IAMG 2001--Cancún Technical Program--Session H

Home Page Technical Program Session H Intro Session H Papers

Geologic Modeling and Simulation of Exogenic Processes of Destruction on the Earth's Surface by Alexander S. Bykovtsev, University of California, Institute for Crustal Studies, USA, and Alexander A. Katz*, St. John's University, New York, USA In some regions of the Earth there often present a common phenomenon of formation of quasi-regular system of fractures which impart the landscape characteristic appearance. Such are takyrs in deserts, dry-up cracks in the permafrost regions, frost cracks in tundra "polygonal tundra". Suggested concepts, algorithms and results are a general theory of these phenomena based on fracture mechanics. The main idea of this theory consist of the investigation of tensile stresses in the Earth's upper layer which are determined as a function of dimensions of the fracture polygonal lattice. Polygonal structures--forms of micro- and meso-terrain in the seasonal frost and permafrost ground regions--arise as a result of formation of frost cracks and dry-up cracks propagating in the form of 5 to 6-sided polygonal or rectangular lattices. Frost cracks are abundant in the North of Eurasia, in the North America, in the Antarctic (in regions of geocryoson). They impart a characteristic appearance of "polygonal tundra" to the landscape. Frost-crack polygons form cell structure with cells in the shape of rectangles (tetragonal lattice) or polygons of from 5 to 6 sides. The most common size of cells (units) varies approximately in the range of from 10m to 100m, and the cells are separated from each other by cracks with opening of about 1m. Polygons of dry-up cracks represent analogous structures of much smaller scale (from ten centimeters to 2 - 3 meters across). Investigations of the modern exogenic processes of formation of the Earth's surface natural features is an urgent problem of geomorphology. A full-scale study of modern conditions of terrain configuration formations connected with the processes of jointing on the Earth's surface, collection of geomorphological data and creation of quantitative computation theories, most properly describing these data, must serve as a scientific base for development and creation of effective safety measures against destructive processes of jointing which occur on the Earth's surface as well as to be a contribution to their prediction. The main way of study of these processes consisted, as a rule, in field geomorphological observations and their comparative geographical juxtapositions. Those observations make possible: to reveal the main modern terrain-forming exogenic factors exerting effects on the formation of some or other terrain configuration, to classify the respective configurations of the Earth's surface and to define their geographical location, to state the main regularities in the by-stage development. It is rendered that the most characterizing terrain-forming exogenic processes connected with the Earth's surface jointing processes can be the processes of formation of contraction joints. Processes of contraction jointing formation are connected with appearance of quasiregular systems of polygonal ruptures on the Earth's surface. Contraction joints are formed as a result of reduction in rock volume, as well as due to the cooling of igneous rocks. Rocks reduce in volume, as a rule, due to the sharp temperature variations or due to the drying-out processes. Therefore, contraction joints are usually related to regions with sharp continental climate. Thus, in the desert regions the typical representatives of such joints are polygonal systems of takyrs, and in the regions of geocryoson--frost cracks, drying-out cracks and the so-called "polygonal tundra". Especially important have become the study of processes of appearance of contraction joints in connection with the practical economic activity of humans in deserts and in the regions of Subarctic. Expansion of the areas for cultivation in the North regions leads to construction activity on arrangement of populated settlements and industrial objects. Construction of such objects cannot be started without a preliminary estimation of the rock mechanical properties, since under conditions of share temperature differences the formation of considerable fields of tensile stresses is possible in the Earth's crust upper layers. The appearance of tensile stress fields will cause the formation of fracture faults in the upper layers of the Earth's surface, which, in its turn, will cause destruction of the objects constructed. Therefore, investigation of the fracture processes in the Earth's surface under conditions of omnidirectional tension and construction of mathematical models which allow us to study poligonal jointings are undoubtfully interesting for many of specialists dealing with the problems of cultivation of new territories having complex climatic conditions. The two structural models of Hexagonal lattice and Rectangular lattice, simulating the above said phenomena of exogenic destruction of the Earth's surface, will be considered in greater detail.