Dotsero is the most recent of four quaternary volcanic centers in NW Colorado. The locations and relative ages are: Dotsero (4,150 years ago), Willow Peak (undated), McCoy (0.64 Ma) and Triangle Peak (1.98 – 1.87 Ma). Triangle Peak was the only one of these not monogenetic as it produced at least 11 lava flows before activity stopped. Willow Peak is described as a young cinder cone with a narrow lava flow extending 6 – 7 km E.
Dotsero is the only hydrovolcanic eruption, creating a maar, a small line of cinder cones, a short lava flow and at least one lahar. It is located near the Dotsero railroad junction in central Colorado, a bit over a two-hour drive west from Denver, some 220 km.
Physically, the region is in the Rocky Mountains to the west of the Front Range of Colorado, with higher mountains to the east and west. The mountains to the east host popular ski areas Keystone, Breckenridge, Copper Mountain and Vail. They are midway between Dotsero and Denver. Normally, the ski season closes in April – May. Over the last couple years, due to the amount of snow deposited during the year and the cold spring, several of the ski areas have not closed.
Due to the altitude, the mountains normally get significant snow, which provides runoff for local rivers, streams, lakes and most importantly, a relatively shallow water table, which was involved in the creation of the maar. Interstate 70 running west out of Denver cuts across the lava flow.
Dotsero is located in Eagle County, Colorado, near the junction of the Colorado and Eagle Rivers. Most of the surrounding area are the White River, Arapaho and Roosevelt National Forests.
The valley region is relatively dry, with under 45 cm/year of precipitation. From this, they get on average 115 cm of snow in the winters. Average highs range between 2 – 31 C. Trees are typically pine trees, with Ponderosa Pine and Aspen being relatively widespread.
Like many small towns in Colorado, this one exists simply because a railroad chose this location to lay its tracks. It was where a cutoff connecting two existing rail lines started. The other end of the cutoff is named Orestod, Dotsero spelled backwards.
The town is so small, there is not even a post office. It is simply a point on a census map. For years, the largest local economic activity was making cinderblocks from volcanic tephras mined in a small quarry. In 2014 a pair opened a distillery in what used to be part of the quarry. There is fly fishing available along the Eagle River and hiking off the highway. There is a dirt road to the crater drivable by a minivan. Campers can use the nearby White River National Forest.
Like most places in Colorado, recreational hiking is a big part of the way people explore the countryside. One such post in OutThere Colorado by Sophie Goodman in 2017 described hiking Dotsero. She notes that the crater is located along the Dotsero-Ute trail 1.5 km east of town. The Utes first used the trail. It was later followed by prospectors looking for silver ore in the Carbonate mining area at the turn of the 20th Century. The long trail is around 5 km one way with a total elevation gain of 600 m. A shorter version of it is less than 2 km for the round trip.
The eruption of Dotsero is something of an anomaly in Colorado, as most major volcanic activity in Colorado stopped 27 – 30 Ma. We covered part of that activity, the caldera outbreak in the original Volcano Café in 2013, and a pair of 2016 posts in Volcano Hotspot, one discussing the caldera outbreak in the SW US as it progressed westward and the other discussing volcanic plugs (Colorado has a very nice one).
It appears the eruption sequence started with a small chain of scoria cones and issued a small a’a lave flow that flowed 3 km southward and buried around 0.7 km2. Maar creation appears to have taken place late in the sequence, creating the current crater and producing a tephra fall. It also destroyed part of the scoria cone chain. The tephra fall included a substantial amount of red sandstone bedrock fragments. It is at least 20 m thick around the vent. Prevailing winds during the eruption blew the plume eastward. The eruption was dated from wood buried underneath the erupted scoria.
The current crater is 750 m in diameter and 76 m deep. It may have been as deep as 400 m when formed.
One localism are things called Dotsero Diamonds. These are bits of quartz embedded in chunks of the lava as it moved. They are generally tiny, slightly melted on the outside, and can be as large as a centimeter or so across.
The USGS created a stir in Colorado 2018 with release of its 2018 Volcanic Threat Assessment. This in turn nicely spun up local media with hair-raising coverage of what they referred to as “Colorado’s only active volcano.” Dotsero was listed as number 82 on this list, a moderate threat, due to its proximity to Interstate 70 and air traffic routes in and out of Denver.
The USGS Lists a volcano as active if it has erupted within the last 10,000 years.
While there has been no follow-on volcanic activity since the eruption sequence, there are still rumors of active magma chambers beneath the region. A concentration of these was thought to be in the neighborhood of Glenwood Hot Springs some 30 km WSW from Dotsero. As the Dotsero eruption was monogenetic, It would occur to me that distance is a bit far to be the same magma source that powered the Dotsero complex. On the other hand, multiple intrusive bodies in the region are certainly possible.
Glenwood Hot Springs bills itself as the world’s largest mineral hot springs pool. Commercial activity at Glenwood has taken place since 1888. Today, the most famous hos spring is at Glenwood Hot Springs Resort, the historic attraction and pool. The newest resort, Iron Mountain Hot Springs opened in 2015. The third geothermal attraction in the area is the Yampah Spa and Vapor Cave, where the visitor can experience a natural geothermal steam bath with mineral-rich steam in three underground caverns.
In addition to these resorts, there are multiple hot springs in river gravels at Glenwood Springs. Here is a spring of relatively hot water found in a valley some 8 km W from Glenwood Springs. This activity suggests an intrusive body (or bodies) may be present beneath the region. Water may find its way through the system based on local thrust faults.
Basalts of the four volcanic centers are primarily intraplate basalts that show contamination of the juvenile alkali basalts by partial melting of the crust during their ascent. The original basalts are chemically similar to ocean island basalts (OIB).
NW Colorado has a 20,700 km2 basalt area that dates 24 – 20 Ma. Its basalts divided into four groups. Group 1 is up to 210 m thick and is primarily basalt flows and basalt flows interlaced with sandstones. These were OIB melts sourced in the asthenosphere. Group 2 dates 14 – 9 Ma and is up to 180 m of basalts, andesites and interlaced with sedimentary rocks, meaning that eruptions were followed by extended periods of erosion.
Group 2 basalts are regarded as partial melts of lithospheric mantle under Colorado. Starting around 10 Ma, the region underwent major tectonic activity including uplift, reactivation of some Laramide structural features, warps, folds and faults. The Colorado River system started its downward erosion during the uplift. By 8 Ma, the Roaring Fork River had cut a 600 m deep canyon and created a broad flood plain. Thin Group 3 basalts erupted across this flood plain. Uplift and subsequent downcutting slowed 8 – 1.5 Ma. These basalts appear to come from asthenosphere chemically modified by subducting ocean plate (Farallon). Group 3 also appears to have been trapped for at least 8 Ma in an upper asthenospheric mantle thermal boundary layer. Sporadic volcanism took place over the last 1.5 Ma, erupting Group 4 nepheline alkali basalts.
Miocene volcanic activity (Group 1 basalts) in the Glenwood mapped Quadrangle deposited 6 – 10 flows in the northern third of the area. These flows are separated in places by volcanic ash. Single vent sources for these flows include Sheep Mountain, Shingle and Trapper Peaks, Marvine Mountain and a pair of unnamed peaks a couple kilometers SE of Marvine Lakes. Much of this basalt has been uplifted quite high, nearly 3,000 – 4,000 m above sea level. Some of the later basalts were erupted following general uplift of the region and valley cutting by local streams and rivers.
The current Rocky Mountain uplift started some 70 – 50 Ma in a broad region in the Western US. There was an extended caldera / ignimbrite outbreak due to magmas from the subducted Farallon Plate 40 – 20 Ma from Colorado to Nevada. The Colorado Front Range is one of multiple uplift regions during this event and is bounded to the east by a series of parallel faults along the range. Ore bodies in the mineral belt of the Rocky Mountains to the west of the Front Range are associated with the youngest intrusive 30 – 25 Ma. Renewed uplift and tilting of the Front Range took place during the Miocene. Erosion cut deep canyons in the uplifted mountains. Higher parts of the Front Range were carved by glaciers over the last 2 Ma.
To the west, uplift of the Colorado Plateau began some 2 Ma. The Dotsero region is located in the basin between the two uplifted regions. It receives much of the erosional material from glacial, ice, snow and water runoff in the region. Over the last 20 Ma, rivers have carved kilometers-deep canyons across the Colorado Plateau as they drain the region and the basins between the two uplifted regions.
The Dotsero volcanic complex is an example of a relatively simple and recent monogenetic eruption. It was the last one in Colorado. Unless something surprising happens, I would think it will be the last one for a long time.