By Afroz Ahmad Shah
The Earth’s internal structure is mostly studied through indirect means because the deepest hole though the Earth can only be drilled down to a few kilometers. This is because of the prevailing complex physiochemical conditions within the Earth and the lack of a robust scientific gear to drill down to the greater depths. This however is changing with the help of latest drilling techniques and it seems possible that within a decade scientists will be able to investigate the deeper portions of the Earth.
Geologists are trying to study the deep interiors of the Earth, because it will provide a wealth of information about its origin and evolution. One of the first such attempts was made through “Project Mohole”; an endeavor to drill down the Earth through the crust and into the mantle. This idea was primarily given by Harry Hess, one of the founding fathers of the plate tectonic theory and Walter Munk, an American physical oceanographer and who pioneered studies of how winds drive ocean currents and also, explained why one side of moon is always locked towards the Earth. This project came up in April 1957 on a Saturday morning in Munks house over a party.
In those days it was hard to drill deeper into the Earth, because the drilling techniques were not efficient to do this kind of work. However; this project got funding from the United States National Science Foundation and commissioned the best available ship, the drilling barge CUSS 1, named after the oil companies that had developed it, Continental, Union, Shell and Superior. It was between March and April, 1961, when a first sample (a few meters of basalt; an oceanic rock) was taken from the uppermost portion of the ocean crust, off Guadalupe Island in the eastern Pacific Ocean. It was pulled out from 3800 meters of water and 170 meters of sediments and it cost around US$1.5 million. This was the first and the last sample obtained during this project, because after the expenditure, the management changed and some poor decisions were made, which ultimately lead to its collapse in 1960, the time when the US Congress voted to cancel its funding. However, this project motivated the world scientific community to keep exploring the secrets of oceanic crust and this lead to the establishment of international collaboration in scientific ocean drilling, which is successfully running since the last four decades. The Integrated Ocean Drilling Program (IODP) and its predecessors, the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP), are the most successful, long-term international scientific collaboration in any field.
Until now no one has ever drilled deeper than ~2kms into the oceanic crust and now another Mohole campaign is on the way and will probably come up with some new insights into the treasure under our feet. However, if not direct, the indirect means are increasingly popular and have given us a fair idea what is under our feet. One of the best ways in which scientists have achieved this is through the use of earthquake waves, which can propagate through the interior of Earth, because some of these waves can pass through solid, liquid or a gaseous medium. These waves are therefore successfully used by scientists to know the internal configuration of Earth and its layered structure, which otherwise was quite hard to infer.
When seismic waves enter Earth, these are able to gather an “X-ray” and tell us about the inside story, because, these waves travel and behave differently in different kinds of materials, therefore, they are very helpful to obtain its indirect picture, which is made possible through the study of their travel times. For example, during an earthquake S waves do not reach to the opposite side of the Earth, which indicate the presence of a liquid core. These waves can either be reflected or refracted within the Earth. When these waves hit a surface (boundary) between very different materials they may bounce off this surface or get partly refracted. For example, an echo forms when sounds waves are reflected from a surface. Similarly, within the Earth, seismic waves reflect at the boundaries between the major Earth layers. Refraction can also occur within the Earth. Here the seismic waves are bent because of the difference of the velocity within different layers. In general, the seismic velocity in Earth increases with depth (there are some important exceptions to this trend) and refraction of waves cause the path followed by body waves to curve upward.
Imagine a homogenous Earth, where the seismic waves can travel equally in all possible directions. However, the Earth is heterogeneous and is composed of different materials, which have different physical and chemical properties. Therefore, seismic waves are caught in a complex situation and what it means is that scientists always have to work hard to differentiate the waves and interpret the structures within the Earth. Generally, during an earthquake, seismic waves reach quickly to a station, which is close to the epicentre (epicentre is the point on the Earth’s surface that is directly above the hypocenter or focus) than those which are farther away. This suggests that these waves have to travel a long distance before reaching far-off stations (Figure A). For example in 1906 scientists discovered that whenever an earthquake occurs on the Earth, a larger portion on its opposite side doesn’t see seismic waves, this region is called a shadow zone. For example when an earthquake occurs on a particular location, a huge shadow zone for S waves invariably exists beyond 103 degrees from the Earthquake’s focus. This suggests something stops these waves, so that they can’t pass through. This is important because it suggests that there is some liquid portion outside the Earth’s core, which does not allow the S waves to pass. As we know, these shear waves cannot move through liquids and are transmitted only through solids that have enough elastic strength to return to their former shapes after being distorted by the wave motion. This is taken as strong evidence to suggest that the Earth’s outer core is liquid. Therefore, this along with the magnetic field and a high density material suggests a molten outer core exists within the Earth, which should primarily be made up of iron.
Similar to S waves, P waves are also affected by the outer liquid core. However, these are more complex and their shadow zone forms a belt around the Earth between 103 and 143 degrees (Figure A) away from the earthquake’s focus. This suggests that Earth has a central core through which these waves travel relatively slow.
The evidences gathered from the Earthquakes waves, meteorites, which fall on Earth, magnetic fields, some rare exposures of the deeper section of the Earth’s layers, and other physicochemical properties, show that Earth is made up of three major layers, the crust, the mantle and the core. This stratification within the Earth was achieved through differentiation, which is a process by which materials are arranged in a sequence according to their densities. The Earth’s uppermost layer, the Crust is lighter and less dense; whereas the Mantle is relatively dense and the Core is very dense (Figure B). Most of the Earth is Mantle, which constitutes about 82% of its volume and ~68% of its mass. The Core constitutes 16% of the Earth’s volume and ~32% of its mass, which is due to its high density. The Crust is a very thin layer and constitutes only ~ 1% of the Earth’s volume. Its thickness varies from oceans to continents.
Afroz Ahmad Shah is a research fellow at Earth Observatory of Singapore, Nanyang Technological University, Singapore.