The Dotsero Volcano


Written by Richard Busch

This last weekend I was on a little trip out to Glenwood Springs and decided to make a little detour while I was out there – to visit somewhere I’ve been meaning to go for nearly 15 years. This place is Dotsero, exit 133 off of I-70.  The town itself, however, was not why I was there – the volcano to the north of it was.  Just a mile north of I-70, as cars and trucks speed past at 75 miles an hour, lies Colorado’s youngest volcano – its last eruption was just 4,200 years ago. It wasn’t a huge eruption but it created almost a 4 mile long lava flow, which, I-70 cuts across the southern edge of today.

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The Dotsero volcano and lava field as it is today.

The volcano isn’t as spectacular as Mount Saint Helens, or Cotopaxi, or even our own Spanish Peaks here in Colorado. It is a small cinder cone about 2,300 feet in diameter.  It’s called a maar volcano because this last eruption was what is known as a pharetomagmatic eruption – meaning that the explosive force came as a result of water coming in contact with a magma body. Think about the dangers of old pressure cookers if the steam wasn’t released properly. Same sort of thing here – except on a geologic scale.

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The Dotsero Volcano Crater Today.

The volcano has laid dormant ever since.  It’s once 1,300 foot deep crater is now only a few hundred feet deep as the surrounding slopes erode into, and fill it.  You can walk down (just be sure you can navigate and have the stamina and foot wear to hoof it back up the very step sides) and poke around in the crater if you like.  Most of what the casual observer will find today are chunks of red and black scoria and some black basalt. Scoria is the “froth” if you will of lava, lava froth.  As pressure is released as the volcano erupts, gasses come out of solution and, well, froth – much like your soda when you open a bottle. The reds usually indicated the oxidation of iron minerals, and the blacks usually indicate the oxidation of magnesium minerals. Basalt, at least the way I’m using that term here, is the actual cooled lava.  Some of the more dense basalts at Dotsero can have diamonds in them – now, before you go out diamond hunting – these are not gem quality inclusions, and they are not actually diamonds, they are a type of tiny quartz crystal. 

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Some black scoria and basalt together from Dotsero – the tiny white speck near the center is a diamond.

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A piece of reddish scoria from Dotsero – the cavities from the gas bubbles are clearly visible on this piece.

Can, or will this volcano erupt again? Who knows. There is no activity now and the United States Geological Survey Volcano Hazards Program and the Global Volcanism Program do not have this volcano on their watch lists. So, everyone is most probably safe for a good bit of the future. The magma body is still down there somewhere – it’s what makes the hot springs in Glenwood Springs hot – but right now it’s just hanging out.

Here is a video of a phetomagmatic eruption, its from the much, much larger volcano in Japan, Mount Mt.Usu:

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