The black triangle become a source of acid rain when?

When factories and power plants in that area burned coal, fossil fuels, sulfur dioxide and nitrogen oxides

Geologists use  seismic waves to  locate an earthquake's epicenter.. 

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.