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2010-06-29 04:33:43
livescience.com Mon Jun 28, 3:00 pm ET
Hot blobs of magma - the searing liquid rock beneath the Earth's crust - can
spread slow-moving ripples that soar hundreds of meters high across the Earth's
surface, a new study suggests.
This phenomenon, which works on geologic time scales, may explain relatively
rapid pre-historical changes in sea level that occurred without the typical
waxing and waning of the polar ice sheets, which hold and release water on
scales of thousands and millions of years. This unexplained sea level rise is
one of geology's oldest mysteries.
During the Paleogene era (65 million to 23 million years ago), the land under
Scotland moved up and down like a geological yo-yo. The surface rippled up to
1,640 feet (500 meters) over the course of only a million years - a relatively
short time span, geologically-speaking.
"It appears to be caused by something deep within the Earth, moving sideways -
almost like rats running underneath a carpet," said study author Bryan Lovell
of the University of Cambridge.
The sideways movement is actually caused by what's known as convection currents
in the mantle. These currents are created when the cooler, denser material in
the mantle sinks and the warmer, less dense material rises.
"The hot blob would have spread out from the center of the hot spot rather like
a doughnut," Lovell said.
A magma hot spot beneath Iceland, known as the Iceland plume, which is
responsible for the European island's volcanic activity, could be behind this
movement.
Data from oil exploration near the Iceland magma plume has revealed that a blob
in the Earth's flowing mantle caused a pulse beneath the North Atlantic about
55 million years ago. This pulse raised and lowered the land surface and the
sea floor in a rippling effect
Although scientists were confident that hot blobs exist in the mantle, evidence
for the ripples at the Earth's surface have been hard to spot. While the
Scotland uplift is the only example of the hot blob effect, Lovell said the
movement is a natural effect of mantle convection, so there's no reason to
think it isn't happening all the time, even now.
"I'm delighted that we have what appears to be a decent geophysical explanation
for such a longstanding and significant geological problem," Lovell said. "I
hope we'll be able to collect more observations from the geological record that
will explain otherwise cryptic features of the Earth's mantle."
The study was detailed in the June 25 edition of the Journal of the Geological
Society.