The small Dotsero maar in NW Colorado, 2 km NE of the small town Dotsero, near the junction of the Colorado and Eagle Rivers west of the Gore Range, is the only volcano in Colorado that has had activity in the past 10,000 years.
It erupted ca. 4000 years ago, producing an explosion crater (maar), lahars, and a 3 km long lava flow.
The crater of Dotsero is 700 m in diameter and 400 m deep with a flat floor covered by debris. It exposes a lithic-rich pyroclastic deposit, a strombolian scoria-fall deposit, lahars deposits, and a lava flow.
Although easily accessible by Interstate highway 70 which cuts across a lava flow from Dotsero, this volcanic center is one of the least known in the western United States.
The youngest feature, the Dotsero maar, was erupted along a ridge consisting of evaporites and reddish oxidized sandstones of Pennsylvanian age.
Small scoria cones were constructed along a NNE-SSW line on either side of the maar. Small lahars preceded the eruption of a basaltic lava flow that traveled 3 km down two narrow V-shaped valleys and spread out onto the floodplain of the Eagle River, diverting the river to the south side of the valley.
Older Pleistocene basaltic lava flows occur nearby at Willow Peak, McCoy, and Triangle Peak.
Source: GVP Dotsero volcano information