Isostasy

Isostasy Isostasy describes the naturally occurring balance of the mass of the Earth’s crust. Isostasy is essentially based on the concept of buoyancy. When attempting to study isostasy, the first scientific principle one needs to know is Archimedes’ Principle. This principle states that, as an object is partially submerged in a fluid, the weight of the fluid that is displaced is equal to the weight reduction of the object. When the weight of the object submerged exactly equals the displaced fluid, the object floats. This concept can be then applied to the Earth. The asthenosphere acts fluidly and the lithosphere is a rigid body that floats on top. The lithosphere is in isostatic equilibrium, therefore, the mass of the asthenosphere (fluid) displaced equals the mass of the floating lithosphere (solid). This means that the asthenosphere under two different isostatic blocks will be at the same pressure. This allows you to calculate the effects of isostasy due to an increase or decrease of weight on the surface due to erosion or deposition. There are two primary models for isostasy. Both the Pratt and Airy models are about local isostasy. There is no rigidity because any load is perfectly compensated. Both are end member models and there are others possible. The main difference between the two models is that in the Airy model all blocks have the same densities but different thicknesses, whereas in the Pratt model all blocks have different densities but float to the same depth. Generally speaking though, the Airy model is used for continental topography, especially mountain ranges; and the Pratt model is used for mid-ocean ridges. Continental mountain ranges have thick crustal roots which are more easily explained using the Airy model. In the Airy model, the elevation is proportional to this root. The higher the elevation is, the thicker the block and root. At mid ocean ridges the topography is supported by density changes. This is due to increased temperature at the ridges which causes the rocks to expand resulting in a lower density. According to the Pratt model, all blocks float at the same depth. This depth is where the asthenosphere begins. The difference in elevations is due to the density of the rocks. Higher elevations indicate lower density rocks and this higher ground means the lithosphere is thicker. The compensation depth is always the same in the Pratt model. The height equation is the same for both models. Below, figure #1 shows the Airy model and figure #2 shows the Pratt model. The asthenosphere under two different isostatic blocks will be at an equal pressure. This is due to the fact that the asthenosphere acts fluidly, and a liquid will flow from high to low pressure. This allows us to calculate the effects of the isostatic compensation between two blocks. For them to be in isostatic equilibrium, the pressure at the compensation depth (the bottom of the deepest lithosphere) must be equal. To calculate the effects of isostasy, it is necessary to create a height equation, as well as a weight equation. In order to do this you have to set up two blocks showing both the height and weight. For the height equation, the distance from the upper to lower level is the same. Likewise, for the weight equation the weight of the blocks is the same from the upper to lower level. If this were not true the pressure in the asthenosphere would be different. Neither model can be applied over a regional scale. Over large expanses the plate acts elastically. When a weight is added in the center of a plate, it causes a flexural bulge to form around it. A primary example of the effects of this weight on the surrounding area can be seen in islands such as Hawaii. In this situation, volcanoes continually add weight onto the ocean plate which in turn causes the plate to sink around it. Edging the island you see a trough develop due to regional compensation. Similarly, mountain chains are areas of thickened crust and they cause the land around them to sink. This often leads to the formation of sedimentary basins on the edge of the highlands where sediments eroded off of the mountain settle in the basin.