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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
Are There Number 1 Pencils?
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Almost every syllabus, teacher, and standardized test points to the ubiquitous No. 2 pencil, but are there other choices out there?

Of course! Pencil makers manufacture No. 1, 2, 2.5, 3, and 4 pencils—and sometimes other intermediate numbers. The higher the number, the harder the core and lighter the markings. (No. 1 pencils produce darker markings, which are sometimes preferred by people working in publishing.)

The current style of production is profiled after pencils developed in 1794 by Nicolas-Jacques Conté. Before Conté, pencil hardness varied from location to location and maker to maker. The earliest pencils were made by filling a wood shaft with raw graphite, leading to the need for a trade-wide recognized method of production.

Conté’s method involved mixing powdered graphite with finely ground clay; that mixture was shaped into a long cylinder and then baked in an oven. The proportion of clay versus graphite added to a mixture determines the hardness of the lead. Although the method may be agreed upon, the way various companies categorize and label pencils isn't.

Today, many U.S.  companies use a numbering system for general-purpose, writing pencils that specifies how hard the lead is. For graphic and artist pencils and for companies outside the U.S., systems get a little complicated, using a combination of numbers and letters known as the HB Graphite Scale.

"H" indicates hardness and "B" indicates blackness. Lowest on the scale is 9H, indicating a pencil with extremely hard lead that produces a light mark. On the opposite end of the scale, 9B represents a pencil with extremely soft lead that produces a dark mark. ("F" also indicates a pencil that sharpens to a fine point.) The middle of the scale shows the letters and numbers that correspond to everyday writing utensils: B = No. 1 pencils, HB = No. 2, F = No. 2½, H = No. 3, and 2H = No. 4 (although exact conversions depend on the brand).

So why are testing centers such sticklers about using only No. 2 pencils? They cooperate better with technology because early machines used the electrical conductivity of the lead to read the pencil marks. Early scanning-and-scoring machines couldn't detect marks made by harder pencils, so No. 3 and No. 4 pencils usually resulted in erroneous results. Softer pencils like No. 1s smudge, so they're just impractical to use. So No. 2 pencils became the industry standard.

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Big Questions
What Are Curlers Yelling About?
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Curling is a sport that prides itself on civility—in fact, one of its key tenets is known as the “Spirit of Curling,” a term that illustrates the respect that the athletes have for both their own teammates and their opponents. But if you’re one of the millions of people who get absorbed by the sport once every four years, you probably noticed one quirk that is decidedly uncivilized: the yelling.

Watch any curling match and you’ll hear skips—or captains—on both sides barking and shouting as the 42-pound stone rumbles down the ice. This isn’t trash talk; it’s strategy. And, of course, curlers have their own jargon, so while their screams won’t make a whole lot of sense to the uninitiated, they could decide whether or not a team will have a spot on the podium once these Olympics are over.

For instance, when you hear a skip shouting “Whoa!” it means he or she needs their teammates to stop sweeping. Shouting “Hard!” means the others need to start sweeping faster. If that’s still not getting the job done, yelling “Hurry hard!” will likely drive the point home: pick up the intensity and sweep with downward pressure. A "Clean!" yell means put a brush on the ice but apply no pressure. This will clear the ice so the stone can glide more easily.

There's no regulation for the shouts, though—curler Erika Brown says she shouts “Right off!” and “Whoa!” to get her teammates to stop sweeping. And when it's time for the team to start sweeping, you might hear "Yes!" or "Sweep!" or "Get on it!" The actual terminology isn't as important as how the phrase is shouted. Curling is a sport predicated on feel, and it’s often the volume and urgency in the skip’s voice (and what shade of red they’re turning) that’s the most important aspect of the shouting.

If you need any more reason to make curling your favorite winter sport, once all that yelling is over and a winner is declared, it's not uncommon for both teams to go out for a round of drinks afterwards (with the winners picking up the tab, obviously). Find out how you can pick up a brush and learn the ins and outs of curling with our beginner's guide.

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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