By Erin Wayman
A hidden hot spot deep within the Earth could have set off seismic activity causing some earthquakes in the eastern United States.
Researchers have discovered a track of hot rock at the base of the North American plate, stretching from Missouri to Virginia. It marks a trail, they say, where the westward-drifting continent passed over a hot spot tens of millions of years ago. A hot spot is the top of a plume of abnormally hot, buoyant rock in the mantle that wells up like a blob in a lava lamp.
Hot spots can fuel volcanoes, and scientists think a hot spot in the Pacific Ocean created the Hawaiian Islands (SN: 10/22/11, p. 8). Under much thicker continental rock, mantle plumes may heat up but rarely leave a mark on the surface, Risheng Chu of the Chinese Academy of Sciences and colleagues argue September 15 in Nature Geoscience.
The hot spot’s heat, however, might have stretched and pulled the North American plate, causing the brittle crust to crack and initiating seismic activity in the eastern U.S. “If a hot spot indeed exists, it’s plausible that the structure could trigger seismic activity,” says Matthew Fouch, a seismologist at the Carnegie Institution for Science in Washington, D.C. For instance, the hot spot might have activated the New Madrid zone, which is famous for unleashing a series of powerful quakes in 1811 and 1812 that rattled the Midwest (SN: 12/3/11, p. 22).
Finding a hot spot track beneath North America means that many other hot spots could lurk beneath continents, says study coauthor Don Helmberger, a seismologist at Caltech. The energy from such hot spots could be an unrecognized force behind continental breakups, he says.
Helmberger, Chu and colleagues discovered the hot spot track with the help of the magnitude 5.8 earthquake that struck Virginia in 2011. Seismic instruments in the eastern United States recorded the quake’s vibrations. Seismic waves traveling directly from Virginia to Missouri were much slower than vibrations traveling in other directions. Because seismic waves travel slowly through weak, hot rock, the researchers concluded that a band of anomalously hot rock spans the two states. Computer simulations demonstrated that a hot spot could have caused the anomaly.
One of the few surface signs that North America did once drift over a hot spot is a 75-million-year-old kimberlite deposit in eastern Kentucky. Kimberlite is volcanic rock that originates deep within the mantle and erupts rapidly to the surface — in this case, perhaps from the hot spot.
Knowing the age of the kimberlite and the direction of North America’s motion, the team estimated the hot spot’s location in relation to the surface over the last 100 million years. The track goes east from Missouri to Virginia before taking a bend to the northeast. The researchers estimate the hot spot was beneath New Hampshire roughly 50 million years ago and is currently below the North Atlantic.
“Their interpretation makes a compelling case,” Fouch says. Other scientists are skeptical. “I’m not totally convinced,” says Randy Cox, a geologist at the University of Memphis in Tennessee. He says the researchers need a lot more seismic data to verify the proposed hot spot track.
