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| Marcus Landslide Virtual Field Trip |
| McDowell Mountain Regional Park, AZ |
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Joints - Granite dominates this portion of McDowell Mountain Regional Park, and with granite comes jointing. Joints are cracks in rock, not what college students sometimes think:
Compression, shearing, and temperature changes all act in different ways to create these joints. Imagine a body of magma a kilometer or more across buried deep in the earth. The magma cools very slowly to produce the granite rock. That cooling, however, does not produce granite joints. Extreme stresses come afterwards when regional pressures act to squeeze, twist, and pull apart the body of magma until it cracks — making the fractures or joints. In the case of granite, the cracks tend to break granite into roughly cube-shaped blocks, no matter whether you are in the McDowell Mountains (below):
or Joshua Tree National Monument seen here:
In your travels you will encounter may different kinds of
jointing.
For example, when a lava flow cools, it contracts. The
contraction
gives you vertical joints that form columns. This is a view of
Devil's
Postpile National Monument, where glacial erosion helps you see see all
the way through the lava flow:
 In the Chiricahua Mountains of Southeastern Arizona, the cooling of volcanic rock also caused vertical joints:
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