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How Can We Tell When a Volcano is Dormant or Extinct?

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When a volcano makes the news for erupting, you’ll often hear mention of other kinds of volcanoes that are dormant or extinct. But what do those terms mean, exactly, and how do scientists figure that out?

First, a little bit about how volcanoes work: Magma collects in reservoirs beneath the earth’s surface, and as it accumulates, pressure in the chamber increases; if it gets high enough, the rocks over it will break, and an eruption will ensue. The silica content of the magma determines what kind of volcano you’ll have. Low silica magma makes shield volcanoes, like Kīlauea on Hawaii’s big island, and cinder cones, like Mexico’s Paricutín, which have lava that flows easily, like molasses. High silica magma creates stratovolcanoes, like Mount St. Helens in Washington, and calderas like the one under Yellowstone National Park, which have lava that is more viscous and flows like taffy.

The definitions of what constitutes dormant and extinct volcanoes aren’t exact, and can differ depending on the volcano or the scientist. Typically, says Jim Webster, Curator of Department of Earth and Planetary Sciences, Division of Physical Sciences at the American Museum of Natural History, scientists will look at recent history and the geologic record. If a volcano has erupted since the last Ice Age—in the last 10,000 years or so—and is still showing activity like lava and ash flows or gas emissions, it’s considered to be active. If a volcano hasn’t erupted in the last 10,000 years, but scientists think it will erupt again, it’s considered dormant. “If it has been more than 10 thousand years [since the volcano has erupted],” and the volcano is cut off from its magma supply, “it’s considered to be extinct,” Webster says.

Another indicator is seismicity—or lack thereof. “Often times the magma bodies, or chambers—the source that feeds eruptions—are three to eight miles below the surface, and you can monitor seismic behavior there,” Webster says. “If there’s a repeated concentration of seismic activity immediately below the opening, there’s probably hot, active magma down there. If that goes quiet and dormant, and its just background kind of seismic activity—shifting of the rock or weathering, how the planet’s always kind of creaking and moving—for an extended period, perhaps years, that would indicate that, if there’s magma there, it’s not moving. It’s not working toward surface, it’s not generating pressure to try to break the rocks, and perhaps the system is going extinct.”

But even a volcano that’s been deemed extinct could erupt again. “Magma bodies can cool and crystallize below the surface,” Webster says. “The magma may find a fracture and move off someplace else, and come up miles away or a long distance away.”

Because of this, scientists keep tabs on volcanoes whether they’re considered active, dormant, or extinct. “They do thermal monitoring, just to see if there’s heat,” Webster says. They also place tilt monitors on the volcano, which notify scientists if the ground starts to swell—a sign that an eruption could be imminent. (Before Mount St. Helens erupted in 1980, there was “a huge bulge visible to the eye,” Webster says.) And they also use instruments in space to monitor how things are going on the ground: “InSAR,” Webster says, “is a special type of radar that’s done from satellites. It can detect centimeter motions from space, on a land surface.”

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Big Questions
What's the Difference Between Gophers and Groundhogs?
Gopher or groundhog? (If you chose gopher, you're correct.)
Gopher or groundhog? (If you chose gopher, you're correct.)
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Gophers and groundhogs. Groundhogs and gophers. They're both deceptively cuddly woodland rodents that scurry through underground tunnels and chow down on plants. But whether you're a nature nerd, a Golden Gophers football fan, or planning a pre-spring trip to Punxsutawney, Pennsylvania, you might want to know the difference between groundhogs and gophers.

Despite their similar appearances and burrowing habits, groundhogs and gophers don't have a whole lot in common—they don't even belong to the same family. For example, gophers belong to the family Geomyidae, a group that includes pocket gophers (sometimes referred to as "true" gophers), kangaroo rats, and pocket mice.

Groundhogs, meanwhile, are members of the Sciuridae (meaning shadow-tail) family and belong to the genus Marmota. Marmots are diurnal ground squirrels, Daniel Blumstein, a UCLA biologist and marmot expert, tells Mental Floss. "There are 15 species of marmot, and groundhogs are one of them," he explains.

Science aside, there are plenty of other visible differences between the two animals. Gophers, for example, have hairless tails, protruding yellow or brownish teeth, and fur-lined cheek pockets for storing food—all traits that make them different from groundhogs. The feet of gophers are often pink, while groundhogs have brown or black feet. And while the tiny gopher tends to weigh around two or so pounds, groundhogs can grow to around 13 pounds.

While both types of rodent eat mostly vegetation, gophers prefer roots and tubers (much to the dismay of gardeners trying to plant new specimens), while groundhogs like vegetation and fruits. This means that the former animals rarely emerge from their burrows, while the latter are more commonly seen out and about.

Groundhogs "have burrows underground they use for safety, and they hibernate in their burrows," Blumstein says. "They're active during the day above ground, eating a variety of plants and running back to their burrows to safety. If it's too hot, they'll go back into their burrow. If the weather gets crappy, they'll go back into their burrow during the day as well."

But that doesn't necessarily mean that gophers are the more reclusive of the two, as groundhogs famously hibernate during the winter. Gophers, on the other hand, remain active—and wreck lawns—year-round.

"What's really interesting is if you go to a place where there's gophers, in the spring, what you'll see are what is called eskers," or winding mounds of soil, Blumstein says [PDF]. "Basically, they dig all winter long through the earth, but then they tunnel through snow, and they leave dirt in these snow tunnels."

If all this rodent talk has you now thinking about woodchucks and other woodland creatures, know that groundhogs have plenty of nicknames, including "whistle-pig" and "woodchuck," while the only nicknames for gophers appear to be bitter monikers coined by Wisconsin Badgers fans.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Why Does Santa Claus Give Coal to Bad Kids?
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The tradition of giving misbehaving children lumps of fossil fuel predates the Santa we know, and is also associated with St. Nicholas, Sinterklaas, and Italy’s La Befana. Though there doesn't seem to be one specific legend or history about any of these figures that gives a concrete reason for doling out coal specifically, the common thread between all of them seems to be convenience.

Santa and La Befana both get into people’s homes via the fireplace chimney and leave gifts in stockings hung from the mantel. Sinterklaas’s controversial assistant, Black Pete, also comes down the chimney and places gifts in shoes left out near the fireplace. St. Nick used to come in the window, and then switched to the chimney when they became common in Europe. Like Sinterklaas, his presents are traditionally slipped into shoes sitting by the fire.

So, let’s step into the speculation zone: All of these characters are tied to the fireplace. When filling the stockings or the shoes, the holiday gift givers sometimes run into a kid who doesn’t deserve a present. So to send a message and encourage better behavior next year, they leave something less desirable than the usual toys, money, or candy—and the fireplace would seem to make an easy and obvious source of non-presents. All the individual would need to do is reach down into the fireplace and grab a lump of coal. (While many people think of fireplaces burning wood logs, coal-fired ones were very common during the 19th and early 20th centuries, which is when the American Santa mythos was being established.)

That said, with the exception of Santa, none of these characters limits himself to coal when it comes to bad kids. They’ve also been said to leave bundles of twigs, bags of salt, garlic, and onions, which suggests that they’re less reluctant than Santa to haul their bad kid gifts around all night in addition to the good presents.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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