or 7-sided fractures, may occur. Contraction may not be equally spaced if, for example, the thickness or composition of the lava flow varies. The fracture pattern that forms at the cooling surface will tend to be propagated down the lava as it cools, forming long, geometric columns. Thus, as lava cools to form basalt, it may crack in a hexagonal (or other) shape and form columns. These columns form in a variety of sizes– some are fairly small, and some are wider and much taller than people!
The formation of columns is particularly enhanced by water… Where water cooling has played a significant role, often when lava flows are ‘ponded’ in river valleys and are cooled by river water flowing over them, a predominantly two-tiered set of columns can develop, known as entablature and colonnade. The colonnade columns rise straight up from the basal cooling… whereas the ingress of water in the upper parts of the flow sets up a variety of different angles of cooling fronts. This leads to an irregular and sometimes hackly jointing called entablature in the upper parts of the flow.
Here’s a picture of some entablature (upper) and colonnade (lower) columnar jointing structures in basalts in Iceland:
Columnar jointing isn’t restricted to basalts, however. This structure can also form in other types of rocks which undergo cooling and contraction. For example, here is some columnar jointing in the Bishop Tuff of the Long Valley Caldera in California:
I’d like to end this post with a question from me for the geoblogosphere: are there any other conditions (other than cooling of igneous rocks) that lead to the formation of columnar jointing in rocks? Could, perhaps, contraction related to the drying out of a sedimentary rock lead to columnar jointing? I know that mudcracks, for instance, are often hexagonal in shape. Put your brains to work and leave a comment below.
***Thanks to Cian Dawson, Shaun Eaves, Dougal Jerram, and many, many others for providing pictures of columnar jointing. I didn’t have time or space to share all of your pictures in this post, but stay tuned for an upcoming “Columnar Jointing in Pictures” post in which I’ll share a few more pictures. Meanwhile, you can enjoy this amazing collection of columnar jointing Gigapans compiled by Ron Schott.***