A Thin Spot on the Roof of the World
The Tibetan Plateau, often called the "Roof of the World," is the product of the ongoing collision between the Indian and Eurasian tectonic plates. This immense geological pressure has thickened the crust to an average of 70 kilometers. Yet, within the Nyainqentanglha Range, the Earth's crust is unusually thin and hot. This thermal anomaly manifests at the surface as one of the most active geothermal areas in the Himalayas, with numerous hot springs, boiling pools and geysers dotting the area at altitudes over 4,500 meters.
These hot springs are not heated by the normal geothermal gradient. The heat source is different. The continental collision that thickened the plateau also created zones of east-west extension, forming large north-south trending rift valleys. The Yadong-Gulu rift, in particular, has stretched and thinned the crust. This thinning allows hot material from the Earth's mantle to well up, creating zones of partially molten rock just 15 to 20 kilometers below the surface. This shallow magmatic activity is the engine driving the region's intense hydrothermal systems. Meteoric water, primarily from rain and snowmelt, percolates deep into the crust through extensive fault networks, gets superheated by the shallow magma, and then rises back to the surface.
Geochemical Fingerprints of the Mantle
Scientists confirm the mantle connection by analyzing the gases dissolved in the spring waters. Specifically, they measure the ratio of two helium isotopes: helium-3 (³He) and helium-4 (⁴He). Helium-3 is largely primordial, trapped in the Earth's mantle since the planet's formation. Helium-4, conversely, is continuously produced in the crust by the radioactive decay of uranium and thorium. Most continental hot springs show very low ³He/⁴He ratios, indicating a crustal origin.
In the Nyainqentanglha region, however, the helium isotope ratios are an order of magnitude higher than typical crustal values. This high ratio shows mantle-derived volatiles, confirming that these hydrothermal systems are directly fed by heat and gases from a shallow magma body. Further evidence comes from the sheer thermal output. The Yangbajain geothermal field, a major field in the range, has a natural heat flow of up to 107,000 kilocalories per second. Boreholes drilled into the geothermal reservoirs have recorded temperatures as high as 329°C at depths below 1,850 meters. The total carbon dioxide (CO2) emission from the Gulu-Yadong rift is estimated at a significant 1.76 × 10⁷ tons per year, indicates large-scale degassing from a deep geological source.