Local garden centers sell river rocks for landscaping, stones that range from the size of a fist to the size of a basketball. These are rocks that were once irregular and angular, but their corners have been rounded off by physical weathering in the form of years of bouncing and rubbing against their neighbors in the beds of streams and rivers. On hillsides far from any stream, though, there are also rounded boulders far larger than those river rocks. These boulders have never moved, yet their surfaces are smooth and round because of spheroidal weathering.
Mechanical weathering consists of abrasion and other physical action that breaks large rocks into small ones. Rocks are also affected by chemical weathering, processes that break them into smaller pieces by changing some mineral grains into different, weaker minerals. Chemical weathering changes both the composition and the appearance of the rocks. This type of weathering happens when a rock is exposed to air and water near the Earth's surface. In general, the igneous and metamorphic rocks that form at high temperature and pressure deep underground are more subject to chemical weathering because they are chemically unstable at the conditions found at the surface.
Almost all rocks found at or near the surface are broken up by fractures called joints. Rocks buried deep beneath the surface are under great pressure, but when the rock is no longer deeply buried, this pressure releases and the rock can expand a little. Because rocks are brittle, they break instead of stretching. The resulting breaks or joints form a network of near-vertical cracks that cross at high angles.
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In rocks that are at or near the surface, water seeps along the joints, attacking unstable minerals. This causes rocks to decompose and disintegrate at their edges, opening the joints wider and allowing even more water to reach the surfaces. At corners where two or more joints meet, water attacks from more than one direction, causing more rapid decomposition by chemical weathering. This extra disintegration at joint intersections tends to change sharp corners into rounded surfaces. When the decomposed rock is removed from the widened joints by running water, wind or gravity, the unweathered portions of the rock form a complex of rounded boulders in their original positions.
Spheroidal weathering is most common among coarse-grained igneous rocks, especially granite and similar rock types. It is more likely to be found in warm climates, where mechanical weathering by freezing ice is less likely.
Weathering of Man-Made Structures
Blocks of rock used to build some of mankind's oldest structures have been subject to spheroidal weathering after placement. Granite blocks used to build pyramids in Mexico and a Roman aqueduct in Spain show the effects of spheroidal weathering after 2,000 years of exposure to wind and rain.