XPost: alt.astronomy
OK. I'll admit, I'm confused by this.
They say it's a " “gravity hole” in the Indian Ocean — a spot
where Earth’s gravitational pull is weaker, its mass is lower
than normal, and the sea level dips".
If the pull is weaker, why does not the water bulge out?
from
https://www.cnn.com/2023/07/24/world/gravity-hole-geoid-low-indian-ocean-scn/index.html
There is a ‘gravity hole’ in the Indian Ocean, and scientists now think they know why
Jacopo Prisco
By Jacopo Prisco, CNN
Updated 10:33 AM EDT, Mon July 24, 2023
In 2011, GOCE delivered a model of the 'geoid' pictured here. At the
time, it was the most accurate ever produced. The colours in the image represent deviations in height (--100 m to +100 m) from an ideal geoid.
The blue shades represent low values and the reds/yellows represent high values.
The sea level dips by over 328 feet (100 meters) in a "gravity hole"
first discovered in 1948. Today, scientists hypothesize that the hole in
the Indian Ocean was shaped by plumes of magma coming from deep inside
the planet, according to a new study.
ESA/HPF/DLR
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CNN
—
There is a “gravity hole” in the Indian Ocean — a spot where Earth’s gravitational pull is weaker, its mass is lower than normal, and the sea
level dips by over 328 feet (100 meters).
This anomaly has puzzled geologists for a long time, but now researchers
from the Indian Institute of Science in Bengaluru, India, have found
what they believe is a credible explanation for its formation: plumes of
magma coming from deep inside the planet, much like those that lead to
the creation of volcanoes.
To come to this hypothesis, the team used supercomputers to simulate how
the area could have formed, going as far back as 140 million years. The findings, detailed in a study published recently in the journal
Geophysical Research Letters, center around an ancient ocean that no
longer exists.
A disappearing ocean
Humans are used to thinking about Earth as a perfect sphere, but that’s
far from the truth.
“The Earth is basically a lumpy potato,” said study coauthor Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth
Sciences of the Indian Institute of Science. “So technically it’s not a sphere, but what we call an ellipsoid, because as the planet rotates the
middle part bulges outward.”
Our planet is not homogeneous in its density and its properties, with
some areas being more dense than others — that affects Earth’s surface
and its gravity, Ghosh added. “If you pour water on the surface of the
Earth, the level that the water takes is called a geoid — and that is controlled by these density differences in the material inside the
planet, because they attract the surface in very different ways
depending on how much mass there is underneath,” she said.
The “gravity hole” in the Indian Ocean — officially called the Indian Ocean geoid low — is the lowest point in that geoid and its biggest gravitational anomaly, forming a circular depression that starts just
off India’s southern tip and covers about 1.2 million square miles (3
million square kilometers). The anomaly was discovered by Dutch
geophysicist Felix Andries Vening Meinesz in 1948, during a gravity
survey from a ship, and has remained a mystery.
Earth core structure. Elements of this image furnished by NASA
Hidden molten rock layer found beneath Earth's tectonic plates
“It is by far the biggest low in the geoid, and it hasn’t been explained properly,” Ghosh said.
To find a potential answer, Ghosh and her colleagues used computer
models to set the clock back 140 million years in order to see the big
picture, geologically. “We have some information and some confidence
about what the Earth looked like back then,” she said. “The continents
and the oceans were in very different places, and the density structure
was also very different.”
From that starting point, the team ran 19 simulations up to the present
day, recreating the shifting of tectonic plates and the behavior of
magma, or molten rock, inside the mantle — the thick layer of Earth’s interior that lies between the core and the crust. In six of the
scenarios, a geoid low similar to the one in the Indian Ocean formed.
The distinguishing factor in all six of these models was the presence of
plumes of magma around the geoid low, which along with mantle structure
in the vicinity are believed to be responsible for the formation of the “gravity hole,” Ghosh explained. The simulations were run with different parameters of density for the magma, and in those in which the plumes
were not present, the low did not form.
The plumes themselves originated from the disappearance of an ancient
ocean as India’s landmass drifted and eventually collided with Asia tens
of millions of years ago, Ghosh said.
“India was in a very different place 140 million years ago, and there
was an ocean between the Indian plate and Asia. India started moving
north and as it did, the ocean disappeared and the gap with Asia
closed,” she explained. As the oceanic plate went down inside the
mantle, it could have spurred the formation of the plumes, bringing
low-density material closer to Earth’s surface.
More humans have visited the moon than the bottom of the ocean
01:31 - Source: CNN
The future of the geoid low
The geoid low formed around 20 million years ago, according to the
team’s calculation. It’s hard to say whether it will ever disappear or shift away.
“That all depends on how these mass anomalies in the Earth move around,” Ghosh said. “It could be that it persists for a very long time. But it
could also be that the plate movements will act in such a way to make it disappear — a few hundreds of millions of years in the future.”
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Huw Davies, a professor in the School of Earth and Environmental
Sciences at Cardiff University in the UK, said the research is
“certainly interesting, and describes interesting hypotheses, which
should encourage further work on this topic.” Davies was not involved
with the study.
Dr. Alessandro Forte, a professor of geology at the University of
Florida in Gainesville who was also not involved with the study,
believes there is good reason to carry out computer simulations to
determine the origin of the Indian Ocean geoid low, and that this study
is an improvement over earlier ones. Past research only simulated the
descent of cold material across the mantle, rather than including hot
rising mantle plumes as well.
However, Forte said he found a couple flaws in the study’s execution.
“The most outstanding problem with the modeling strategy adopted by the authors is that it completely fails to reproduce the powerful mantle
dynamic plume that erupted 65 million years ago under the present-day
location of Réunion Island,” he said. “The eruption of lava flows that covered half of the Indian subcontinent at this time — producing the celebrated Deccan Traps, one of the largest volcanic features on Earth —
have long been attributed to a powerful mantle plume that is completely
absent from the model simulation.”
Another issue, Forte added, is the difference between the geoid, or
surface shape, predicted by the computer simulation and the actual one: “These differences are especially noticeable in the Pacific Ocean,
Africa and Eurasia. The authors mention that there is a moderate
correlation, around 80%, between the predicted and observed geoids, but
they don’t provide a more precise measure of how well they match
numerically (in the study). This mismatch suggests that there may be
some deficiencies in the computer simulation.”
Terra Bella, CA - October 14: Matt Davis's company drills a 1300 feet
deep well in an orchard at Setton Farms on Thursday, Oct. 14, 2021 in
Terra Bella, CA. (Irfan Khan / Los Angeles Times via Getty Images)
Humans pump so much groundwater that Earth’s axis has shifted, study finds Ghosh said that not every possible factor can be accounted for in the simulations.
“That’s because we do not know with absolute precision what the Earth looked like in the past. The farther back in time you go, the less
confidence there is in the models. We cannot take into account each and
every possible scenario and we also have to accept the fact that there
may be some discrepancies on how the plates moved over time,” she said. “But we believe the overall reason for this low is quite clear.”
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