Isostacy

• Isostasy is a fundamental concept in the Geology.
• It is the idea that the lighter crust must be floating on the denser underlying mantle.
• It is invoked to explain how different topographic heights can exists on the Earth’s surface.
• Isostatic equilibrium is an ideal state where the crust and mantle would settle into in absence of disturbing forces.
• The waxing and waning of ice sheets, erosion, sedimentation, and extrusive volcanism are examples of processes that perturb isostasy.
• The physical properties of the lithosphere (the rocky shell that forms Earth’s exterior) are affected by the way the mantle and crust respond to these perturbations.
• Therefore, understanding the dynamics of isostasy helps us figure out more complex phenomena such as mountain building, sedimentary basin formation, the break-up of continents and the formation of new ocean basins.

There are two main ideas, developed in the mid-19th century, on the way isostasy acts to support mountain masses.

• In Pratt’s theory, there are lateral changes in rock density across the lithosphere. Assuming that the mantle below is uniformly dense, the less dense crustal blocks float higher to become mountains, whereas the more dense blocks form basins and lowlands.
• On the other hand, Airy’s theory assumes that across the lithosphere, the rock density is approximately the same, but the crustal blocks have different thicknesses. Therefore, mountains that shoot up higher also extend deeper roots into the denser material below.

Both theories rely on the presumed existence of a denser fluid or plastic layer on which the rocky lithosphere floats. This layer is now called the asthenosphere, and was verified in the mid-20th century to be present everywhere on Earth due to analysis of earthquakes – seismic waves, whose speed decrease with the softness of the medium, pass relatively slowly through the asthenosphere.

Both theories predict a relative deficiency of mass under high mountains, but Airy’s theory is now known to be a better explanation of mountains within continental regions, whereas Pratt’s theory essentially explains the difference between continents and oceans, since the continent crust is largely of granitic compostion which is less dense than the basaltic ocean basin.

Difference between Airy and Pratt’s views on Isostasy

The laws of buoyancy act on continents just as they would on icebergs and rafts.

An iceberg will rise further out of the water when the top is melted, and a raft will sink deeper when loads are added. However, the adjustment time for continents is much slower, due to the viscosity of the asthenosphere. This results in many dynamic geological processes that are observed today. The following paragraphs illustrate some of these examples.

• The formation of ice sheets could cause the Earth’s surface to sink. In areas which had ice sheets in the last ice age, the land is now “rebounding” upwards since the heavy ice has melted and the load on the lithosphere is reduced.
• Evidence from geological features include former sea-cliffs and associated wave-cut platforms that are found hundreds of meters above the sea level today.
• In the Baltic and in Canada, the amount and rate of uplift can be measured. In fact, due to the slowness of rebound, much of the land is still rising.
• Isostatic uplift also compensates for the erosion of mountains.
• When large amounts of material are carried away from a region, the land will rebound upwards to be eroded further.
• Due to drainage patterns, the erosion and removal of material is more prominent at plateau edges.
• Isostatic uplift may raise the edge higher than it used to be, so the ridge tops can be at an elevation considerably higher than the plateau itself.
• This mechanism is especially probable in mountain ranges bounding plateaus, such as the Himalayas and Kunlun Mountains bounding the Tibetan Plateau .
• Interestingly, given enough time and reaction kinetics, due to chemical transformations, the thick crustal root underneath mountains can become denser and founder into the mantle.
• The removal of the dense root can happen by the convection of the underlying asthenosphere or by delamination.
• After the root has detached, the asthenosphere rises and isostatic equilibrium leads to more mountain building at that region.
• For instance, this is thought to be the reason behind the late Cenozoic uplift of the Sierra Nevada in California.
• In fact, seismic data provide images of crust-mantle interactions during the supposed active foundering of the dense root beneath the southern Sierra Nevada.
• It appears that dense matter flowed asymmetrically into a mantle drip beneath the adjacent Great Valley.
• At the top of this drip, a V-shaped cone of crust is being dragged down tens of kilometers into the center of the mantle drip, leading to the disappearance of the Mohorovicic discontinuity (the boundary between crust and mantle) in seismic images.
• Likewise, at the northern Sierra Nevada, there is also a seismic “hole” known as the Redding anomaly, lending to the assumption that lithospheric foundering occurred there as well.

In conclusion, isostasy is yet another example of a deceptively simple idea in physics that provides crucial and sweeping explanatory power for other sciences.