The Sub-Surface of the Planet: Observations vs Inferences - Coursework Example

HЕ SUB-SURFАСЕ ОF THЕ РLАNЕT: ОBSЕRVАTIОNS VЕRSЕS INFЕRЕNСЕS By: Course: Instructor: Institution: City: Date: Introduction The earth’s surface is made up of several components, some of whose existence is merely theoretical. Most of these theories are derived from observations made by scientists which help them to come up with dependable inferences that describe those observations. The two major categories of observations that are usually made in order to come up with specific theories can be classified as Geochemistry (measuring the chemical composition of the various materials of the earth and then using the chemical rules of the distribution of chemical elements to infer the chemical composition) and Geophysics (inferring the composition of the earth’s interior based on the physical rules governing the variation in the physical properties). Each of these shall be discussed in detail later on in this paper. This therefore leads us to the main point of discussion of this paper. The paper seeks to have a look at the various observations made by the scientists and their related inferences that have been used to build the presently- accepted model for the composition and internal structure of the earth. In order to do this, it is also prudent that we get introduced to the three major layers of the earth’s surface i.e. crust, mantle and the core. A better structure is shown in fig. 1 below: Figure 1 We shall discuss the observations and inferences of the composition of the earth with regard to each of these three layers. Crust Geochemical Observations and Inferences The crust is the outermost layer of the earth’s surface. It is basically made of rocks which are ideally basalt, quartzite and pyroxenes. It may also contain metamorphic rocks. The silica content of these rocks is usually very high, though their typical densities are very low. Rocks are basically mixtures formed of minerals, many of which are ideally elements. The most prevalent element in the earth’s crust is oxygen followed by silicon. The other elements present in the earth’s crust include aluminium, iron, sodium and potassium. Since the crust is practically observable due to its closeness to the earth’s surface, many of the inferences with regard to the composition of its materials are practicable. They are not mere theories arising from assumptions, but proven facts. Geophysical Observations and Inferences According to Sunderland, J., Bott, M. H. & Smith, P. J., 1974, thorough and informed research by the scientists has shown that the temperature of the crust increases with depth ranging from about 160 degrees Celsius to 360 degrees Celsius. It is from this kind of observation that the inference the very interior of the earth must be more molten than the crustal layers. Mantle Geochemical Observations and Inferences Compared to the rocks in the crust, the rocks here have slightly smaller amount of silica as well as increased densities. Studies have shown that the rocks observed on the earth’s surface are not necessarily the ones found in the mantle. In order to infer the chemical composition of the earth from the components of the rocks close to the earth’s surface, a theory of chemical differentiation is used (Chang, K., 26 Aug 2005). According to this theory, the partial melting of the mantle materials leads to the formation of basalt. In fact, it goes ahead to infer that this is the most important volcanic activity on earth. Following the observation above, pyrolite rock was proposed; a rock which is said to be the major component of the earth’s mantle. This in itself is basically theory. It was then further proposed that the different densities as well as elasticity of the materials in the mantle ought to be interpreted from compression, thermal expansion and phase transitions of those materials. Another geochemical model put forward was the chondrite model which is based on the consideration that the earth has a chemical composition similar to that of chondrite. Chondrite is basically made of carbon. In the pyrolite model, it is important to mention that there is less silicon in the mantle compared to that in the chondrite model. Assuming the chondritic model therefore, the upper mantle has less silicon than the lower one. Geophysical Observations and Inferences Usually the properties of the mantle just like the core and crust can only be derived from the study of earthquake waves that tend to propagate through the layered rocks within the structure of the earth. The time taken by these seismic waves to reach the surface of the earth is largely dependent on the velocity of propagation at each point along the path as well as the length of the path. Based on this seismographic information, the scientist is able to know the physical properties of the mantle such as temperature, pressure and density of the materials in the mantle (Sunderland, J., Bott, M. H. & Smith, P. J. et al., 1974). Scientific experiments have shown the presence of Iron dioxide in the mantle. This oxide was melted and its melting temperature found. The data found suggested that the temperature at the lower mantle ranges between 4000K and 5000K. The upper mantle is characterized by an increased seismic velocity as opposed to the lower mantle which is more of plastic. An experiment was recently done in which a small probe that generates heat was directed into the mantle while its position and continuity is monitored by the acoustic signals from the rocks. This particular experiment led to the theory of the distribution of the mineral deposits such as iron in the mantle. Core Geochemical Observations and Inferences It is an observable fact that our solar system is endowed with quite a number of chemical elements. Based on this fact, there’s the theory that the core of the earth is entirely made of iron and nickel as well as their alloy. It is also a proven fact that the core of the earth contains materials that have high density (Chang, K. et al., 26 Aug 2005). It can also be observed that some high density elements such as gold and platinum may also be part of the equation in the composition of the core. The theory behind this observation is the iron catastrophe in which it is believed that iron and nickel sank to the center of the earth which led to it allegedly containing more metallic elements that have high densities. Geophysical Observations and Inferences By observation and common knowledge, it is expected that the density of the materials at the core of the earth must be higher than that of the rocks at the surface. This inference comes from the fact that the overlying weight of the rocks above the core will definitely tend to compress interior rocks to some extent (Herndon, J. M., 2005). The density required however needs that there be some comparatively abundant dense elements such as iron and nickel. This is basically the inference from that observation. Additionally, the presence of lighter outer materials and dense inner materials actually implies that at some point in history, the earth must have been molten to some extent so as to ensure dense materials to settle into the interior. Conclusion In conclusion therefore, one has to realize that there are quite a number of theories that attempt to explain certain observations as far as the earth’s sub-surface layers are concerned. However, one has to note that each of these theories cannot exist in isolation of the others. They are only meant to back up one another in case of a deficiency in strength. It is critically important that a certain theory is clearly pegged on a believably existent fact. In order to fully understand the evolution as well as the general dynamics of the composition of the earth’s surfaces, the rocks are used. The rocks on the earth’s surface have very low densities and as a result, it therefore becomes obvious that they cannot be the main components of the interior of the earth’s surface. It therefore goes without saying that the materials in the core of the earth are fundamentally different from those on the surface. References Chang, K., 26 Aug 2005. Scientists Say Earth's Center Rotates Faster Than Surface. The New York Times, pp. 13. Herndon, J. M., 2005. Scientific basis of knowledge on Earth's composition. Current Science, 88 (7), pp. 1034–1037. Jordan, T. H., 1979. Structural Geology of the Earth's Interior. Proceedings of the National Academy of Sciences, 76 (9), pp. 4192–4200. Martin, H. P. B. 1982. The Interior of the Earth: its structure, constitution and evolution, London: New York. Sunderland, J., Bott, M. H. & Smith, P. J., 1974. Evidence for continental crust beneath the Faeroe Islands. Nature 248, Issue 5445, pp. 202–204. Read More