c. at depths of about 50 to 200 km below earth's surface
Explanation:
Rocks begins to melt when sufficient temperature and pressure is applied on them.
To attain these conditions of temperature and pressure, a rock must be taken deep into the earth. With depth, the geothermal gradient of the earth increases. The geothermal gradient determines the temperature conditions a rock might exposed to deep within the crust. Rocks usually melt below the crustal levels and at the mantle.
The crust runs from about 0-100km in depth and the mantle takes off from there. We expect melting to fall beyond this region.
More hydrothermal resources exist in the Western regions of the United States because lack of groundwater and extended drought has reduced the number of vents.
