Spheroidal weathering is a type of physical and chemical weathering that creates rounded boulders and helps to create domed monoliths. This should not be confused with stream abrasion which also creates rounded rocks on a much smaller scale. A good example of spheroidal weathering can be found in the Alabama Hills area of eastern California.
Spheroidal weathering occurs whenever a mass of rock (most typically granitic in composition), experiences a drastic reduction in ambient heat and pressure (such as when a batholith is exposed at the surface). Since the rock formed at great temperatures and pressures (1,400 ° F and ~3,000 atm. for granitic rock). In granites there are three mutually perpendicular sets of joints that develop when this overburden is removed. Two things cause this in granites; the quartz crystals expand about 5% in size and acidic water attacks the feldspar minerals, turning them into clay. The corners become rounded because angular edges provide more than one area of attack by exposing a greater amount of surface area. Thus edges and especially corners of an angular block weather faster than flatter surfaces. The ultimate result of this process is a rounded bolder or a domed monolith (such as the rounded back of Half Dome in Yosemite National Park; exfoliation is a related type of weathering that aids in dome creation).
However, the process of spheroidal weathering is slower than other common types of weathering such as frost wedging, and it becomes lower still at progressively lower temperatures (this slows down the chemical process of feldspar breakdown). Thus many granitic mountain peaks are jagged and craggey instead of rounded.
