Studying how radon enters buildings from the soil and through the water system is also an important part of the geology of radon. In general, the uranium content of a soil will be about the same as the uranium content of the rock from which the soil was derived. The bright-yellow mineral tyuyamunite is one of the most common uranium ore minerals.
Some types of rocks have higher than average uranium contents.
These include light-colored volcanic rocks, granites, dark shales, sedimentary rocks that contain phosphate, and metamorphic rocks derived from these rocks. These rocks and their soils may contain as much as 100 ppm uranium. Layers of these rocks underlie various parts of the United States. The higher the uranium level is in an area, the greater the chances are that houses in the area have high levels of indoor radon. But some houses in areas with lots of uranium in the soil have low levels of indoor radon, and other houses on uranium-poor soils have high levels of indoor radon.