Inside Science

The where, why of geological dangers

CorrespondentFebruary 2, 2014 

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Emlyn Koster, PhD, is a geologist and the director of the N.C. Museum of Natural Sciences.

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Seared into our minds are images of Indonesia’s earthquake and Indian Ocean tsunami the day after Christmas 2004, Haiti’s earthquake under its capital in 2010, and Japan’s earthquake, tsunami and nuclear power plant damage in 2011. These massive disasters resulted in more than a half-million fatalities and more than $100 billion in repairs.

Only since the 1960s have we understood that the Earth’s surface consists of large, relatively stable areas separated by narrow zones prone to disastrous geological events. Pointing to the jigsaw fit of the coastlines of Africa and South America, the “Thesaurus Geographicus” of 1596 suggested that the Americas “were torn away from Europe and Africa … by earthquakes and floods.” Though this explanation was wrong, the valid idea of “continental drift” had been born. By the early 20th century, a variety of evidence was mounting that continents had split up.

Then during World War II, as a result of remote sensing technology to detect submarines, a breakthrough occurred in our knowledge of the ocean floor. Earth scientists quickly realized that continents are fixed parts of larger crustal plates that include ocean floor and that these plates move at rates of a few inches per year, driven by giant forces below the crust.

We also discovered that adjacent plates move in three ways. As we can glimpse in Iceland, one kind of movement involves separation along mainly ocean-floor mountain ranges with frequent, but rarely dangerous, volcanic activity. The second kind of movement, as we see off the coasts of Indonesia and Japan, involves convergence along trenches as deep as Everest is high. The third kind of movement, as is the case in Southern California, is when plates grind past one another in opposite directions.

These last two situations build up massive stresses, resulting in occasional but catastrophic earthquakes, tsunamis and volcanic eruptions (examples: Indonesia, Japan, Mount Etna in Sicily). It is also these situations that lead to new mountainous regions (like the Andes and Himalayas).

Earth scientists also now know that the map of continents and oceans has changed continuously over eons. When the Virginia region shook and the Washington Monument cracked on the afternoon of Aug. 23, 2011, we were reminded that old fault lines are very slow to heal and only seldom move. Other good geological news is that the last active volcanoes in the area we now know as North Carolina were about 225 million years ago when the Appalachians were rising next to a long-gone convergent plate boundary.

Emlyn Koster, Ph.D., is a geologist and the director of the N.C. Museum of Natural Sciences. He can be reached at emlyn.koster@naturalsciences.org.

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