Mudcracks - when everything shrinks

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Mudcracks - when everything shrinks - a mudcrack tells its story

My Story - I am an ancient set of  mudcracks
I am an ancient set of mudcracks from Elliot Lake in Northern Ontario Canada.
At first I was a mixture of mud and water on a shoreline, but as the water evaporated my volume decreased and I shrank evenly in every direction leaving hexagonal mud cracks behind.
Some time later these cracks filled with rock to make me look as I do now.

So if you see my shape in the rocks you are looking at a location that in the past was muddy place that dried out.

mudcracks explained
shrinkage from cooling
The shrinking because solids occupy less space than liquids so when mud dries or lava cools it shrinks.
If the shrinkage is gradual and even, it results in polygonal (stop-sign) shapes.
In mud the edges of the little stop-sign shapes will curl upwards and we know which was was up when the mud was deposited.

basalt column

Molten basalt lava will shrink in a similar fashion
most often if it is buried by an insulating layer of ash and is able to cool slowly and evenly
In this instance it will form three dimensional columns
- the most famous the Giant's Causeway in Ireland
or the Devil's Tower in South Dakota
Granite rock cools a little differently , cracking (the cracks are called joints) in shapes that are nearly cubes.
If you try to store liquified greenhouse gases or nuclear waste in a rock that has dried or was once molten
then that rock will contain many many cracks that would have to be sealed (if that is possible).