The world’s largest gravity hole in the Indian Ocean. The Reason is Revealed
The Indian Ocean has the world’s largest gravity anomaly. This was first discovered in 1948, but was finally clarified in May of this year. According to Indian scientists, this is caused by debris from the ancient Tethys seafloor.
Don’t take offense, Indian Ocean, but this question has puzzled scientists ever since the hole was discovered in 1948. And now a research team at the Indian Institute of Science (IISc) thinks they have found the answer. The “hole” in the Indian Ocean is due to debris that sank to the bottom of another, much older ocean.
In this enigmatic part of the Indian Ocean, gravity is weaker than anywhere else on Earth. This gravity hole is the largest (and deepest) gravity anomaly in the world and is officially called the Indian Ocean Geoid Low (IOGL). (The geoid is a theoretical model of sea level around the world, whose irregularities correspond to variations in the Earth’s gravity.)
The variations in gravity correspond to differences in the density of subsurface (and seafloor) rock layers, which in itself is nothing strange. What cannot be explained is the magnitude and amplitude of the anomaly in the Indian Ocean.
Air rather than water
The center of the gravity hole is about 1,200 km (750 miles) southwest of Kanyakumari (also known as Cape Comorin), the southernmost point of the Indian subcontinent. This circular depression has an area of about 3 million square kilometers (1.2 million square miles), about the same size as India. Due to the region’s low gravity, sea level in IOGL is 106 meters (348 feet) lower than the global average. This means that there is a large amount of air where there should be water. Estimates are that about 100 cubic kilometers (25 cubic miles) of water has been discharged because of this anomaly.
The anomaly was discovered by Felix Andries Benning Mines, a Dutch geophysicist who invented a device that measures gravity over the ocean. (The device was nicknamed the “golden calf,” a premium given to the device because of its color and the annoyance that the crew of the submarine carrying it had to remain motionless in their bunks during the measurement.)
As a pioneer in submarine gravity measurements, Venning Mainz spent most of his career roaming the oceans in submarines and research vessels. He discovered several anomalies in the Earth’s gravity field, which he attributed to plate tectonics, but none as large and strong as the anomalies in the Indian Ocean.
The actual cause of the IOGL was discovered last May in Geophysical Research Letters, published by IISc researchers.
The IISc team used a supercomputer to run various simulations from 140 million years ago to explore how tectonic and volcanic forces shaped the world as we know it today. The models that produced gravity holes that closely resembled the actual IOGL had one thing in common. That is, low-density magma erupted and displaced high-density material, weakening the gravity in the area.
Why did it collapse?
About 120 million years ago, the Indian plate separated from the supercontinent Gondwana and collided with the Eurasian plate. That collision would eventually give rise to the Himalayas. Before reaching there, however, the Indian Plate passed over the Tethys Plate, closing off an ancient sea called the Tethys.
As fragments of that plate, the so-called Tethyan Slab, were pushed deep into the Earth’s mantle, some of the material trapped in the so-called African Blob, a continent-sized magma bubble about 1,000 km beneath East Africa, was displaced. About 20 million years ago, this relatively light material rose toward the surface as a plume. Together with mantle structures near the geoid low, these plumes are responsible for gravity anomalies off the southern tip of India, the IISc team writes. The team was able to estimate the age of the geoid low, but it is difficult to predict when it will move and dissipate.
Source: The world’s largest gravity hole in the Indian Ocean. The Reason is Revealed