Hotspot once ate across the US

Seismic Anomaly reveals trace of an ancient plume under North America

Hotspot trail in North America © Chu et al., / Nature Geoscience
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A volcanic hotspot could have left a trail across North America. While the land mass over millions of years pulled over this geological welding torch, this melted the crustal stone from below and deformed it. Indications for such a trace have now been discovered by researchers using seismic measurements. The hotspot zone could also be responsible for the intrabble earthquake near New Madrid in Missouri, the researchers report in the journal Nature Geoscience.

Hotspots are places where the mantle is particularly hot and rising molten rock therefore burns like a torch through the solid crustal parts. Lying under the thin oceanic crust, these hotspots often form whole chains of volcanic islands as the crust, driven by plate tectonics, moves across the hotspot. This then drills a number of different old volcanic vents into it over time - a typical example is the islands of Hawaii.

Unrecognized in the underground

Because the earth's crust is much thicker among the continents, the heat of the hotspots is usually not enough to burn through them and form volcanoes. Therefore, such hotspot tracks in the countryside are also far less easily recognizable. As an indication are Kimberlite, rocks from the upper mantle, which have risen in the crust and remained there as frozen vents.

However, Risheng Chu from the State Laboratory for Geodesy and Earth Dynamics in Wuhan and his US counterparts have found another way to track down old hotspot trails. To do so, they analyzed the seismic waves that emanated from an earthquake in Virginia in 2011 using the USArray monitoring network. Because different rocks break and reflect the waves differently, it is possible to identify faults and other structures in the subsurface rock.

Effects of the Hotspot in Hawaii © Public domain

Seismic anomaly pulls once across the country

And, indeed, when analyzing the data, the researchers discovered a linear, west-to-east seismic anomaly. It reaches from Missouri to Virginia - but is not visible on the earth's surface. "This unexpected anomaly is characterized by a reduced speed of the P-waves and a strong weakening, " explain the scientists. "We interpret that as a hotspot's track." Display

This lane probably goes even further west and could continue in the east to the western Atlantic - the place where the hotspot is expected to be today. The hotspot track probably originated 100 to 50 million years ago, the researchers suspect. They see additional evidence in a roughly 75-million-year-old kimberlite deposit in Kentucky, which is exactly on the line of this anomaly.

Blame for the New Madrid earthquake?

But would a hotspot really make such a seismic signal? In order to verify this, the researchers carried out a model simulation. In this they let a picture of the continental crust of North America over a virtual hotspot wander, whose temperature was 300 to 400 degrees above the environment. It showed that the hotspot passage would temporarily melt the lower lithosphere and cause rocks to collect from the earth's mantle. Both could produce the seismic anomalies millions of years later that have now been measured.

The newly discovered hotspot track may also explain why in the center of the USA - miles away from each plate boundary - unusually often intraplate quakes occur. Here, four of the world's largest earthquakes occurred. The most famous of these, in 1812, produced earthworks to the power of 8.6, which completely destroyed the city of New Madrid and continued to reach New York and Boston.

As the evaluations showed, the responsible rejection at New Madrid is crossed by the hotspot lane. Scientists speculate that this unfavorable collision of two swell zones might have activated the New Madrid Fault. (Nature Geoscience, 2013; doi: 10.1038 / ngeo1949)

(Nature, 16.09.2013 - NPO)