Map of U.S. Natural Radioactivity
U.S. Geological Survey image
Natural radioactivity arises from several different geological materials. In addition, higher altitude means a higher level of natural radiation from cosmic rays. The following explanatory text about the numbers on this map is from the US Geological Survey.
1. Great Salt Lake: Water absorbs gamma rays so it shows as no data area on the map.
2. Nebraska Sand Hills: Wind has separated the lighter quartz from the clay and heavier minerals that usually contain uranium.
3. The Black Hills: A core of granites and metamorphic rocks high in radioactivity is surrounded by less radioactive sedimentary rocks and gives a distinctive pattern.
4. Pleistocene glacial deposits: The area has low surface radioactivity, but uranium occurs just below the surface. Thus it has a high radon potential.
5. Deposits of glacial Lake Agassiz: Clay and silt from a prehistoric glacial lake have higher radioactivity than glacial drift surrounding it.
6. Ohio Shale: Uranium-bearing black shale with a narrow outcrop zone was scooped up and spread over a large area in west-central Ohio by glaciers.
7. Reading Prong: Uranium-rich metamorphic rocks and numerous fault zones produce high radon in indoor air and in ground water.
8. Appalachian Mountains: Granites contain elevated uranium, particularly in fault zones. Black shales and soils above limestone also contain moderate to high levels of uranium.
9. Chatanooga and New Albany Shales: Uranium-bearing black shales in Ohio, Kentucky, and Indiana have a distinctive outcrop pattern clearly defined by radioactivity.
10. Outer Atlantic and Gulf Coastal Plain: This area of unconsolidated sands, silts, and clays has one of the lowest radon potentials in the United States.
11. Phosphatic rocks, Florida: These rocks are high in phosphate and associated uranium.
12. Inner Gulf Coastal Plain: This area of the Inner Coastal Plain has sands containing glauconite, a mineral high in uranium.
13. Rocky Mountains: Granites and metamorphic rocks in these ranges contain more uranium than sedimentary rocks to the east, resulting in high radon in indoor air and in ground water.
14. Basin and Range: Granitic and volcanic rocks in the ranges, alternating with basins filled with alluvium shed from the ranges, give this area a generally high radioactivity.
15. Sierra Nevada: Granites containing high uranium, particularly in east-central California, show as red areas.
16. Northwest Pacific Coastal Mountains and Columbia Plateau: This area of volcanic basalts is low in uranium.