Radioactive decay is spontaneously happening in all rocks. All rocks are radioactive, because they contain radioactive uranium, thorium, potassium and rubidium.  This radioactivity is part of earth’s natural system and is the main cause of heat inside the earth and the driving force for earth’s tectonic system which leads to volcanoes, earthquakes and plate tectonics.  Understanding radioactive decay leads us to understand the age of the earth, the solar system and the universe.