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Peering inside the Earth, one quake at a time

Maven geophysicists dive deep into the Earth’s crust to find a lot more than just rocks.

Did you know that the sealevel in the Indian Ocean just south of the Indian peninsula drops down by about a kilometer? This is because the surface of the ocean there is pulled down by some deep seated structure in the Earth’s mantle. Such “gravity anomaly” also exists in the North Atlantic where the sealevel is 50 m higher than average.

Attreyee Ghosh’s lab at IISc is trying to understand this, and a host of other interesting aspects about the way our Earth works. And they are attempting this from inside out. An Assistant Professor at IISc, Ghosh is currently setting up a “Geodynamics Lab” with high-end computers and computational software. The lab studies “dynamics” of the Earth – physical processes within the Earth by using a mix of geology, mathematics and physics.

We have all studied that the Earth is made of a central core of hot iron, an intervening zone of molten rocks called “mantle” and the upper crust, on which we live. Of the different mechanisms of heat transfer, convection -- hot fluid moving from a region of high temperature to a region of low temperature in the presence of gravity -- is an important process shaping the Earth. Something like how hot water at the bottom of a vessel on a burning stove moves to the top and how cold water on top sinks to the bottom. Convection also happens in the Earth’s mantle, in an effort to bring the energy from the very hot core to the cool crust as well as due to radioactive decay.

The Earth’s crust can be divided into “plates”, and the study of how these plates move around constitutes the theory of Plate Tectonics, a paradigm that was understood only in the recent past (1960’s). Many geophysical activities (genesis of mountains, oceans and islands, rifting of continents, earthquakes, volcanism, etc.) are beautifully explained by this theory. Currents caused by convection in the Earth’s mantle result in the plates moving around – like pizza slices on a thick, hot broth. The mantle is made up of rocks, but even rocks move around over billions of years. Like fluids, they flow and bend and mix with each other. This mantle flow drags the Earth’s crust from underneath, pulling them in all directions and crashing them into each other. There are bands of oppositely polarized rocks on the ocean floor which can be explained by the Earth’s magnetic poles switching between north and south in the past. Plate Tectonics helps us understand how these rocks are formed in the oceans. Magnetic fields are caused by the Earth’s north and south poles. They are the reason why your little compass points north.

"During my masters in Columbia University, I took a course called Plate Tectonics. It was the best course I have ever had and it led me towards my chosen research direction," says Ghosh, "The course dealt with fundamental questions about earth’s processes, driving forces behind Plate Tectonics, and the controversies behind each theory. It was an eye opener about all the things we are yet to learn about this planet!"

The geodynamics lab is a menagerie of people from different educational backgrounds: two research students (Srishti and Smruti) and one research assistant (Shree). Srishti and Smruti are only a semester old, and are currently mowing through papers exploring research ideas.

Shree is looking to find a definite explanation for a rather peculiar problem, a mass deficit just south of the Indian peninsula which manifests in the form of a giant “negative gravity anomaly”. What is even more interesting is that the source of this anomaly lies deep in the Earth’s lower mantle, more than 700 km below the Earth’s surface. Studying these anomalies is one of the few ways we can learn about what is happening deep inside our planet. The specific cause of this gravity anomaly is still a matter of controversy. "The big question that we are trying to address in our lab by doing these kinds of studies is: how much of the processes that we see on the surface of the Earth, such as earthquakes, movement of plates, formation of mountains, rifting of continents, volcanic eruption, and so on, can be explained by what is happening deep inside the Earth as opposed to what is happening in the shallow part (top 100 km)? Which one dominates and can we quantify this contribution?” asks Dr. Ghosh.

Peering into the earth

"You can't really look into the Earth as you can look into space," Ghosh explains, "The field of optics, that lets you build very powerful telescopes to look into the far reaches of the universe, is very well developed, but the Earth is opaque".

Hence, indirect methods are used to understand the Earth’s internal processes. Large dedicated GPS stations are used to monitor motions of tectonic plates. Seismic tomography is used to peer inside the Earth. The principle of seismic tomography is similar to that of a CT-Scan: If you want to image your brain, there are machines that shoot X-rays from different directions around your brain and sensors that detect them after they travel and bounce through the brain. Slices of your brain are imaged, and weaved together to create a 3-dimensional model. Instead of X-rays, geophysicists use seismic waves to get a 3-dimensional picture of the Earth’s interior. When an earthquake occurs, it generates seismic waves, much like ripples in water by dropping a pebble. These waves travel through the Earth collecting information about the layers through which they pass. Seismic stations all over the world log this information round the clock and store it. Geophysicists, like those from Ghosh’s lab, use the data, coupled with numerical modeling to create a 3-dimensional picture of the innards of the Earth.

"Seismic tomography has been growing fast for the past twenty to thirty years and new innovative algorithms along with more powerful computers are giving us a clearer picture of the earth’s internal processes" adds Ghosh.

Peering into the universe

Geophysics isn't confined to the Earth. We are already aware of moonquakes, earthquakes on the moon, detected by seismometers installed there during the Apollo missions. "There are a lot of geophysicists studying other planets, especially Mars. Some of my colleagues look at topography and gravity anomalies on Mars to understand its internal dynamics and find evidence of tectonic activity," says Ghosh, “The main drawback is the lack of data. No one is willing to send a seismometer to Mars due to monumental costs."

The story of Earth is a peculiar one. Eons ago there was just a lot of solar nebular dust. A few million years later, the dust started to accrete together. As the dust accreted, large chunks of rocks formed in space, and these attracted more dust and space rocks called “bolliods” onto and into themselves and grew bigger. The largest cosmological entity born from these processes, which took tens of millennia, was the sun. Second largest were the planets, followed by asteroids and comets. A simple understanding of the universe using thermodynamics (the art of figuring out how energy is likely to be transferred), reveals that almost every mass that exists, would like to cool down. The sun and planets are understood to have formed in an effort to reduce the energy that the nebular dust had. They are still cooling down today.

Hence, understanding the Earth is imperative to understanding the outer planets. But since very little is known about the Earth’s internal processes new theories that explain and re-explain various observations are hazarded every year. “It was only in 2004 that the sudden change in seismic wave velocity occurring close to the Earth’s core-mantle boundary was attributed to a newly discovered mineral phase called post-perovskite. Nowadays people take it as a given,” Ghosh mentions. There are a large number of theories rejected by the scientific community too.

"Fundamental and basic physics can be used for addressing a huge myriad of problems about the Earth," remarks Dr. Ghosh, "Newton's second law (F = ma) is one of the underlying physical laws used to investigate the driving forces behind Plate Tectonics."

Between dividing her time between classes, research and setting up her lab, Attreyee Ghosh can be contacted at: aghosh@ceas.iisc.ernet.in. Her website is http://ceas.iisc.ernet.in/~aghosh/.

The author, Gautham S. B. is a PhD student at the Centre for Earth Sciences, Indian Institute of Science. You can look him up at www.gauthamsb.wordpress.com.