NAVIGATION:  Introduction > Back to Stop 3

Marcus Landslide Virtual Field Trip
McDowell Mountain Regional Park, AZ

Stop 3 of 11 (Site 3b)

Stop 3 of 11 (Site 3b) 

Spheroidal Weathering -  Almost all of the granite boulders littering this portion of the McDowell Mountain Regional Park have a spheroidal shape.  It is almost as though a sculptor started with a cube or a cube-like shape and rounded the corners.  That is exactly what happened, but agent of change was and is weathering.

Granite starts out in the subsurface already fractured by "joints".  The pattern of fracturing splits granite into blocks that are cube-like in shape.  That's step 1.

Step 2 happens when the joint fractures come into contact with ground water that seeps into the cracks.  This all takes place meters beneath the surface, where chemical decay weathers the minerals into grus (granite sand).  Corners of the cube have three joints that intersect, so the corners decay the most.  Decay on the corners gives the boulder a "spheroidal" shape. This rounding still takes place all in the subsurface where you can't see the boulders (except in road cuts).

Step 3 is erosion.  Erosion of the decayed granite (grus) exposes these boulders at the surface.  Spheroidally-weathered boulders are "born rounded" by subsurface mineral decay (subsurface weathering).

This same spheroidal weathering process does not just operate on boulders.  Entire granite mountains are rounded as the largest joints intersect.  You can see larger rounded forms, like Pinnacle Peak, all over the McDowell Mountains.  You can also see domes (spheroid weathering) in such places as Mount Rushmore and Yosemite National Parks.