Columnar Basalt :: Shared Description

Natural columns of basalt are a fairly familiar sight in views of Staffa and the Giant's Causeway, but they occur in many other places as well.

When molten rock, called magma, is erupted on the surface of the earth, it is termed lava. Lava cools relatively quickly, so there is no time for large crystals to develop, and the resulting rock is fine-grained.

Basalt forms from cooling of lava with a low silica content and a high content of heavy elements like iron and magnesium. Such lava is very runny, and spreads out to cover large areas with successive flows, building up a flat-topped pile of layers that fills in and covers over the pre-existing landscape.

As it cools, the rock contracts, and it develops cracks. It forms hexagons because hexagons are the only shape that can completely fill a void without sharp corners. If you like, they are Nature's compromise between circles, where all parts of the surface are equidistant from the centre, and squares, which can also fill voids but have corners much further from their centres than the middles of their sides are.

The cracks propagate at right angles to the cooling surface, resulting in the formation of hexagonal columns. As the cracks go deeper into the lava flow, they can be deflected, for example if they lie on top of an uneven cold surface, forming curves and complicated patterns of columns.

Not every column is exactly hexagonal, but if you counted the number of sides per column in any given flow, the average number of sides would be six.
by Anne Burgess
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6 images use this description:

NM4986 : Columns Close-up by Anne Burgess
NM4986 : Columnar Basalt by Anne Burgess
NM4986 : Collapsing Columns by Anne Burgess
NM4986 : Basalt and Sandstone by Anne Burgess
NM4885 : Druim an Aoinidh and An Sgùrr by Anne Burgess
NM4986 : Basalt Columns by Anne Burgess

These Shared Descriptions are common to multiple images. For example, you can create a generic description for an object shown in a photo, and reuse the description on all photos of the object. All descriptions are public and shared between contributors, i.e. you can reuse a description created by others, just as they can use yours.
Created: Tue, 13 May 2014, Updated: Tue, 13 May 2014

The 'Shared Description' text on this page is Copyright 2014 Anne Burgess, however it is specifically licensed so that contributors can reuse it on their own images without restriction.

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