Eruption Episode 1 rocks, Clear Lake Volcanic Field, California

Eruption episode 1 contains the oldest rocks in the Clear Lake Volcanic Field. The rock on the ridge in the background., located off Butts Canyon Road, is a lava flow that erupted around 2 million years ago. Episode 1 lava flows filled pre-existing valleys when erupted, but over time, weathering has eroded these pre-existing sediments, leaving the lava flows on the ridges that were once valleys – this phenomenon is called inverted topography. Eruption episode 1 lavas are primarily of basaltic andesite composition. USGS photo by Dawnika Blatter.