Isostasy
Introduction
Isostasy is a fundamental concept in the field of geology and geophysics that explains the state of gravitational equilibrium between the earth's crust and the denser, underlying mantle. This principle is crucial to understanding various geological phenomena, including mountain formation, sea level changes, and the behavior of the earth's crust during glaciation and deglaciation.
Concept of Isostasy
The concept of isostasy is based on the understanding that the earth's crust, or lithosphere, "floats" on the denser, semi-fluid asthenosphere beneath it. This balance is similar to the way an iceberg floats on water, with the amount of iceberg above the water surface balanced by a much larger volume below the surface. In the context of the earth's crust, this balance is maintained by the vertical movement of crustal blocks, which rise when material is eroded away and sink when additional material is deposited.
Theories of Isostasy
There are two primary theories of isostasy: the Airy-Heiskanen model and the Pratt-Hayford model. Both models were developed in the 19th century and continue to be used today to explain the isostatic adjustments of the earth's crust.
Airy-Heiskanen Model
The Airy-Heiskanen model, named after British scientist George Airy and Finnish geodesist Veikko Heiskanen, proposes that the earth's crust varies in thickness depending on the topographic height of the land. According to this model, areas with high mountains have a thicker crust to support the additional weight, much like the deep keel of a heavy ship.
Pratt-Hayford Model
The Pratt-Hayford model, named after American geologist John Henry Pratt and British geologist John Hayford, suggests that the earth's crust has a uniform thickness, but the density of the crustal material varies. In this model, areas of high topography are made up of less dense material, allowing them to "float" higher on the asthenosphere.
Isostatic Adjustments
Isostatic adjustments refer to the vertical movements of the earth's crust as it seeks to maintain equilibrium with the underlying mantle. These adjustments can occur over long geological timescales and are driven by processes such as erosion, sediment deposition, ice cap formation, and melting of glaciers.
Erosion and Sediment Deposition
Erosion and sediment deposition can cause significant isostatic adjustments. When a large amount of material is eroded from a mountain range, the crust beneath the mountains will rise in response to the decreased load. Conversely, when large amounts of sediment are deposited in a basin, the crust beneath the basin will sink to accommodate the added weight.
Glacial Isostatic Adjustment
Glacial isostatic adjustment refers to the vertical movement of the earth's crust in response to the formation and melting of large ice caps. During an ice age, the weight of the ice causes the crust to sink. When the ice melts, the crust rebounds, a process that can continue for thousands of years after the ice has disappeared.
Isostasy and Plate Tectonics
Isostasy plays a significant role in the theory of plate tectonics. The movement of tectonic plates can cause changes in the distribution of mass on the earth's surface, leading to isostatic adjustments. For example, the collision of two tectonic plates can create a mountain range. The weight of the new mountains causes the crust to sink into the asthenosphere, while the removal of material from the collision zone causes the crust to rise.
Conclusion
Isostasy is a key principle in geology and geophysics, providing a framework for understanding the vertical movements of the earth's crust in response to changes in surface load. While the concept is simple, its implications are profound, influencing everything from the formation of mountain ranges to the rise and fall of sea levels.