Looking inside earth: a geophysical approach

Lately, we have been flooded with discoveries of exotic phenomena in the other planetary bodies of our solar system, be it the icy moons of Jupiter and Saturn or our next door planetary neighbour, Mars. But, how much do we really know about our own planet, especially when it comes to the processes occurring deep inside it?

Secrets of deep earth

The deepest borehole into the earth goes down to only about 12 km. Between the earth's surface and the core lie impenetrable and highly heterogeneous rocks whose properties are largely unknown. So how do we know what is going on inside the earth? A part of our knowledge about the earth's interior comes from studying the surface rocks that had been dredged from the deep earth by some tectonic activity - earthquakes, volcanoes, mountain building, etc. Another way of learning about the interior comes from studying the behaviour of rocks in laboratory experiments under high pressure and temperature conditions that occur within the earth. A rapidly progressing field today is geodynamical modelling, which provides a third way of unravelling the secrets of the deep earth.

Seismic tomography, which uses the same technique as CT scans in medical imaging, can provide three-dimensional images of the structure of the earth's interior. Information from these images, when fed into computational models that simulate the earth's internal processes, provides us with predictions that can be compared with observations and measurements on the surface of the earth. A perfect match between the model predictions and observations would imply that our models of the deep earth are correct. However, lack of knowledge about the properties of the highly heterogeneous rocks, especially at high pressure and temperature conditions that exist in the deep earth, makes it difficult to come up with an accurate picture of the interior.

Tremendous progress

However, the advent of more powerful computers and cutting-edge technology is changing that. We have come to know that the earth is not just a simple layer cake structure as we previously used to think; there's a lot more going on. We have learnt that the rocks that we see around us, which seem to be hard and cold and indestructible, behave like fluid over millions of years. They sink and rise and flow, and in the process split apart continents, build mountains and form ocean basins.

In the last decade, we have made tremendous progress in understanding the deep earth processes and how those affect what we experience on the surface, including earthquakes. However, every year, we lose hundreds of lives because of earthquakes. Though we understand the physics behind earthquakes, our knowledge so far has only enabled us to know which regions are prone to them, but not how to predict them.

The best we can do is making long-term forecasting and mitigating potential hazard. A better understanding of the earth's internal processes, about how rocks behave under stress, could one day help us in considerably narrowing the time window for an earthquake forecast and save lives and properties.

In recent years, we have come to know that the phenomenon of 'plate tectonics', the process by which the blocks in the earth's outer brittle layer drift apart, collide and move past each other, has been crucial in making this planet habitable, thus leading to the birth of life. In our recent forays into other planets and planetary bodies of the solar system, we are learning that plate tectonics might have been active in the past on some of those. As our quest for earth-like planets in the universe goes on, could we link plate tectonics to one of the crucial processes that makes a planet habitable, and perhaps narrow down our search for planets that are liveable?

As we discover more and more about this planet that we call home, we are often filled with wonder. Relating that sense of wonder to our day-to-day lives is not always straightforward. But, in a world rife with problems, starting from climate change to conflicts due to scarcity of resources, solution to a lot of them will lie in how we utilise that knowledge and that sense of wonder in an effort to find a way out of those problems.

(The author is an assistant professor, Centre for Earth Sciences, Indian Institute of Science, Bengaluru)