isostasy
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What is isostasy in geology?
Isostasy is the theoretical balance of large portions of Earth's lithosphere. It is as though they are floating on the denser asthenosphere, which is the upper mantle composed of weak, plastic rock. Isostasy controls the regional elevations of continents and ocean floors in accordance with the densities of their underlying rocks.
The concept of isostasy considers that columns of equal cross-sectional area rising from the asthenosphere to the surface have equal weights everywhere on Earth, even with different constituents and surface elevations. This balance means that an excess of mass above sea level, such as a mountain, is due to a deficit of mass, or low-density roots, below sea level. High mountains, therefore, have low-density roots that extend deep into the underlying mantle.
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How does isostasy help explain the shape of the Earth's crust?
Isostasy explains the Earth's crust shape by describing the balance of the lithosphere floating on the denser asthenosphere. This balance controls regional elevations of continents and ocean floors based on the densities of underlying rocks.
The concept suggests that areas with excess mass above sea level, like mountains, have a deficit of mass (low-density roots) below sea level. This means high mountains have low-density roots extending deep into the mantle.
Isostatic movements also occur due to the formation and melting of large ice sheets, where the crust subsides under the weight of the ice and rebounds upon melting.
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What does it mean when we say the Earth's crust is “floating” on the mantle?
When we say the Earth's crust is "floating" on the mantle, it refers to isostasy, the equilibrium between the Earth's crust and the denser mantle beneath it. The lithosphere, composed of the crust and the uppermost part of the mantle, is rigid and rests on the weaker, partially molten asthenosphere.
Because continental crust is less dense than both the oceanic crust and the mantle, it has greater buoyancy, causing it to float much higher in the mantle. This is similar to how icebergs float on water, where a larger part of the iceberg is underwater. The concept of isostasy is important to the theory of plate tectonics.
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How do mountains and valleys affect isostasy?
Isostasy describes the equilibrium between Earth's crust and mantle, where the lithosphere floats on the denser asthenosphere. Mountains and valleys influence this balance.
Mountains, being large masses above sea level, have low-density roots extending deep into the mantle to compensate for their weight. This is similar to how a large iceberg has a greater part of its mass underwater.
Valleys, on the other hand, are often formed by river erosion or tectonic activity. The removal of material in valley formation reduces the load on the crust, leading to a relative uplift due to isostatic adjustment. This interplay maintains a balance, reflecting the densities of underlying rocks and regional elevations.
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What happens to the Earth's crust when glaciers melt according to isostasy?
When glaciers melt, the Earth's crust responds through a process called isostatic rebound. During glaciation, the weight of massive ice sheets causes the crust to subside, displacing material in the Earth's mantle. The amount of subsidence is roughly one-third the thickness of the ice sheet.
When the ice melts, this load is removed, and the crust begins to rise or rebound. This uplift is initially rapid but slows over time. For example, the eastern Hudson Bay area has risen more than 300 meters since deglaciation, and uplift continues today at a rate of about 1.3 cm per year. This process demonstrates isostasy, where the lithosphere seeks balance by floating on the denser asthenosphere.
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Why is understanding isostasy important for geologists studying earthquakes, volcanoes, and land changes?
Understanding isostasy is crucial for geologists studying earthquakes, volcanoes, and land changes because it explains how Earth's lithosphere maintains balance, influencing regional elevations of continents and ocean floors.
Isostasy helps in understanding:
- How high mountains have low-density roots that extend deep into the mantle.
- Vertical movements of Earth’s crust caused by the formation and melting of large ice sheets. The area beneath an ice sheet subsides during glaciation because the crust is not able to sustain the weight of the glacier.
- The location of most volcanoes in the so-called “Ring of Fire” made up of island arcs and mountain ranges bordering the Pacific Ocean.
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isostasy, ideal theoretical balance of all large portions of Earth’s lithosphere as though they were floating on the denser underlying layer, the asthenosphere, a section of the upper mantle composed of weak, plastic rock that is about 110 km (70 miles) below the surface. Isostasy controls the regional elevations of continents and ocean floors in accordance with the densities of their underlying rocks. Imaginary columns of equal cross-sectional area that rise from the asthenosphere to the surface are assumed to have equal weights everywhere on Earth, even though their constituents and the elevations of their upper surfaces are significantly different. This means that an excess of mass seen as material above sea level, as in a mountain system, is due to a deficit of mass, or low-density roots, below sea level. Therefore, high mountains have low-density roots that extend deep into the underlying mantle. The concept of isostasy played an important role in the development of the theory of plate tectonics.
In 1735, expeditions over the Andes led by Pierre Bouguer, a French photometrist and the first to measure the horizontal gravitational pull of mountains, noted that the Andes could not represent a protuberance of rock sitting on a solid platform. If it did, then a plumb-line should be deflected from the true vertical by an amount proportional to the gravitational attraction of the mountain range. The deflection was less than that which was anticipated. About a century later, similar discrepancies were observed by Sir George Everest, surveyor general of India, in surveys south of the Himalayas, indicating a lack of compensating mass beneath the visible mountain ranges.
In the theory of isostasy, a mass above sea level is supported below sea level, and there is thus a certain depth at which the total weight per unit area is equal all around the Earth; this is known as the depth of compensation. The depth of compensation was taken to be 113 km (70 miles) according to the Hayford-Bowie concept, named for American geodesists John Fillmore Hayford and William Bowie. Owing to changing tectonic environments, however, perfect isostasy is approached but rarely attained, and some regions, such as oceanic trenches and high plateaus, are not isostatically compensated.
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The Airy hypothesis says that Earth’s crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddell Airy, an English mathematician and astronomer, assumed that the crust has a uniform density throughout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are buoyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is underwater.
The Pratt hypothesis, developed by John Henry Pratt, English mathematician and Anglican missionary, supposes that Earth’s crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density, such as mountain ranges, project higher above sea level than do those of greater density. The explanation for this was that the mountains resulted from the upward expansion of locally heated crustal material, which had a larger volume but a lower density after it had cooled.
The Heiskanen hypothesis, developed by Finnish geodesist Weikko Aleksanteri Heiskanen, is an intermediate, or compromise, hypothesis between Airy’s and Pratt’s. This hypothesis says that approximately two-thirds of the topography is compensated by the root formation (the Airy model) and one-third by Earth’s crust above the boundary between the crust and the substratum (the Pratt model).