Exfoliation joint
Exfoliation in geology is a weathering process, mainly caused in arid areas by differential heating and cooling of rock surfaces. There needs to be a high diurnal temperature range. Hard rock such as granite forms plutons (a rising blob of lighter rock similar in form to those seen in a lava lamp, but far larger) several kilometers below the surface as magma slowly cools and crystallizes. The granite is under great pressure due to overlying rock.
Then, granite is rapidly uplifted to the surface during a mountain-building event. During the mountain building process, the overlying rock is stripped away by erosion as the granite is uplifted.
With the overling rock removed, the pressure on the granite is reduced. The granite expands and fractures. These fractures, called sheet joints, develop parallel to the exposed surface. The granite subsequently erodes in concentric layers (similar to the way an onion peels) forming rounded masses called exfoliation domes. Many such domes are found in the Sierra Nevada range in California, which includes the most famous exfoliation dome in the United States, Half Dome. Granitic surfaces in this region that have been exfoliated are identified by their lack of glacial polish.
