Earth's mantle rises, lifting ancient shoreline

By James Maynard , May 27, 2013 07:38 AM EDT

Sea levels are often measured by looking at evidence of ancient shorelines. Remnants of such an ancient coast, called Orangeburg Scarp, stretch today from Florida to Virginia, rising up to 280 feet above today's sea level. That ancient shoreline was created through the action of waves crashing on the land three million years ago. It is believed that the higher shoreline of that time was due to the melting of massive ice sheets. However, researchers have now discovered another reason that line is so far above today's sea level - the ground itself has risen.

Robert Moucha, assistant professor of Earth science in The College of Arts and Sciences, and a team of researchers investigated movement of the Earth's mantle in the southeast United States over the last three million years. Their study revealed that three-quarters of the apparent change in sea levels over the last three eons was due not to the lowering of sea levels, but the rising of the Earth's crust.

This new evidence of the swelling of mantle in the region indicates that the melting of ice sheets that caused the rise in sea levels back then was not as great as was once believed. Average sea surface temperatures back then were three to five degrees Fahrenheit warmer than they are today.

"If we can estimate the height of the sea from 3 million years ago, we can then relate it to the amount of ice sheets that melted," Moucha said. "This period also serves as a window into what we may expect in the future."

Much of the ice covering Greenland is believed to have melted during that time, and traditional thinking held that Antarctica suffered loss of ice sheets all over the continent. This new research provides evidence that the Eastern Antarctic Ice Sheet remained largely intact while other ice sheets melted.

As the mantle rose, it created bends in the shoreline, creating evidence of the line's tectonic causes. These wave-like formations were caused at the interface between the Earth's crust and the mantle which moves beneath it. This process is called dynamic typography.

"You simply can't go somewhere and look at the height of the shoreline and infer anything about the amount of water in the oceans or the height of sea level without already knowing an awful lot about what the mantle is doing," David Rowley, geologist at the University of Chicago, and lead author of the study, said.

Learning more about how changes in the Earth's mantle can affect measurements of ancient shorelines as well as help refine predictions of future sea levels due to climate change.

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