The history of the "pinnacles" below began
about 7,700 years ago when the eruptions of
Mt. Mazama were reaching their climax. Torrents
of red-hot, gas-charged pumice poured down
Mazama's slopes at speeds of up to 100 mph
(160kph). On top of this came a flow of heavier
rocks called scoria. These glowing avalanches
flooded downslope for many miles, leaving
deep deposits in their wake.
Temperatures in the deposits may have exceeded
750 F (400 C). Plumes of vapors appeared, as
gasses escaped from the settling rocks through
vents called fumaroles. Minerals in the gasses,
combined with extreme heat, welded the sides of
the fumaroles in the shape of slender cones. Since
then, streams have eroded a canyon through the
deposits, exposing the cones. Many of these fossil
fumaroles are hollow.
The glowing avalanche deposits took years to cool. Hot gasses escaped
through fumaroles whose shapes are preserved in the pinnacles.
Glowing avalanches from Mt. Mazama
filled surrounding valleys with deposits
of pumice and scoria. Today, streams
are cutting new valleys.