How Can Molten Rock Transform into Geometric Columns?

Figure 1: (A) Columnar jointing on mars. (NASA, 2007). (B) Tops of columnar joints at Giant’s Causeway, Northern Ireland. (Wikimedia: Code Poet, 2005). (C) Sides of columnar joints at Devil’s Tower, Wyoming. (Wikimedia: Jonathunder, 2017).

Marte Vallis of Mars (Figure 1a), Giant’s Causeway of Northern Ireland (Figure 1b), and Devil’s Tower of Wyoming (Figure 1c) give us just a few examples of the beautiful features formed when lava flows cool and contract to produce geometric columns, a phenomenon called columnar jointing. When columnar joints form, solidified igneous rocks crack in a particular way that causes them to develop columns that resemble  hexagonal  honeycombs from above and a bundle of posts from the side (Figure 2a and 2b). These cracks, called joints, form in lava as it cools and act as pathways for water to flow through the hot rock. Water can flow into the spaces between the columns and react with the hot rocks, even forming valuable mineral deposits such as copper and zinc. This makes them a valuable resource for geothermal and hydrothermal energy, as well as mineral deposits. Columnar joints are common features that can form in any kind of igneous rock, but the conditions under which they develop aren’t perfectly understood.

Figure 2: (A) Close up view of the tops of columnar joints at Giant’s Causeway, Northern Ireland (Wikimedia: Mayer, 2003). (B) Close up view of the sides of columnar joints at Devil’s Tower, Wyoming (Wikimedia: Konstantin, 2003).

Most materials expand when they’re heated up and then contract when they’re cooled down. For a molten lava flow, this means that it takes up less space after it cools. Cracks or joints form in order to compensate for that change in volume. If the contraction happens under specific  conditions, columnar joints will form. But what exactly are these conditions?

Researchers, Lamur et al. 2018, brought rock samples from Iceland’s Eyjafjallajökull volcano into the lab and performed specialized heating and cooling experiments. Their goal was to understand how and why columnar jointing takes place by focusing on the temperature window in which they form. To do this, they heated the field samples to simulate the natural pressure conditions expected in a typical lava flow. They observed how the rocks changed as they cooled to figure out how temperature affects the formation of columns and the size of the gaps between them. Lamur and his team found that columnar jointing happens at 840-890ºC (1544-1634ºF). This means that columnar joints only form when the  molten rock has begun to solidify, but is still hot enough for the rock to be somewhat flexible. They also used their observations to predict how wide the gaps between columns would become under different temperature conditions. The size of those gaps tells us about how well fluids would be able to circulate within the rocks. The larger the gap, the greater its contribution would be to secondary mineral deposition and to geothermal and hydrothermal resources.

References

Code Poet. "Giant's Causeway, Co. Antrim, Northern Ireland." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 28 May 2005. https://commons.wikimedia.org/wiki/File:Causeway-code_poet-4.jpg

Jonathunder. "Devils Tower National Monument at sunset in Wyoming, United States." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 23 August 2017. https://commons.wikimedia.org/wiki/File:SquareDevilsTower.jpg

Konstantin, Phil. "Closeup photo of the columns on Devil's Tower." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 24 May 2003. https://commons.wikimedia.org/wiki/File:DevilsTowerCloseupByPhilKonstantin.jpg

Lamur, Anthony, et al. "Disclosing the temperature of columnar jointing in lavas." Nature communications 9.1 (2018): 1432. https://www.nature.com/articles/s41467-018-03842-4

Mayer, Matthew. "Close up of Giant's Causeway." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 25 April 2003. https://commons.wikimedia.org/wiki/File:Giants_causeway_closeup.jpg

NASA. “Marte Vallis, Mars.” Nasa Earth Observatory, 31 October 2007. https://earthobservatory.nasa.gov/IOTD/view.php?id=38904