A hidden hotspot in the Eastern US
This image shows seemingly plain fields and houses from the interior of the United States. They seem to be just average residential plots, but look closely at the fields. They’re dotted with patches of lighter material.
Those patches of lighter material are sand that shot upwards out of the ground. Their remnants on the surface today testify to the events of 1812, when the largest earthquake in recorded history struck the interior of the United States at a place called New Madrid, Missouri. They were produced after the earthquake when sand layers liquefied and were shot upwards into the air due to the shaking; they’re known as sand blows. The New Madrid system has long been difficult to explain. Unlike places along the Pacific Coast where earthquakes strike, New Madrid is in the center of a plate, far away from major tectonic boundaries, yet it is capable of producing earthquakes large enough to devastate major cities.
New research just published in the journal Nature Geoscience puts out a provocative new hypothesis for New Madrid; that the entire seismic system is being driven by a plume in the mantle, hidden by the crust in the Eastern U.S.
Hotspot tracks created by mantle plumes are something people might be familiar with in the ocean. The classic example is Hawaii, where volcanism has produced a chain of islands thousands of kilometers long, with new islands created as the Pacific Plate moves across the hotspot.
Hotspots and plumes of various types have been proposed to happen at many places on the Earth’s surface, and they can hit continents as well. The Yellowstone supervolcano and the Snake River Plain are probably a plume/hotspot track, a mark left by the interaction of that hotspot with the continental crust.
A team of researchers at the California Institute of Technology, led by a scientist who is now at the Chinese Academy of Sciences in Wuhan, China, took advantage of another recent earthquake in the Eastern U.S., the Virginia earthquake of 2011. That Earthquake sent seismic waves through rocks across the entire Eastern U.S., and those waves were detected by dozens of seismic stations positioned throughout the U.S.
When the scientists looked at the data from this earthquake, they found that the seismic waves in an area of the continent stretching from Missouri through Kentucky and Virginia moved more slowly than the same seismic waves outside this area. The difference was only a few seconds, but that is more than enough to be measured by modern seismometers. There is a long path of seismically slow material underneath the Eastern U.S, stretching across this entire region, cutting across the Appalachian Mountains and then turning to the north along the coastline.
Seismically slow material can have several causes, but the most common assumption is that seismically slow material is hotter than the mantle material around it. If that assumption is right, then sometime, perhaps 100 million years ago, a plume of hot mantle rose up beneath the United States and hit the bottom of the continental crust. The crust was so thick and cold that it didn’t produce large volcanoes at the surface but it left a remnant path of hot material at the top of the mantle.
That plume kept supplying hot material as the continent drifted over it, leaving a trail of hot mantle stuck to the continent from Missouri to Virginia and then turning north along the coastline heading towards Maine.
Without direct sampling of the rocks it’s hard to know for sure what the seismic data is telling us but it is clear there is something very weird about the rocks in this region and the area where the track starts: Missouri, Kentucky, Arkansas, and Southern Illinois, is right at the heart of the New Madrid seismic zone.
If there is a hot plume of material sitting beneath the continent, pushing up against the crust and trying to break through, one consequence of that plume could be earthquakes as the crust strains and cracks.
This is a very interesting story but it will need a lot of effort to confirm it. The authors did note that there are samples of kimberlite rocks in Kentucky, produced in small volcanic eruptions which sample the mantle and could possibly be consistent with their hypothesis. If their story is correct, then one certainly wonders what other impacts this possible plume might have had on the geology of the Eastern United States.
-JBB
Image credit: http://web.mst.edu/~rogersda/nmsz/
Original paper: www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1949.html