maandag 16 november 2015

PhD almost over, so lets write a little about it

The ice sheets and glaciers on our Earth are currently melting, which causes the global sea level to rise. This has of course major impacts on the 44 percent of the world’s population living in coastal areas. This melting is happening already since 24,000 years ago, during that time enormous ice sheets, up to 3-4 km thick, covered lands like Scandinavia and Canada. Scientists have done measurements of this rising sea level using the coral reefs at Barbados that show a sea level rise of 120-130 meters since the start of the de-glaciation. Imagine this for a moment; England was not an island at that time.

Currently, we see a global sea-level rise of 3 mm/year, however locally the rate of sea-level change can vary significantly. For example, at the coastlines of the Netherlands we observe a 2.3 mm/yr sea-level rise, whereas the center of Sweden and Finland has a 7.8 mm/yr sea-level drop. The cause of these particular variations is that not only the sea-level changes, but also the solid ground underneath us moves up and down.
Anual Tide Gauge record of two stations: Hoek van Holland (blue) and Ratan (red, middle of Scandinavia). I fitted a linear trend line through the data to estimate the relative sea-level change of those two locations. Data can be found here.
The huge ice sheets of 3-4 km thickness, mention earlier, have pressed the Earth’s crust downward which caused mantle magma to flow outwards. Just like when you would lie on a waterbed. Some part of the water beneath you would flow away from you, such that other parts of the bed rise. When you get out of the bed, the water will flow to its equilibrium state. Similar effects will happen to the surface of the Earth after the big ice sheets have melted. So, the land is now rising again where the historical ice sheets were situated (Scandinavia), resulting in a sea-level drop in that area. This sea-level drop is caused, because the solid Earth is faster moving upwards than the sea level. Therefore, relatively in Scandinavia the land is coming up. One funny consequence, somebody once told me, is that Sweden and Finland have world’s best legal regulations for land division, because every year they get more land and need to divide it between them.

A schematic representation of post-glacial rebound of the crust.
(courtesy of the Canadian Geodetic Survey, Natural Resources Canada)
This motion is called post-glacial relaxation of the Earth’s crust and it can bias local observations of the actual ongoing sea-level change. So, to really understand the ongoing sea-level change we need to understand this relaxation. To do so we need information about the physical state of the magma underneath our feet. This region is called the mantle of the Earth. We need to know the temperature, density, and viscosity of the mantle. Temperature (how hot something is) and density (how heavy something is) are easy to recognise, but viscosity is sometimes difficult to understand. Viscosity is a way to tell how sticky a certain fluid will flow. For example, maple syrup has a higher viscosity then water. It takes maple syrup much longer to get out of a bottle than, for example, water. If we would fill our waterbed with maple syrup, the sloshing of the bed while you were moving on top of it, would go much slower. You can imagine that the viscosity of the mantle is therefore important to know when studying the relaxation motion.

However, the exploration of the mantle is extremely difficult. NASA scientists often say that humanity knows more about the farthest places in outer space, than the interior of the Earth. The deepest hole mankind has ever constructed is only 12 km deep (Kola superdeep borehole), whereas the mantle in continental areas (like Scandinavia) starts at 30 km deep until the core of the Earth (2890 km deep). We have only scratched the surface. So, instead of going there and study the mantle in-situ, we have to think of observation techniques that can remotely tell use something about the physical state of the mantle: measurements that use seismic waves, gravity field, and magnetic field, or old geological outcrops that give information about the composition of mantle rocks.

Unfortunately, ground measurements and satellite observations give us incomplete information and do not show us the complete picture. Seismic observations give us information about seismic wave speed, which are affected by composition, temperature, and density, whereas satellite gravimetry can penetrate to deeper layers of the Earth, but only shows us the density structure. Geological field studies give us hints about the history and composition of the Earth crust, but only give us information about what is found on the surface. Somehow, we need to combine all these different techniques in an ingenious way to successfully explore the deep Earth. So, how can we improve the exploration of the deep mantle, by combining gravity, seismic, and geological observations? With the end goal to better understand the relaxation of the Earth’s crust, such that we can improve our predictions of the current sea-level change?

This is where my PhD research is hopefully answering a few questions. By studying the gravity field of earth, we know that continental plates are floating on the mantle just like an iceberg. Here, a small tip above the water line means a large submerged root to keep the whole iceberg floating. This information can be used to estimate the density of the mantle and validate it with the observed gravity field. Geological studies give us information about the type of rocks and their composition found in the area. Also, geological theories state that old continental areas are colder than young continental areas. These clues about the temperature can be validated with seismic studies. For instance, certain seismic waves are more sensitive to the temperature profile of the magma than compositional changes. By combining the initial estimated density of the mantle with seismic measurements of temperature and composition, we are able to construct a complete physical model of the deep mantle. Finally, the explored physical state of the deep mantle can be used to simulate the vertical motion of the Earth’s crust due to the melting of the huge ice sheets. By changing the physical parameters within their uncertainty, different results of the vertical motion can be obtained. These results are then compared to geological and geodetic observations the vertical motion. Hopefully after many iterations, we hope that these new computer models of the mantle can reduce the uncertainty in the physical state and better predict the motion of the solid Earth.

In the end, by combining the gravity observations and crustal structures from geological and seismic studies in sophisticated computer models, we can simulate the slow movement of the solid Earth and will improve our estimation of current sea-level change by removing the bias in local sea-level change measurements.

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