Some 600,000 years ago, basalt lava at 2000 degrees Fahrenheit flowed down the Portneuf River canyon from vents near Bancroft in Gem Valley. Two flows, advancing up to several miles per day, travelled the 60 miles to Ross Park in southern Pocatello. Years of slow cooling formed vertical columnar joints seen on exposed edges of the lava flows. The Gem Valley Lava Field is related to the much larger Snake River Plain – Yellowstone Volcanic Field, which has been active for the last 17 million years. White volcanic ash present in the Pocatello foothills and the rhyolite lava that underlies Howard Mountain west of the city erupted from the Snake River Plain. Rhyolite lavas on the plain were covered by extensive basalt flows, leaving a nearly flat landscape. The three large buttes in the desert 45 miles north of here are rhyolite domes which intruded into and through the basalt about half a million years ago. Volcanic activity created the Snake River Plain and the Yellowstone area as the continental crust drifted slowly across a hot spot within the earth’s mantle. Eruption of basalt on the plain may continue; the youngest lava at the Great Rift and Craters of the Moon National Monument, north of American Falls, is only about 2000 years old.