<html>The rocks deep down are hot and provide heat for deep water. But, the rocks near the surface are heated by rising hot water. Bear in mind that rocks are poor conductors of heat (we put them around our campfires).  So only the rocks near the surface that are in contact with hot water are hot, and they can only get as hot as the water that contacts them. -- Will Moats  ********************  ... the result is that the rocks around the interstices will increase significantly in temperature. My understanding has been that the water is heated a great depth and then carries the heat upward.  If that's true, the rocks close to the surface (within the geysers plumbing region) will never get hotter than the water and they won't heat without water moving through. I don't have a reference, so speak up if you have contrary evidence.  Are the rocks heated by the water, or the other way around? </html>