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Earth Sense: Atlantic subduction could lead to more quakes
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Nov. 1, 1755, started out with a fine morning. Bright sunshine and balmy temperatures greeted churchgoers as they prepared to celebrate All Saints Day.

The cathedral of Lisbon, Portugal filled with worshippers, and mass was in progress when the earth began to shake violently between 9 and 10 a.m. One of the most violent earthquakes in human history unfolded, making churches and lesser buildings collapse.

In his written account, the Rev. Charles Davy recalled that he “expected nothing less than to be soon crushed to death, as the walls continued rocking to and fro in the frightfullest manner, opening in several places; large stones falling down on every side from the cracks, and the ends of most of the rafters starting out from the roof.”

Screams of people buried under the rubble were heard everywhere, and those who had run to the seashore for safety were attacked by huge waves from the tsunami the quake produced. When it was over, the once-splendid city of Lisbon lay in ruins, and an estimated 10,000 to 100,000 lives had been lost.

Conventional knowledge of plate tectonics has the North American continent drifting away from Europe and Africa as a solid plate, riding on half-molten rock. The resulting gap in the center of the Atlantic Ocean gets filled with fresh lava.

But recent research, just published this July (geology.com) identifies a new subduction zone forming between Africa and Portugal. Subduction is the process of a plate, such as the African one, diving beneath another, like the Eurasian Plate on which Spain and Portugal are located. The result is movement of the ground on the surface, with accompanying earthquakes, and possibly an ocean trench of the kind you see near the Philippines.

Some geologists predict that we’re observing the start of a new cycle of continental drift. Instead of the Atlantic Ocean getting wider over time, what’s happening in Portugal could draw that part of Europe closer to America, and even unite the two continents again in roughly 200 million years time — which isn’t very long in geologic terms. On a more human scale, this means that Spain, Portugal, and Morocco could be slated for another earthquake of the 8.5 to 9 magnitude that hit Lisbon in 1755.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.

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