Pyramid Mountain erupted about 12,000 years ago. It was once mistaken for a cinder cone and is now known to have erupted underneath about of glacial ice. The volcano erupted vigorously and cooled rapidly when the hot lava contacted the surrounding ice and melted it, creating an envelope of water around the rising volcano. The result was a cement-like surface with smoothed cobbles of granite, schist, phyllite and other non-volcanic pebbles which were carried by the ice from many kilometers distant. As a result of this type of eruption, Pyramid Mountain has no crater and no lava flows stretching away from its base. The secondary cone to the east (best seen from Clearwater Valley Road near Hemp Creek) was not formed by a separate eruption, but was simply a slump when the ice melted away from Pyramid, releasing the pressure against its slopes.