hydrothermal alteration makes Lava Creek Tuff appear different

Example of how the same rock type can look very different due to hydrothermal alteration. Left image is of the Lava Creek Tuff, which erupted during the formation of Yellowstone caldera 631,000 years ago, near Tower Junction. Right image is the same geological unit -- the Lava Creek Tuff -- at Monument Geyser Basin, where it has been heavily altered by hydrothermal activity.