Non-welded Rhyolite Tuff

In the upper levels of the Bishop Tuff, pressure was low. Pumice and ash cooled quickly, retaining their original shapes.

Slightly Welded Rhyolite Tuff

In the last 2.1 million years, three enormous eruptions have occurred in and near Yellowstone National Park. Each produced huge flows of hot debris that came to rest to form hard volcanic rocks like this tuff.

Slightly Welded Rhyolite Tuff

In the last 2.1 million years, three enormous eruptions have occurred in and near Yellowstone National Park. Each produced huge flows of hot debris that came to rest to form hard volcanic rocks like this tuff.

Strongly Welded Rhyolite Tuff

Each of Yellowstone's three major eruptions produced huge flows of hot debris that formed thick hot deposits. The deposit interiors were hot enough to compress, squeezing once-rounded pumices into flattened disks, such as in this strongly welded tuff.

Welded Rhyolite Tuff

The thin, dark streaks in this specimen were once rounded pumice lumps. Pressure and heat deep within the Bishop Tuff welded pumice and ash together.