A granite dome is a dome of granite, formed by exfoliation.
Granite forms plutons of igneous rock several kilometers below the surface as magma slowly cools and crystallizes. The granite is under great overhead pressure.
Then, granite is uplifted to the surface during a mountain-building event. During the mountain building process, the overlying rock is eroded as the granite is uplifted, and the pressure on the granite reduced. The granite expands and forms fractures or sheet joints parallel to the surface. The granite then erodes in concentric layers (similar to how an onion peels) forming rounded masses called exfoliation domes.
While found worldwide, 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.