Blocks

Blocks are boulders being born. Solid rocks, like this granitic outcrop on Mount San Jacinto in southern California, fracture into blocks under a set of conditions lumped under the name physical weathering. The most important of these conditions is temperature, which fluctuates above and below freezing almost every day. Water in cracks will expand as it freezes at night, forcing cracks wider, then trickle further into the expanded crack during the day. To a lesser extent, the cycling of temperature also affects the various minerals in the rock, causing the grains to loosen. Another factor that guides the direction of cracking is internal stresses in the rock, which cause jointing and exfoliation. Between these forces, the work of tree roots and earthquakes, mountains are steadily dismantled into blocks that tumble down the slopes.

As blocks work their way loose and form steep deposits of talus, their edges begin to wear down and they officially become boulders. When erosion wears them down smaller than 256 millimeters across, they become classified as cobbles.

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