arizona, geomorphology

How do you form a mushroom rock?

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Thanks to a variety of weathering and erosive processes, rocks can come in all shapes and sizes. The above rock from the McDowell Mountains near Phoenix, AZ has a peculiar shape similar to that of a mushroom!

This rock is an example of a tor–an exposed mass of rock that abruptly rises above the surrounding ground surface.

Tors most commonly form in granite, like the mushroom-shaped tor above. The original large granite rock mass has fractures and joints throughout. Water can travel down through these fractures and begin to erode the rock either through freeze-thaw weathering or groundwater weathering. As water freezes, it expands, thus widening and weakening cracks. The water over time can also ‘eat away’ at the rock over time to erode it.

As the cracks in the rock are widened, portions of the rock may begin to break away into boulders or erode into finer material. Tors usually occur when the original jointing in the rock is spaced farther apart so it can remain connected to the bedrock below instead of detaching as a boulder.

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The ground surface continues to lower via erosion and the tors begin to emerge. However, not all tors come shaped like mushrooms! To get a top heavy tor, the tor needs to be shaped by wind erosion. Wind can pick up sand particles from the ground that will abrade the rock. This type of erosion occurs closer to the ground (sand can get a bit heavy to carry!) so the lower half of the tor becomes more eroded than the top.

Voila, you then have a mushroom-shaped rock!

Tor image by author, McDowell Mountains.
Tor diagram source unknown.

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