John Joly introduced the Radioactivity Theory to explain the “thermal cycles” of the Earth. He argued that radioactive minerals in the continental crust generate immense heat that cannot easily escape. This heat eventually melts the underlying basaltic layer (sima), causing the continents to sink slightly and the oceans to encroach. As the heat eventually dissipates, the rocks re-solidify and contract, exerting lateral pressure that folds the sedimentary layers into mountains. While later superseded by Plate Tectonics, Joly’s work was pioneering in recognizing that internal heat driven by atomic decay is a primary engine for the Earth’s surface geomorphology.