How were basalt columns formed?

• The formation of basalt organs is due to a phenomenon of "thermal contraction", according to our partner The Conversation.

• But this explanation is not enough for all the observations… Materials science then provides valuable clues.

• This analysis was conducted by Bernard Guy, teacher-researcher at the Mines Saint-Etienne school (Institut Mines-Télécom).

This photograph shows basalt columns.

It can be found all over the world, where flows of magma emitted by volcanoes have solidified and cooled.

These spectacular and fascinating structures have been, and continue to be, the subject of numerous studies, both for characterization in the field and in the laboratory, and for physico-chemical and mathematical modeling.

A well-known example is the Giant's Causeway in Ireland.

Here, as elsewhere, you can see the columns of the organs, but also the structure in section, and therefore the interior of each hexagonal column of basalt.

This helps to better understand how these magnificent structures are formed.

Indeed, the most common explanation for the formation of basalt columns is that of "thermal contraction".

When the basalt rises from the depths of the earth's crust, the solid mass cools: it is still hot, not yet structured but already solidified.

Then, the material contracts and shrinkage fractures appear and form the rectilinear contours of the different hexagons.

The analogy put forward is that of the dried mud which retracts and reveals tilings of polygons.

Organs grow perpendicular to the edges of lava flows, and parallel to thermal gradients.

But this explanation is not enough for all observations: in the section of the hexagonal columns, we sometimes see a circular design.

Structures radiating from the center can also occur, also respecting a cylindrical symmetry.

​When small-scale structures make it possible to understand large-scale ones

How to explain these circular and radiated structures?

They show an early internal structuring of the solid mass, contrary to the hypothesis of contraction of a homogeneous solid mass.

Materials science provides valuable clues.

Indeed, metallurgists specializing in the solidification of metal alloys observe similar columnar structures in their ingots.

Under particular conditions of thermal gradient, the interface between the liquid and the solid presents “fingerings”: fingers of solid develop and move forward in the liquid.

They end up welding together, which is what makes the organs appear.

The possible posterior thermal contraction of the solid mass can, depending on the case, disjoin the previously formed prisms.

Our “VOLCANOES” file

The previous mechanism of fingering can play in the case of the prismation of volcanic rocks: it makes it possible to explain the circular and radiated structures, as well as the large organization of the flows where the two mechanisms (contraction/digitation) can play, and finally to report on the characteristics of the rather special organs discovered in the slag heap of La Ricamarie near Saint-Étienne, where remains of coal burned within sandstone and argillites and formed magmas.

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This analysis was written by Bernard Guy, teacher-researcher at the school Mines Saint-Etienne – Institut Mines-Télécom.

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