Millaa Millaa's Columnar Basalts EarthCache

Millaa Millaa's Columnar Basalts

During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns.

The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are often erroneously described as being predominantly hexagonal.

The size of the columns depends somewhat on the rate of cooling; very rapid cooling may result in very small columns, while slow cooling is more likely to produce large columns.
 

The Waterfall

Text taken out of: "Rocks, Landscapes & Resources of the Wet Tropics" p.19
pictures and graphics: Vulkanpark Eifel

1. Go to the border of the litte lake. You find pieces of the columns. The ends of the columns are in the shape of a polygon. Are they all hexagons? If not, what do you see?
2. Measure the width of some colums and decide whether they cooled rapidly or slowly.
3. Pictures are welcome.

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