Alan Stahler: Understanding volcanoes
May 23, 2018
The surface of the Earth is a jig-saw puzzle. A glance at the map reveals the near-perfect fit of Africa and South America — once joined, now sliding apart. All of Earth's jig-saw pieces are in motion, some moving toward each other, some moving apart, others (for example, in the San Andreas Fault Zone) sliding sideways.
Where ocean floor and continent collide — Western North America and the Pacific; South America and the Pacific; East Asia and the Pacific — volcanoes erupt. (The granite of the Sierra cooled and crystallized, before it could erupt, beneath volcanoes active some hundred million years ago.)
Geologists have a good handle on what drives the volcanoes of the Pacific "Ring of Fire." But other volcanoes erupt nowhere near such collisions — Kilauea, on the Big Island of Hawaii, for instance.
Fresh rock — rock that hasn't been long on the surface — sports shiny crystals and crisp edges. Weathered by wind and rain, however, crystals grow dull, and edges are rounded. Such clues led nineteenth-century geologist James Dana to spot a pattern: Traveling northwest from the Big Island, the rocks/volcanoes/islands grow older.
Modern radioactive dating puts the volcanoes of Maui (northwest of the Big Island) at around a million years old. Further northwest, Molokai's volcanoes are close to two million years old. Oahu's volcanoes clock in at two-and-a-half to three-and-a-half million.
The floor of the Pacific, like the other jig-saw puzzle pieces, is in motion.
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What lies beneath
Geologist J. Tuzo Wilson suggested, in the 1960s, that the Pacific, might be acting as a gigantic conveyor belt … not collecting anything from above, but pierced, from below, by a plume of hot rock.
Hot air rises … so does hot rock, for the same reason: hot rock is less dense than cold, so it floats. Could a plume of hot rock be rising up from deep within the Earth … melting … and erupting?
A volcano would then grow where the plume pierced the floor of the Pacific. And over millions of years, the Pacific conveyor would move on, carrying old volcanoes with it, toward the northwest, as new volcanoes formed over the plume.
The rocks above Earth's liquid metal core lie a couple thousand miles beneath the surface. The deepest borehole we've yet drilled reaches down barely seven miles. We need other ways to look into the Earth.
If you want to hang a mirror on the wall, you don't just pound a nail into the sheetrock. Mirrors are heavy, sheetrock is weak. You want to put that nail into a stud.
But you can't see the studs, so you knock on the wall. Your knocks sound hollow, until one knock returns a dull thud: where sheetrock is nailed to a stud, it changes the sound of the knock.
When Earth quakes, it sends vibrations — waves — through the surrounding rock. Like the stud behind the sheetrock, different rocks tweak these waves in different ways, depending on their temperature, their density, the different ways their minerals crystallize. Studying waves coming from many different directions — many different earthquakes — seismologist deduce what sort of rock lies beneath a volcano.
Kilauea has, for years, been the poster-child for volcanoes outside the Ring of Fire. But Kilauea is also among the most mysterious.
Just a few months ago, geologists reported seeing (with seismometers) a plume of hot rock, rising up from the region just above Earth's core — the plume driving Yellowstone's volcanism. But seismologists looking at what underlies Kilauea have reported contradictory findings … just what drives Kilauea remains controversial.
If you're making sugar candy, you want to dissolve as much sugar as possible in the water, so you make the water hot. Solids dissolve best in hot solvents.
Gases, on the other hand, dissolve best in cold solvents … which is why a warm beer or soda goes flat so fast
Hot-spot volcanoes erupt the hottest and runniest of lavas — over 2000 degrees Fahrenheit has been recorded. Lava that hot does not have to touch wood to set it aflame — thermal radiation will ignite wood from a distance.
Gases do not want to remain dissolved in such a hot solvent. Gases escape easily from the runny lava, creating fire fountains of lava.
Al Stahler enjoys sharing nature with students of all ages — his science stories can be heard on radio station KVMR (89.5 FM). He also welcomes e-mails at firstname.lastname@example.org.
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