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From a 2-dimensional bird's eye view, these columnar joints,
or cracks on the tops and bottom of the rock, will look roughly
like hexagons. As the basaltic magma continues to slowly and
uniformly cool into basaltic rock, the cracks will penetrate up
and down the rock and eventually meet. These cracks are called
joints because the movement is within a rock instead of along
a fault. The vertical columns will separate through natural
erosional processes to display pristine columnar basalts. The
more uniform and closely packed columnar basalts are, the less
eroded they are.
Why do Columnar Basalts form?
It is not obvious why we observe these highly symmetri-
cal geometric rock formations initially. Columnar basalts are
repeatedly consistent in shape and size, suggesting that it is
not a random act of nature. Like all processes under the laws
of physics, nature will always choose to minimize energy. Since
the magma has a long time to cool, the rocks naturally crack
at the lowest possible energy geometry: intersections of ap-
proximately 120 degrees. This suggests that when we see other
basaltic rock formations not columnar in shape, the magma
cooled more rapidly and irregularly jointed.
Where are Columnar Basalts found?
Columnar basalts are found where a mantle plume impinges
upon the Earth's outer crust. During this type of volcanism, the
volcanoes outpour thick, heavy, and high volumes of basaltic
lava. These places are called flood basalt provinces. They
are uncommon on Earth's surface today, but geologists have
observed them sporadically throughout the rock record for
much of Earth's history. Modern places to observe columnar
basalts are Iceland, Ireland, and California, where active fault
creates conduits for the deep mantle basaltic lava to access the
outer crust.
Columnar Basalt (continued)
Columnar basalts at the Devil's Post Pile Natural Monument in California, USA.
The bottom chunks are eroded column bits from the enact stack on top.
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Fig. 1: Diagram illustrating the relationship between inward cooling and
columnar joints.
Bird's eye view of columnar joints, demonstrating the first fractures to form in the
jointing process.
