The superheated steam is raised in geothermal plants by the heat of? 

The superheated steam is raised in geothermal plants by the heat of Magna or lava.

 Magma is a high-temperature and highly viscous fluid. When magma breaks the earth's crust and rises as lava on the surface, a volcano is created. Any surface water that leaks into the earth's crust through cracks and comes into contact with magma gets heated and escaped to the surface as hot springs. The steam trapped in rocks is used by geothermal plants and it is routed through a pipe to a turbine and used to generate electricity.