A radiogenic engine
Deep within the rugged Northern Flinders Ranges, the hot springs near Mount Gee are heated by radioactive decay, not volcanoes. their warmth comes from the slow, steady radioactive decay of elements within ancient granite. This geological region, a 610-square-kilometer private conservation reserve called the Arkaroola Wilderness Sanctuary, sits on what is known as the Mount Painter Inlier. The basement rocks here include Mesoproterozoic granites, such as the Mount Neill Granite, formed approximately 1.58 billion years ago.
These granites are exceptionally high in the heat-producing radioactive elements uranium and thorium. The rate of heat production from these rocks averages around 16 microwatts per cubic meter (μWm⁻³), which is more than four times the average for typical granites. This constant, low-level release of energy from atomic decay has been warming the deep crust for hundreds of millions of years. Rainwater, known as meteoric water, seeps through deep fractures and faults in the earth's crust, where it is heated by these "hot rocks." The water then rises back to the surface along conduits like the Paralana Fault, emerging as warm, radioactive springs.
A hazardous brew
The most prominent of these springs in the area is the Paralana Hot Springs, which shows this process. The water emerges at a temperature between 57 and 62 degrees Celsius. While geothermally heated, the water is not acidic, maintaining a neutral pH of 7 to 8. As the water circulates through the uranium-rich granite, it leaches out radioactive elements. The emerging water contains dissolved radium, but the most significant health concern comes from the gases bubbling to the surface.
These bubbles are a mix of carbon dioxide, nitrogen, helium, and radioactive radon gas. Radon is a decay product of radium and, being heavier than air, can accumulate in dangerous concentrations near the springs. Prolonged exposure is hazardous, and swimming in or drinking the water is not permitted. Despite the risks, the springs support unique life, including extremophile algal mats that are adapted to the high temperatures and radioactivity. The entire Mount Gee area is a well-preserved fossil of a much larger epithermal system, showing geological processes that are typically hidden deep underground.