From Mammoth Hot Springs we moved to the Old Faithful Inn for another 4 days. This is the busiest part of the park with 3 huge hotels. The Inn was built in the early 1900's with a distinctive use of crooked wood for corner cross pieces.
This part of the park is inside a 50 km diameter volcanic caldera that last erupted 650,000 years ago. A blob of magma a few miles under the surface heats up the water to produce geysers. This time the dissolved mineral is silica (much less soluble in hot water than calcite). It takes 100 years to build up one inch of silica at these geysers.