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Forschungsarbeit

Understanding the Earth’s interior: clues from seismological observations simulated by high-pressure high-temperature experiments

Von Ashima Saika

The Earth is a dynamic planet with a changing surface that is in constant flux as a result of processes driven by two heat sources. The Sun drives the cycle of water through oceans and rivers that erode and shape the surface. The loss of heat from the Earth’s interior, however, also shifts and shapes the surface causing volcanic eruptions and the formation of uplifted and mountainous regions. Aside from creating these phenomena, the heat within the Earth’s interior causes motion, which ensures that a significant portion of the Earth’s surface is constantly being exchanged with the interior. As the interior tries to lose heat hot rocks and lavas are pushed up onto the surface, while colder rocks slip back into the interior cooling it in a process called subduction. This motion is a form of convection, which also results in an exchange of chemical compounds between the surface and the interior. This cycling of chemical compounds is of great scientific interest because the interior is such a huge reservoir with an enormous potential to influence the concentration of compounds at the surface over geological time. Cycling of compounds include, for example, greenhouse gases and aerosols that can and may have influenced the Earth’s climate throughout history. The process of subduction that returns portions of the surface to the interior is also the main cause of earthquakes. To understand these natural processes, and to understand why they don’t occur in the same way on other planets, it is essential to have a detailed understanding of the Earth’s behavior as a chemical and physical unit. This must include an understanding of the composition and dynamics of the Earth’s deep interior.

Figure 1: A schematic cross section of the Earth showing it's internal constitution and plate tectonic settings (Courtesy BGI year book modified from GEO 4/94).[Bildunterschrift / Subline]: Figure 1: A schematic cross section of the Earth showing it's internal constitution and plate tectonic settings (Courtesy BGI year book modified from GEO 4/94).

The main source of information on the Earth’s interior comes from seismologists who can measure the speed of sound through the Earth after an earthquake has occurred. A comparison of these speeds with those measured for known materials indicates that the Earth’s interior is rocky down to 2900 km. This rocky region is called the mantle. Below this is the Fe-rich metallic core (Fig. 1). However, sound speed observations provide much more information. For example some sound waves are reflected off layers deep in the mantle. To understand what causes this layering, high pressure and temperature experiments must be performed to simulate the dense mineral structures that occur in the interior.
 
To simulate conditions in the deep interior large hydraulic presses that can produce forces up to 5000 tonnes are used to compress mineral samples, typically one millimetre in diameter such, that pressures equivalent to 250000 times atmospheric pressure are achieved. The device that focuses this force on such a small area is called a multianvil (Fig. 2), as it consist of a series of anvils made of hard materials that compress an inner ceramic pressure cell. Inside the cell is a furnace heated electrically that can reach temperatures over 2000°C and at the centre of the furnace is the material to be investigated. During the experiment the material changes structure to become denser. The most well known example of this type of transformation is that of graphite transforming to diamond, which occurs in the Earth at depths greater than 150 km. In the experiments performed in this study similar transformations that take place in the silicate minerals that compose the Earth’s interior between 500-600 km were studied.

Figure 2: Multianvil press (1200 ton) located at Bayerisches Geoinstitut which can achieve pressure up to 250000 times room pressure.[Bildunterschrift / Subline]: Figure 2: Multianvil press (1200 ton) located at Bayerisches Geoinstitut which can achieve pressure up to 250000 times room pressure.

The experiments performed in this study have shown that the transformation of minerals to denser structures at a depth of approximately 560 km in the Earth can cause sound waves to be reflected at this depth, which is in agreement with actual observations made by seismologists. The experiments indicate further, however, that these reflections will occur strongly in regions of the Earth where material from the surface has been returned to the deep interior. This not only allows Earth scientists to track the presence of these chemically different surface materials as they are cycled  through the interior but to consider how long these regions remain distinct in the interior before being mixed or returned to the surface.

Figure 3: Graduation Ceremony of the Elite Network of Bavaria on 17 November 2008[Bildunterschrift / Subline]: Figure 3: Graduation Ceremony of the Elite Network of Bavaria on 17 November 2008

Dr. Ashima Saika
* 1979
Stationen
2002, M.SC. in Dehli, Doktorandenkolleg: Struktur, Reaktivität und Eigenschaften oxidischer Materialen, Dr. rer. nat., Februar 2008, Universität Bayreuth seit 03/2008 ETH Zürich
Veröffentlichung
“Splitting of the 520-Kilometer Seismic Discontinuity and Chemical Heterogeneity in the Mantle”, Science 14 March 2008: Vol. 319. no. 5869, pp. 1515 - 1518, DOI: 10.1126/science.1152818
Kontakt
ETH Zurich, Institute for Mineralogy and Petrology, Clausiusstr. 25, NW, 8092 Zurich