A plume of magma may explain North America’s odd geology
Earth’s active geologic processes are a blessing and a curse. They help sustain life. Yet, the last sign I saw as I entered the Washington State Fair this summer pointed to an evacuation route in the event of volcanic eruption. It is no secret most destructive events occur at plate boundaries. Earth’s crust consists of dozens of tectonic plates moving against one another, causing earthquakes and volcanoes. But these hazards are much less frequent in the middle of tectonic plates. In Iowa and the Amazon, you worry more about erosion, not volcanoes.
There are exceptions to this rule, and those exceptions are often caused by hotspots. Hotspots are regions of volcanic or earthquake activity caused by superheated plumes of magma that rise up from Earth’s mantle. These plumes melt rock and sometimes pierce the surface, triggering earthquakes and volcanoes. As a plate moves across a stationary plume, the plume can leave a train of volcanoes in its wake. The most famous example of these pock-marked scars is the Hawaiian Islands. The hotspot that created them has punched holes through the Pacific Ocean’s thin oceanic crust since dinosaurs roamed the Earth.
Unlike oceanic crust, continental crust is much thicker, posing a challenge for hotspots that want to be heard. The particularly violent hotspot beneath Yellowstone National Park has managed to breach thick continental crust several times. In its last major eruption 640,000 years ago, volcanic debris fell in Mississippi over 1,300 miles away. But Yellowstone is the exception, not the rule; geologists still struggle to understand just how hotspots behave beneath thick continental crust. In a paper published last week in Nature Geoscience, a team from the California Institute of Technology report evidence that eastern North America recently passed over a hotspot, with surprising implications for the continent’s geologic history.
The evidence surfaced during the 5.8-magnitude earthquake that struck Virginia and Washington, D.C. on 23 August 2011. Seismometers measured the energy released by the earthquake in the form of seismic waves. P waves, one of these seismic waves, propagate through rocks based on their mineral composition and temperature. The Caltech team noticed that waves moving from Virginia west toward Missouri moved slower than they should. After checking their data, exploring local geologic history, and running through computer models, they came to a striking conclusion: eastern North America had passed over a hotspot.
This hotspot would date back to the time of the dinosaurs, at least 80 million years ago. It also never reached the surface, probably since the crust beneath eastern North America is so thick. But, this hotspot would have melted the deepest layers of crust along its path, heated or stretched nearby rock, and left a signature “highway” of deformation along the North American plate. This “highway,” including the Virginia earthquake P wave anomaly, tracks across Missouri, Illinois, Kentucky, and Virginia, before turning north into Pennsylvania, New York, and New England.
If it existed, this hotspot left no surface signatures like the Hawaiian or Yellowstone volcanoes. But, “non-piercing” hotspots can expose ancient faults or weak areas within overlying crust. This putative hotspot likely crossed southern Illinois and Missouri 80 million years ago, just as the area’s ancient, brittle, and violent New Madrid fault zone was reactivated for some unknown reason. It has remained active ever since, with massive earthquakes ringing church bells as far as Massachusetts in the past two centuries. The hotspot’s supposed progression across eastern Kentucky 75 million years ago coincides with the formation of local kimberlite, a rare magmatic rock associated with hotspots. Its effects in Virginia could explain seismic activity like the 2011 earthquake.
Evidence for this hotspot is lacking. Correlations like the New Madrid earthquakes and Kentucky kimberlite aside, few surface features point to its existence. Though the hotspot should now be beneath the Atlantic Ocean, there is no current evidence of its activity. Still, if time and additional seismic data prove this theory true, it could reveal a new role for hotspots in deforming or weakening continental crust over time.
Hotspots do not have a constant history; they depend on the types of oceanic or continental crust that pass over them. Hawaii is persistent and relatively benign. Yellowstone is periodic and catastrophic. The now-benign Réunion hotspot was apocalyptic when India passed over it 65 million years ago, flooding the continent with magma and probably contributing o the demise of the dinosaurs. In comparison, this “new” North American hotspot looks benign. But, who knows what the future holds.