In 2011 Andrés Ruzo began studying the Shanay-timpishka, a boiling river in the Amazon. It is described as “a natural feature: a non-volcanic, geothermal feature flowing at anomalously high rates”, see this National Geographic interview. The non-volcanic claim was interesting to me… Apparently groundwater finds its way in deep-rooted faults in the Earth’s crust and is heated according to the geothermal gradient.
So I wanted to find it on maps. Here it is on Google Maps. The approximate coordinates are -8.81,-74.73 which is in Peru. The area has decent satellite imagery, but nothing about the immediate vicinity appears remarkable. It is ordinary Amazonian jungle.
However, I opened a topographic map and what I saw suprised me:
As I marked with the red dashed line, the river appears to be located within an old oblong caldera of 6-7 km in diameter, with a rim rising about 100-200 meters above the caldera floor. In the center we even see a geological feature consistent with a resurgent dome. The dome rises to about 380 meters in elevation, matching the highest point on the rim also at 380 meters elevation.
Noticing all this would not have been possible without the help of a topographic map. 100-200 meters in elevation difference spread across kilometers in the jungle is not obvious from satellite imagery.
Calderas are typically associated with dormant or extinct volcanoes. So my theory is this was the site of an active volcano, and the “non-volcanic” claim may need revisiting.