Friday, October 23, 2015

Do you live near a maar volcano?

-Alison

I am always going on about maar volcanoes. So where are the maar volcanoes? These volcanoes are formed by explosions underground occur because magma interacts with water and form unassuming craters. The craters are cut below the ground surface so their outer slopes are fairly shallow, and they are frequently filled with a lake. When there isn’t a large majestic volcano to climb and take fancy sunset photos in front of, it can be hard to get excited about what looks like a little hole in the ground.
A picturesque maar crater in Michoacán Mexico.
But remember, they are explosive, it takes a lot of energy to carve a big hole in the ground. I use dynamite to make craters that are only 2 meters in diameter (a tall friend lying on their side). We would need about 10,000 sticks of the dynamite we use to get close to the size of a maar volcano.
Our experimental maar volcanoes would need to be a LOT bigger to match nature.
I have been recently comparing the shapes and sizes of maar volcanoes around the planet. While I knew they were all over the place, I am now better informed on exactly where and how big they are. An average maar crater is as wide as 17 Olympic pools end to end, or 850 m. So if you get bored doing laps in a pool you could find a small maar lake and cross it to get a good work out. The largest maars that have been found on are up to 5000 m, or 100 of those Olympic swimming pools in a row. These exceptionally large maars all occur in a cluster on the Seward Peninsula in Alaska. Most maars are less than a kilometer across with the smallest maars closer to 100 m, or just one trip across a pool and back. You can find these smaller maars in the Eifel Volcanic Field in Germany or in Iceland at Askja volcano.
 
Google Earth image of the exceptionally large Epsenberg maars on the Seward Peninsula, Alaska, USA. The yellow pin is in the largest maar crater called the Devil Mountain Lake. Maar craters can be confused for other lakes. There are five maars in this image and the rest of the lakes are formed in the permafrost through more subtle processes.
 
I have swum around in this maar at Askja Volcano in Iceland, the lake is about 100 m across and a warm(ish) 60 degrees F. If you look at the upper rim of the crater you can see the layered deposits from the explosions that formed the crater. The lower more solid layers are material that was there before the maar formed.

These craters are found all over the planet in a wide variety of environments. The giant Epsenberg maars are in the remote subartic of Alaska. There are maars in the deserts of Turkey and Mexico. You can also find a lot of maars in the tropics, like in the Philippines and Indonesia. While some of them occur in remote locations, like Kamchatka Peninsula Russia or Ukinrek in Alaska that erupted in 1977, many of them are found in the middle of cities. In fact, the city of Auckland in New Zealand is built on top of 53 volcanoes, including ~11 maars. If you live in San Luis Potosi or Morelia in Mexico, Frankfurt in Germany, Rome in Italy or Kagoshima, Japan you are only a short drive away from a maar volcano.

View of Auckland from Google Earth showing some of the maars. You can also spot some scoria cones and shield volcanoes. They are called Lake Pupuke, Onepotu, Orakei Basin, Panmure Basin, Hopua, Mangere Lagoon and Pukaki from upper left to lower right.   

Eruption of Ukinrek Alaska 1971, USGS.
Some maar volcanoes occur in isolation, where they are the only volcano in the area, like Ubehebe crater in Death Valley California. While others occur in volcanic fields mixed with other types of volcanoes, like scoria cones and stratovolcanoes. Some maar volcanoes are covered by more recent volcanic activity. Sometimes scoria cones and lava flows occur at the end of the same eruption that formed a maar. Other maars occur inside, or on the flanks of a bigger volcano like Aniakchak caldera in Alaska.
Google Earth image of Aniakchak volcano, Alaska, USA. Two maars have formed inside the caldera, noted by the yellow pins They are cleverly named Southwest and Northeast maar.
Do we know where all the maars are on Earth? Nope. There are lots of other circular lakes that can, at first glance, look like a maar: permafrost lakes, sink holes, those crazy holes in Siberia and impact craters. Also, as maars get older they are subject to weathering and erosion. Since they are holes in the ground, instead of big fancy cones, they fill up with sediment and get shallower with time. Lots of maars have lakes in them which only helps them fill with sediment. Other maars get eroded along the edges (Hunt's Hole) and stop looking like nice perfect craters. In wetter environments, the shallow slopes of volcanic ash make for fertile soils, which means that you can find a lot of circular lakes in the middle of farmland that are actually volcanoes. In some locations the ash and rocky deposits are mined for abrasives or road materials. This makes it harder to count maars and know how big they can be. My goal is to look at the maars that we have found and see what we can learn about the type of eruptions that form them by comparing shape, size and environment. The more maars I study, the better. Thankfully satellite imagery means that I can study these volcanoes without having to pay for plane tickets to get to all of these locations. But maybe I'll make it a personal goal to see as many as I can. I think I've visited at least ten already, but that there are many more maars to go.
Kilburn (top) and Hunt's Hole (bottom) maars in New Mexico.
If you live in one of the cities I mentioned you can take a hike to your nearest maar volcano and appreciate it for more than just a hole in the ground for me. Keep an eye out for a circular or elliptical crater that may have a lake in it, then check for local tourist or hiking guides to find out if there are any maar volcanoes near you.  

There are 22 maars in this image, plus a stratovolcano and several scoria cones. How many can you spot? Lamongan Volcanic Field on the Island of in Indonesia.


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