What do granite and bananas have in common? Radioactivity. As it turns out, radioactivity is all around us and has been for eons.
About 25 years ago, I was doing radiation surveys at a NASA research site, searching for radioactivity from past research activities. Sure enough, as I started walking towards the first building, the radiation readings began to rise, ending up about three times higher than normal when I was adjacent to the building. “Bingo!”
But as I surveyed around the structure, I noticed I was getting the same readings all over. And that was puzzling – radioactive contamination isn’t that uniform. Eventually, it dawned on me that I wasn’t seeing evidence of contamination or the presence of radioactive materials; those readings came from the building itself. More correctly, the clay making up the building’s bricks. Some types of clay have high potassium levels, and about one potassium in 10,000 is naturally radioactive (K-40); that’s what my meter was detecting. Anything with a lot of potassium – clay, bananas, salt substitute, any number of rocks and minerals will give off radiation.
It turns out that there are types of granite that contain elevated levels of potassium and a few dozen other radionuclides that come from the radioactive decay of uranium and thorium. I’ve also run into elevated levels of radioactivity in coal, black shale, at factories that process rare earth elements, and in the oil fields of North Dakota. On the other hand, when I was in Japan and Hawaii, the radiation dose rates were lower than what I normally see in, say, Ohio and New York.
Can we predict where we’re going to run into elevated levels of natural radioactivity?
A lot of it comes down to geochemistry.
Coal forms in swamps that are choked with decaying leaves, branches, and the remains of other plants. When organic matter decays, it removes oxygen from the water – water near the bottom of the swamp is bereft of oxygen. Uranium is soluble in oxygen-rich water but is insoluble in the anoxic water found in a coal swamp. When water, carrying traces of dissolved uranium, flowed into the ancient wetlands that became today’s coal seams, the uranium precipitated out of solution, adding radioactivity to organic material that became today’s coal.
That same process also explains the radioactivity in black (organic-rich) shales, oil, and natural gas deposits. In general, dark-colored sedimentary rocks are more likely to contain radioactivity than are lighter-colored sediments. Except for travertine (freshwater limestone), that is – but I’ll get to that in a minute!
Ramsar Iran has the highest natural radiation levels of any inhabited place on Earth. Some of the people in Ramsar regularly receive as much as five times as much radiation exposure as workers at nuclear reactors are permitted to receive. The source is not an underground bunker with weapons of mass destruction, but deep underground reservoirs of magma enriched in uranium, as are the rocks on its periphery. Groundwater circulates that deep underground, but by the time it reaches the uranium-rich rocks, it’s lost its oxygen and cannot dissolve uranium out of the rocks. Radium, one of uranium’s decay products, can still dissolve into the now-hot water, along with calcium and other elements. When the water returns to the surface, the calcium and the chemically similar radium precipitates out of the solution, forming travertine. The travertine contains scads of radium – up to a million times as much as we’d find in a shovelful of dirt from the garden. This same process – the substitution of uranium, thorium, radium, and other natural radionuclides – is also why we tend to see high levels of radioactivity in many ore minerals.
And that leaves us with granite!
Picture a pool of molten rock, a magma chamber, deep underground. If it works its way near the surface, it might end up fueling a volcano, but it might also just sit there, cooling and solidifying. But the magma doesn’t solidify randomly – there’s a pattern (called Bowen’s Reaction Series) to the minerals that form and the elements of which it is comprised. Minerals made of the smallest atoms form first, leaving the residual melt enriched in larger atoms. The minerals that form last are made of large ions, including naturally radioactive uranium, thorium, and potassium. This is why granites, the last rocks to form, have more radioactivity than the darker rocks that form first.
But it’s even more interesting than that – over the eons, these large ion-rich rocks have agglomerated, forming the continental crust. Over the Earth's history, the continental crust has become increasingly enriched in these large ions, while the Earth’s mantle has become increasingly depleted. This is why Hawaii and Japan, made from magma that originated in the mantle, are so low in natural radioactivity.
A quick summary might help keep it all straight.
- Uranium precipitates out of solution in places where the oxygen is stripped from the water (such as places where organic material is rotting). This is why dark sedimentary rocks and rocks that host fossil fuels tend to have elevated radioactivity levels.
- Uranium, thorium, and potassium have geochemical properties similar to those of calcium and rare earth elements. So ores of rare earth elements and metals, and in some circumstances, travertine can be radioactive due to the substitution of naturally radioactive atoms for similarly sized stable atoms.
- And in igneous rocks, the large ions, including uranium, thorium, and potassium, are the last to form minerals as a magma chamber cools, making the last-formed minerals, which tend to be lighter in color, to be more radioactive.
- Bonus reason! – many minerals contain potassium, which is why some clays, feldspars, and other potassium-rich minerals are somewhat radioactive.
There are other sources of radioactivity in nature. “Primordial radionuclides” present since our planet formed, and a smattering of radionuclides formed by the interactions of cosmic rays with atoms in our atmosphere. But by and large, this explains why granite countertops are “radioactive,” but marble counters are not; why radioactivity is found in fossil fuel deposits but not in solar cells (which tend to be made of silicon and other smaller ions); and why volcanic islands (comprised of black basalts from the mantle) are less radioactive than continental mountains made of granite.
It’s not only the rocks that are more or less radioactive; it’s also the things made of them. This is why I always got higher radiation readings on the Brooklyn Bridge (made of granite) and when flying over cemeteries (filled with granite headstones) than over other parts of the city where the buildings were made of sandstone or limestone.
There are virtually no naturally radioactive materials that emit enough radiation to be dangerous. The countertops in my apartment kitchen are granite; they don’t worry me in the slightest. Nor the bricks of my building which contain potassium. Just because we can measure the radioactivity doesn’t make them dangerous. It just shows how very good we are at detecting radioactivity.