Natural Near-Spheres at Joshua Tree National Park

July 06, 2018

SpheroidalWeatheringJoshuaTreeNP2018_1.jpeg

Photographer: Thomas McGuire
Summary Author: Thomas McGuire 

JoshuaTreeSunsetWe usually associate weathered and eroded landforms with highly irregular shapes such as the Grand Canyon or the Himalayan mountains. But Mother Nature can also use the same processes to produce forms that come remarkably close to geometric shapes. In this case, rectangular solids changing to near-perfect spheres.

Solid rock often develops cracks due to stress caused by regional forces or shrinkage. These forces break the rock but don't cause relative displacement, such as occurs with faulting. Cracks allow water to penetrate deep into the rock, resulting in both physical and chemical weathering.

Weathering is fastest where the ratio of surface to volume is greatest, such as at the corners of joint blocks. Thus, as a jointed rock weathers below ground, the unweathered portion becomes progressively more spherical. The rocks featured above were observed at Joshua Tree National Park in California. The uniform granite has cracked (forming joints) making near-cubic blocks. Through time these blocks have weathered into near-spheres. The colorful sunset at left shows another reason why Joshua Tree National Park is so special. Photo taken on May 19, 2018.

Photo Details: Top - Camera: SONY DSC-HX80; Exposure Time: 0.0025s (1/400); Aperture: ƒ/4.5; ISO equivalent: 80; Focal Length (35mm): 54.