Geoid-Gravity Map of the Earth 

Decoding a low in the Indian Ocean 

The Earth from its rotation is compressed at the poles and bulged at the equator. This makes the poles closer to the center of the Earth and experiences high gravity than the equator. Similarly, gravity is higher at elevated places like mountains as they are packed with more mass. Elevations and depths are measured relative to a mean sea level. At this level, Earth’s gravitational pull is supposed to be equal around the globe. This gravity-defined shape of the rotating Earth is called a geoid. With more accurate gravity measurements being taken by researchers, finer details of non-uniformities in the geoid are being unravelled. 

Schematic showing  (a) The geoid shape of Earth showing cross-section of the interior and locations of ocean bottom seisomometers (white triangles) deployed by NCPOR in the Indian Ocean geoid low region. (b) Vertical distribution of mantle composition and extent along the study region. (Courtesy: Sanjay Negi, NCPOR) 

A geoid low indicates a mass deficit in the interior of the Earth, especially in the mantle. The lowest of the geoid is in the central Indian Ocean just below Srilanka. Recently, Sanjay Negi and team from the NCPOR obtained details of this low using seismological observations. They deployed 17 ocean bottom seismometers on the ocean floor at the geoid low region. Seismometers record seismic waves sent out by tectonic motions occurring in the Earth’s interior. Among those waves, primary or P waves are faster than secondary or S waves. P waves convert to S waves whenever there is a sudden density or mineralogical change in the mantle. The delay between the direct P wave and the P to S converted wave defines the boundaries of the mantle transition zone - a layer between the upper and lower mantle from 410 to 660 kilometres beneath the surface.  

The researchers used these waves to map the extent and composition of the transition zone below the Indian Ocean geoid low. They found nearly 40 to 50 kilometres of depression in 800 kilometres stretch of low gravity in the transition zone which corroborated with the Indian Ocean geoid low. In this stretch, upper discontinuity depth varied from 386 to 460 kilometres and lower discontinuity from 643 to 710 kilometres.  

Using these depths, the team inferred excess temperatures - relative heating or cooling compared to the surrounding temperature. They found that the temperatures varied from around -133 to 284 degrees Celsius in the upper discontinuity depth and from -67.15 to 726 degrees Celsius in the lower depth. Based on these temperatures, the team inferred dominance of majorite garnet mineral in the transition zone. At upper discontinuity depths, they observed patches of old basalt-rich subducted material. At lower discontinuity depths, the researchers found an increase in temperature and low density. They explained that this low density might have brought up the subducted material from the lower mantle. 

In addition, the researchers observed a delay in seismic waves passing through the lower discontinuity depths. They emphasised that such delays can happen when the waves interact with the matter coming upward due to mantle upwelling. However, if upwelling was happening, why there were no raised features on the surface, contemplated the researchers. Eclogite- a garnet dominated dense rock can occupy deep depths of high temperature and pressure in the lower mantle. But, the upwelled eclogite due to its high density spreads horizontally in the lower discontinuity depths, rather than reaching upper depths. ‘Thus, we found characteristics of a less dense but also a less buoyant mantle in the Indian Ocean geoid low region’, says Dhananjai Kumar Pandey, NCPOR.

Though the gravity variations on the geoid are small to be felt by an individual, there are bigger ways in which the geoid distribution and changes affect us. The measurements of these changes can help predict and understand earthquakes and tectonic motions in advance. Geoid measurements are also vital in understanding the ice sheet mass balance and related isostatic adjustment when the melting of a large ice sheet readjusts geoid height. Such readjustments may also change the sea level height of a place. The measurements are also being used to obtain first-hand information about freshwater stored in reservoirs and their impact on hydrological cycles.