Geology (Research Paper Sample)

Nasser Almosa
Professor’s Name
The Pennsylvania State University
Geosciences
Date: 17th December 2012
Abstract
The study indicates that crevasses can occur at depths of meters beneath the surface of the ice sheets where most was formed in areas without surface crevassing. This supports the hypothesis that these are instances of subsurface crevasse formation in the Greenland. The study exploits the concept of linear elastic fracture mechanics – LEFM, and this research investigates the possibility of crevasse initiation at depth in the Greenland (Nath and Vaughan, 4). The research considers the initiation of isolated crevasses from a subsurface crack, based on the dynamic tensile stress that arise from the deformation associated with ice movement and the weight-induced Litho static stress.
1. Introduction
The contraction cracks in the basalt, perma-frost, mud, and crevasses in glaciers are examples of geological phenomena studied by reference to theoretical models of tension cracks within a semi-infinite solid. In the Greenland crevasse, the effect of the crack to relieve stress at the surface beam that cause crack spacing, and energy dissipation at the advancing crack tip beam on the crack depth (Nath and Vaughan, 7).
Although the stress that cause cracking, it increases the elastic concept and model of the stress adjacent to a crack can play an integral role provided the cracks initiated can propagate rapidly. The Study results are presented for the elastic stress perturbation caused by cracks both in the infinite and semi-infinite medium (Hambrey and Feister, 57). In this case, the initial stress acts as a step function and linear function of depth. Tables, graphs, and charts are presented in this study and can be applied directly to geological studies where the variation of stress with depth is arbitrary (Lachenbruch, 11). The research study has developed key objectives.
To find out the formation of crevasses in the Greenland
To understand the cause of crevasse formation
To find out the rationale behind crevasse formation
The crevasses always form because of brittle fracture in ice or firn that is under tension that are frequently observed on the surface of glaciers or ice sheets within a geographical setting. The use of data from satellite images and field observations, numerous studies of crevassing demonstrates the orientation of newly formed crevasses and the correlation with the surface strain rate. Crevasses in either Greenland or other sites are an indication of the stresses, which caused the crevasse (Hambrey and Feister, 57). This is an integral tool in understanding dynamics of glaciers and ice sheets. Now, there are no elaborate-established criteria for crevasses formation, and the means to control the crevasse formation are not clearly understood. This prompts this research study of crevasses in the Greenland (Nath and Vaughan, 3)
Studies have been undertaken in different glacial areas to find out the aspect of crevasse formation. However, it has been difficult to gain and insightful and universal base of knowledge on the formation of the crevasse in the glacial geological settings. It is essential to find out to find out the rationale behind the formation of the crevasse in the glacial and ice sheet covered areas (Lachenbruch, 11). It solely the use of high-resolution images and charts used to understand of the crevasse formation.
There has been limited information on the formation of the crevasse and this point to scanty information and research on the formation of the crevasse in the Antarctica. In the Greenland, for instance the only hypothesis responsible for the formation of crevasse is the stress exerted. The use of information from satellite images and observations in the field creates a challenge on extensive research rationale. Studies of crevasse formation demonstrate the orientation of crevasse formation and the correlation with the surface strain rate. This creates a dilemma in adopting a common concept and model in any research study (Lachenbruch, 18).
2. Tensile Stress in crevasse formation
The crevasses always form as a response to tensile stresses in glaciers and ice sheets in the geographical areas. It is assumed that crevasses initiates from the starter, cracks up to some centimeters long. When the tensile stress is adequate, the cracks can propagate downwards to the ice and form a crevasse (Nath and Vaughan, 11). This is until the weight-exerts Litho static stress that prevents them penetrating deeper. This study presents the ground-penetrating information and research like one of the Rut ford ice stream (Weertman, 8).
2.1 Starter Cracks
The initiation, as well as the propagation of crevasses, depends mainly on the presence of initial defects and the starter cracks in the firn. Little is well known about the likely origin or dimension of the starter cracks. This trend shows that starter cracks of some few centimeters in length are essential before crack propagation occurs. A starter crack of size five to fifty centimeters is necessary to describe the distribution of crevasses in the Filchner-Ronne Ice Shelf according to linear elastic fracture mechanics – LEFM approach (Lachenbruch, 19).
However, it is not clear how large like starter cracks originate in the firn. Analytical studies and tests have indicated that brittle fractures in both full density ice and firn can arise from micro cracks that develop during plastic flow. In contrast, these micro cracks are just few times the grain’s diameter long. The refreezing of surface melt waters within the interconnected pores can result to sharp cracks in the firn that is few centimeters long.
Nevertheless, there is no evidence of the surface melt water like in the Rut ford ice stream, hence an alternative understanding and knowledge are required. There could be some centimeter-scale defects in firn, which stresses can be concentrated giving rise to the starter cracks formation. One possible source of such defects is the burial of surfaces as a rogue, and this may enclose small air pockets or enclaves. A possible cause of defects can be buried sun crusts that arise from of the glazing of these surface layers of snow. This happens after a longer exposure period to the bright sunlight (Nath and Vaughan, 8). An eye in ice cores can see the sun crusts clearly. Defects can also result from the density variation between firn layers. There is the possibility folding of the internal layers in the burial process might create some centimeter-scale in homogeneities where stresses can concentrate.
Research data from the Rut ford ice stream exhibits that crevasses exist at depths of between three and twenty meters under the ice surface, and this is not unique in the Greenland. Research studies show that these crevasses may be forming at different depths. Investigations on the feasibility of crevasse initiation at varying depth layers use ordinary models based on the linear elastic fracture mechanisms. There are more realistic approaches than the assumption of tensile stress varying with depths and tensile strain rates are constant. Crevasse initiation can occur at depths of up to thirty meters right from the starter cracks of some few centimeters in length.
The studies on crevasse initiation do not have the exhaustive description of the sub-surface crevasse formation. If it is possible to demonstrate that crevasses can form at varying depths without ice surface breaking, then current estimates and models of ice stream stagnation s are credible (Nath and Vaughan, 10).
2.2 Effect of creep deformation
There is the assumption that deformation and stress fields around a crevasse are determined only by elastic stress and the strain-field solutions. This could be true for a short time just after a crevasse suddenly forms. In turn, creep deformation would determine the stresses that exist around a crevasse. It is not universal that the general results obtained by elastic solutions are modified greatly when creep deformation is taken into consideration. The solution for a crevasse in a glacier that follows the Newtonian creep equation is similar in the form to the one found using the elastic equations. In essence, the creep of ice follows the power-law creep equation instead of the linear, Newtonian creep equation (Weertman, 5).
The model of linear elastic fracture mechanics – LEFM approach enables the estimation of the minimum length that a crack must be prior to crack propagation occurs. In other models and theories of crevasse formation, it is assumed that the dynamism of tensile stress is always constant with its depth. A more realistic scenario is considered where the dynamic tensile stress differs with its depth, in a way that the tensile strain rate is constant. In this scenario or case, the crevasse initiation from centimeter-scale point cracks is feasible at depths of between ten to thirty meters and at the surface, as well. In the current studies, the formulation of a reliable predictive mode is challenged by the incomplete knowledge of mechanical properties of a firn. In the past studies, the depths of the buried crevasses have been used in estimating the period since ice was exposed to high stresses or different flow regimes (Lachenbruch, 9).
3. Methods
            In this chapter, the research gives detailed approach of the methods uses to undertake the research study. This helps in research by defining the research design the sample areas covered, the methods and tools used to gather information and data. This shows methods used in gathering the necessary information data as well as the different tools and techniques used to analyze and interpret data. Internet, journals, magazines and have been used to source pertinent.
3.1 Research Design
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