Compared with western North America—mountainous, volcanic, and earthquake-prone—the geologically quiescent East Coast has earned the appellation "passive continental margin." But new geologic models show that Earth's churning interior warps and bends this and many other so-called stable areas.
Three million years ago, Earth was several degrees warmer than it is today—about the same global temperature that we may see by the year 2100. Geologists want to know what continental shorelines looked like during this ancient era, known as the Pliocene, in order to forecast future sea-level change. Scientists assumed that passive continental margins, like the Atlantic coastal plain and offshore sea floor, have no geologic forces pushing them up. The coast instead slowly and relentlessly sinks as the rock beneath it cools and sand and mud washed off the land fill the space created by the sinking continental margin. Without anything pushing the rocks up, the ancient coastlines studied by geologists should remain flat and horizontal, marking the level that the sea once came to. But one of these old beaches, the Pliocene-era Orangeburg Scarp, warps and bends along its course from Florida to North Carolina.
What if something is pushing the land up? David Rowley, a geologist at the University of Chicago in Illinois, and a team of geodynamical modelers simulated the lava lamp-like movement of hot material in Earth's mantle, which is a highly viscous though solid layer of rock between the crust and the molten core. Hot mantle plumes rising up from the core can affect Earth's surface, creating Yellowstone's steaming geysers and Hawaii's spectacular volcanoes.
Rowley's team uncovered subtle mantle movements under eastern North America by including in their model a layer of relatively soupy rock, just beneath Earth's rigid outer shell, called the asthenosphere. It allows the outer shell to slip across the underlying mantle, Rowley says.
A model that doesn't consider the asthenosphere would show the Orangeburg Scarp sinking. "But clearly, the Orangeburg Scarp is going up," Rowley says. It bows and bends "like a magic carpet." Rowley says he's modeled similar effects, dubbed "dynamic topography," in Africa and in the Colorado Plateau of the American southwest. "The whole Earth is riding on a dynamic mantle," he says.
A dynamic mantle may have so distorted passive margins that researchers have been misreading the record of sea-level changes during the Pliocene era and seen them as more dramatic than they were. That may cause climate modelers to rethink future sea-level rise under a warming climate. Dynamic topography doesn't change global sea levels, but moves the "low-water marks" that are used to measure Earth's past climate.
Dynamic topography means that the Atlantic coast can't really be considered a passive margin anymore. "It's really a passive-aggressive margin," says Kenneth Miller, a geologist at Rutgers University in Piscataway, New Jersey, who was not involved in the study. "They give us a mechanism for the aggression." Rowley's modeling work needs to be checked against the geologic record, Miller says. He believes that the current large-scale model needs to be refined to better match the surface geology. At the current resolution of hundreds of kilometers, he says, "it's not really testable."
Paleontologist Harry Dowsett of the U.S. Geological Survey in Reston, Virginia, who was also not involved in the study, says that he sees evidence of Rowley's model in the distribution of surface rock outcrops. "It's such an excellent fit to the geology. It explains what we see," he says.
Better estimates of Pliocene sea levels will help geologists know how much of the ice sheets melted during that balmy era, Dowsett says, which may give us a glimpse of our own climate future.
"It doesn't make my job any easier," jokes Dowsett, who uses microscopic fossils to reconstruct Pliocene shorelines. "Probably made it a lot harder."