Monday, 27 December 2010

Cracking the clues

Your pupils may have noticed that, when a pool dries up, it often leaves a muddy bed, which cracks into regular shapes (polygons) as the wet mud shrinks. Therefore, ancient mudcracks show us that the area where they are found must have been mud that dried out in the past. It must have been surface mud rather than mud laid down under deep water. So the cracks are key clues to the conditions in which the mud was laid down.
Polygonal cracking in natural materials is caused by shrinkage and the shrinkage is caused either by drying out or by contraction on cooling. By trying the Earth Learning Idea, ´Cracking the clues´, we can simulate shrinkage conditions that cause mud cracks in the classroom, and see the stages as they develop. The photo shows polygonal cracking which has occurred by contraction on cooling.
This is one of many Earth Learning Ideas in the Earth Energy category.

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