National Geographic : 2012 Feb
• Tim Folger's article on rare earths appeared last June. His family hails from Nantucket, but he lives now in Gallup, New Mexico, far from the sea. changed history. Some archaeologists have ar- gued, for instance, that a Mediterranean tsunami struck the north shore of Crete a bit over 3,500 years ago; the disaster, they say, sent Minoan civi- lization, one of the most sophisticated of the age, into a tailspin, leading it to succumb to Myce- naean Greeks. In 1755, when an earthquake and tsunami killed tens of thousands in Lisbon, the tragedy had a lasting impact on Western thought: It helped demolish the complacent optimism of the day. In Voltaire's novel Candide the blinkered philosopher Pangloss arrives in Lisbon during the catastrophe, persists in arguing that "all is for the best in the best of all possible worlds," and gets hanged for his trouble. Voltaire's withering satire made it a little harder to be Panglossian--- to believe that a benevolent God designed an optimal Earth. In the h century . . the Greek historian ucydides was the rst person to document the connection between earthquakes and tsunamis. He noticed that the rst sign of a tsunami is of- ten the abrupt draining of a harbor, as the sea pulls away from the coast. "Without an earth- quake I do not see how such things could hap- pen," he wrote. Actually they can. e Minoan tsunami was triggered by the cataclysmic erup- tion of ira, a volcanic island 70 miles north of Crete in the Aegean. And landslides can cause local tsunamis, such as the one that surged 1,700 feet up a hillside in Lituya Bay, Alaska, in 1958 (see page 70). All it takes is a large mass of rock moving abruptly in a large mass of water---not necessarily the ocean. e vast majority of tsunamis, however, in- cluding the Tohoku one, are caused by sea oor earthquakes along faults called subduction zones. Most are in the Paci c and Indian Oceans. Along those boundaries two of Earth's tectonic plates collide, and the one carrying dense oceanic crust dives under the more buoyant continental one, forming a deep-ocean trench. Normally this happens smoothly, at a rate of a few inches a year. But at some times and places the plates become stuck---the peak of a subducting seamount might snag on the bottom of a continent, for example. After centuries the accumulated strain overwhelms the friction, and the plates shudder past each other. O Japan last March the quake began miles below the sea oor and then spread up the sloping contact between the plates to the Japan Trench at the sea oor. It released the energy equivalent of 8,000 Hiroshima bombs. A sizable fraction of that went into motion of the sea oor, which raised and lowered the water above it---thus creating a tsunami. Ordinary ocean waves are mere wind-driven wrinkles in the sea surface, but a tsunami moves the entire water column, from the sea oor up. e initial disturbance spreads out in opposite directions from the fault, in long wave fronts that may be a few hundred miles apart. In deep water o shore they're barely noticeable. ey grow to dangerous heights only in shallow water, as they pile up against a coast---and they can remain dangerous even a er they've crossed a whole ocean, barreling at the speed of a jetliner. The tsunami that savaged Japan last March swept a man in California out to sea; it broke Manhattan-size blocks of ice o the frozen mar- gins of Antarctica. e tsunami that took 41 lives in Minamisanriku in 1960 was triggered by a magnitude 9.5 earthquake o Chile, the largest quake on record. The Indonesian tsunami of December 26, 2004, killed people all around the Indian Ocean. It began o the northwest coast of Sumatra with a sudden, thousand-mile-long rupture---and Tsunamis remain dangerous after they've crossed a whole ocean, barreling at the speed of a jetliner. The one that savaged Japan swept a man in California out to sea.