How were the columns of “basalt members” formed?

This image shows basalt columns. It can be found all over the world, where magma flows emanating from volcanoes have solidified and frozen. These astonishing and fascinating structures have been, and continue to be, the subject of numerous studies, both for field and in vitro characterization, and for physical, chemical and mathematical modeling.

A well-known example is this Giant’s Bridge in Ireland. In this place, as elsewhere also, one can see the columns of the members, but also the sectional structure, and thus the interior of each hexagonal column of basalt. This helps to better understand how these remarkable structures are formed.

In fact, the most common explanation for the formation of basalt columns is “heat shrinkage”. When basalt rises from the depths of the earth’s crust, the solid mass cools: it is still hot, not yet formed but already solid. Next, physical contracts and contraction fractions appear and form the straight outlines of the various hexagons. The analogy offered is that of dry mud that retracts and reveals the roofs of the polygons. Organs grow perpendicular to the edges of the lava flows, and parallel to thermal gradients.

But this explanation is not enough for all the observations: in the hexagonal column section, one can sometimes see circular design. Radiating structures can also occur from the center, also taking into account cylindrical symmetry.

Vertical basalt of the Giant’s Causeway, Northern Ireland – Lara Viduto / Pixabay

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

How do we explain these circular and radiating structures? They show an early endoskeleton of the solid, in contrast to the assumption of contraction of a homogeneous solid.

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Materials science provides valuable clues. In fact, metallurgists who specialize in hardening metal alloys note Vertical structures similar in their alloys. Under certain conditions of the temperature gradient, the interface between the liquid and the solid presents “fingers”: the fingers of the solid develop and move forward in the liquid. They end up soldering together, which is what makes the members pop. Possible reverse thermal shrinkage of the solid mass can, depending on the case, lead to the separation of previously formed prisms.

The former mechanism of the finger can play in the case of igneous rock prisms: it makes it possible to explain the circular and radiating structures, as well as the large organization of the flows where the two mechanisms (contraction / number) can play, and finally to calculate the characters Kind of special devices Slag was discovered in the Ricamare heap near Saint-Etienne, where the remains of coal burned inside the sandstone and argilite and magma formed.

This analysis was written by Bernard Gay, Research Professor at the École Mines Saint-Etienne – Institut Mines-Télécom.
The original article was published on Conversation.

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