Volcanoes: How they work; what they do
Rock is flowing once again on Hawaii’s big island, where geologic change is not a matter of centuries and millennia, but rather of hours and days. Every square inch of these Hawaiian islands owes its existence to a hot spot that conduits molten rock from deep inside Earth to the surface.
Pelé, the god of the volcano, is a hungry god, and as volcano Kilauea oozes red-hot rock, the village of Pahoa watches the lava push closer. Families on the volcano side of town have already packed for evacuation if necessary.
Volcanoes can do much worse than ooze into the backyard and seize territory. On Sept. 27, Ontake, Japan’s second-tallest volcano, erupted without warning, killing at least 56 in that island nation’s worst volcanic tragedy since 1902. Earthquakes, caused by the movement of molten rock, or magma, usually precede eruptions, but little magma moved before the steam explosion at Ontake, so the government issued no warning.
Kilauea’s rock chemistry is not conducive to explosive eruptions, and more inclined to slow-moving oozes. You can walk up to the molten lava. As the cherry-red surface cools to a brittle, glassy, black rock, you hear the unforgettable crinkly, crackly noise of a planet changing shape.
The rare opportunity to watch terra firma forming in Hawaii is misleading: most of the planet’s crust was once rock — magma — that emerged from the deeps and cooled million or billions of years ago. Some estimates say that 80 percent Earth’s cold, hard surface originated as molten rock.
Energetic but enigmatic
Volcanoes are awesome reminders that Earth is not a boring, static hunk of rock, but rather a living planet that coalesced from a hot cloud of gas and dust about 4.5 billion years ago. The skin cooled as heat from the surface radiated to space. But the high-temperature rock inside was insulated by the crust, and actually has gained heat from radioactive decay and gravitational energy.
In short, we’re sitting on a ball of fiery rock wrapped in a dozen kilometers or two of cool rock.
That’s unstable. Heat rises. Hot substances are less dense than colder ones, so magma is always looking for an escape hatch through the crust. And since heat is a form of energy, rising magma brings up oodles of energy — enough to power cataclysmic explosions. Enough to reshape the face of the planet.
Volcanoes don’t just appear anywhere. They require places where
* deep hot rock rises above intra-oceanic “hot spots;” these magma pipelines fuel volcanoes on Hawaii and Reunion Island; or
* water-rich ocean crust is subducted into much hotter mantle rocks at the edges of tectonic plates.
To understand volcanoes, you need to know the Ring of Fire, where most of them are located. The Ring is located where tectonic plates meet and denser ocean crust sinks beneath the lighter continental crust.
That’s the process of subduction, and it’s the major source of magma.
One hundred kilometers deep in a subduction zone, the sinking crust carries seawater under fantastic pressure (about 30,000 times atmospheric pressure) and temperature (1,000° Celsius) into the earth’s hot mantle. When this water mixes with hot mantle rock, the rock melts. (Just as salt melts ice on your sidewalk by lowering its melting point, water lowers the melting point of mantle rock by hundreds of degrees.)
The mantle rock melts, and becomes less dense than surrounding rocks, so they rise through the surrounding solid mantle by convection, eventually reaching a magma chamber a few kilometers below the volcanic vent. As the magma continues rising, falling pressure liberates high-pressure gas that was trapped inside it, vapor bubbles expand and eventually rupture, and the volcano can erupt explosively.
Volcano structure is dependent on the geologic setting and chemistry of the emerging magma.
Composite volcanoes are built of alternating layers of ash and lava. These so-called stratovolcanoes, including Mt. Fuji in Japan, Vesuvius and Stromboli in Italy, and Villarrica in Chile, fulfill the stereotypical volcanic cone.
Shield volcanoes are born from multiple vents that emit flowing (not exploding) lava. Mauna Loa on Hawaii, standing 10 kilometers from ocean floor to summit, is a classic shield volcano. Shield volcanoes typically form over an oceanic hot spot, not at a subduction zone.
A lava dome is built by eruptions of viscous, non-explosive andesitic or dacitic lava.
A cinder cone is built from basaltic ash and lapilli — sand to golf-ball-sized ballistic fragments that pile up around a volcanic vent.
A caldera is a ring-shaped depression surrounded by steep cliffs. A caldera forms when a magma chamber spews out its molten rock, and the mountain above it collapses.
All volcanoes, in one way or another, represent a leak in the Earth’s crust that allows magma to ooze or burst out. Many types of rock can be released, and some flows can threaten locations so distant that the volcano is just innocuous scenery at the horizon.
Volcanic Violence Slide Show Rated R
As rising populations run out of safe places to live in volcano country, sooner or later volcanoes will write headlines of havoc. The dying was already under way 2,000 years ago, when Mt. Vesuvius erupted and buried Pompeii in 79 CE. Today, three million people in Naples, Italy, live in the shadow of Vesuvius.
Plenty of other cities face grave volcanic threats. At the western edge of the Ring of Fire, for example, densely crowded Japan, Philippines and Indonesia are all studded with active volcanoes.
For example, Mexico City, the world’s largest metropolis, is just 55 kilometers from Popocatépetl, a 5,465-meter giant that’s erupted about 17 times since Columbus “discovered” the New World.
For perspective, eruptions like Mount St. Helens, which emitted one cubic kilometer of ash in 1980, happen every decade or so. Krakatau, Indonesia, 1883, put out about 20 times as much stuff. Such event is expectable roughly once in a century.
But consider the 3,000-cubic-kilometer eruption about 75,000 years ago of Mount Toba (also in Indonesia), which likely affected the global climate for years. These volcanic “super-eruptions” can effect the climate and economy of an entire hemisphere, yet we have no human experience of what signals volcanoes might emit before such mega-eruptions.
It’s a principle of geology: What happened before can happen again. So get this straight. Gargantuan eruptions are rare. But gargantuan eruptions can happen.
Volcanoes are fascinating in their own right, but the big money is trying to anticipate, even predict, eruptions. The record is spotty, as emphasized by Ontake’s unexpected and deadly eruption on Sept. 27, but steady progress in understanding the chemistry and dynamics of volcanoes should improve matters.
To reduce the surprise quotient for volcanoes, scientists look at:
* Geologic history: How regular and frequent were this volcano’s past eruptions? Based on the composition of the erupted magma, how explosive were the eruptions? How did past eruptions change the landscape? How far away were the effects felt?
* Deformation of Earth’s surface: Before eruption, rising magma may change the surface above the magma chamber. Broad deformation indicates deep magma, while more focused deformation signifies shallow magma that may be more likely to erupt in the near future.
* Gas emissions: Rising magma releases gases like water, carbon dioxide and sulfur dioxide. The timing and intensity of these gases hints at the magma’s composition and location.
* Chemical and physical structure of the magma: Scientists are fascinated by crystals present in a “mush” in some magma reservoirs capable of super-eruptions. If the crystals vanish or change composition, could that trigger an eruption?
* Earthquakes: Rising magma must bust through rock, and the resulting quakes may be the best single indication of magma movement and near-term eruption.
* Gravity: Magma is hotter, and therefore, less dense than the same rock at lower temperatures.
* Electrical conductivity: Changes in underground electrical conductivity can reveal the presence of hot water or molten rock at shallow depths.
Although scientists do not claim that the volcano prediction problem is near solution, some incremental gains are evident. In 1991, for example, the lessons of Mount St. Helens were tested when the ground began shaking around Mount Pinatubo in the Philippines.
A collaboration of U.S. and Filipino volcanologists watched, waited, and eventually made an accurate warning. The Philippine government evacuated tens of thousands of residents, saving massive casualties. Although gigantic mud flows and ash deposits lead to the closing of two major U.S. military bases, few lives were lost during the largest eruption of the 20th century.
– David J. Tenenbaum