How are volcanoes and other geologic phenomenon (hot springs, geysers, etc) created?
Heat from the Earth
Volcanoes, geysers and hot springs are all the resulting of the effects of the geothermal heat generated by the Earth.
The main source of this heat is the decay of radioactive elements and the residual heat accumulated within the planet during its formation.
The deeper underground the warmer the environment following what is called "geothermal gradient" (which in average is 25 degree Celsius every kilometer). There are areas where such gradient is much higher and here is where volcanoes and hot springs can be found.
The source of the fluids for geysers and hot springs is the groundwater that is then heated by the geothermal heat often at extremely high temperatures due to the fact that the high pressure generated by the lithostatic head (the weight of the rocks) pushes the boiling point of water well above the one at atmospheric pressure.
Hot springs are often considered the last stage of volcanic activity developing once the volcanoes are in their final stage of life slowly cooling down.
In some areas, such as Iceland, this geothermal activity is an important source of energy that can be used for the production of electricity using specifically designed power-plants based on steam turbines.