An Excerpt from Girl Gone Wild
The heat from the hot springs comes from the volcanoes that make up the Cascade Range. The Earth is really just a cooling blob of magma floating in space. We live on the cooled outer crust of that blob on large cracked chucks of cooled rock we call tectonic plates. Every continent is a plate and every ocean has plates making up the seafloor. Where these plates meet, ocean plate to continent, is a subduction zone, a place where the heavy ocean plate meets and then plunges underneath the lighter continental plate. When this happens the oceanic plate melts sending up plumes of magma to the surface much like the rising blobs in a lava lamp. Eventually these blobs melt the continental crust above them and break through, erupting lava and ash and pumice and building volcanoes. A lot of the magma doesn't make it all the way to the surface, this magma fills in cracks and holes and faults and cools slowly over thousands of years. When it rains the water flows through the ground and encounters the heat from this leftover magma, it warms up and rises back up to the surface picking up dissolved minerals from the rock along the way, the closer the magma is to the surface, the hotter the water in the springs.
Photograph Courtesy of the USGS