Isostasy

Topic: Geomorphic Processes: Weathering, Mass Movement, Erosion and Soil Formation · NCERT: Beyond NCERT

Meaning

Isostasy is the state of balance in which blocks of the earth's crust "float" on the denser mantle below, much as wood floats on water. Thick or light blocks stand high, and thin or heavy blocks sit low. It explains three things:

• why high mountains have deep "roots"
• why land sinks when a load is added and slowly rises when the load is removed
• why mountains last much longer than erosion alone would allow

Explanation

How it works: cause → process → result

• Cause: the crust is lighter than the mantle.
• The crust is made of less dense rock. The mantle below is denser.

• So the crust floats on the mantle, like a log on water.

• Process: each block finds its own floating level.

• A thick or light block floats higher, like a big log.
• A thin or heavy block floats lower.

• High mountains therefore push deep "roots" down into the mantle to hold them up.

• Result: land moves up or down when the load changes.

• A load is added (an ice sheet, a thick pile of sediment) → the crust sinks.
• A load is removed (ice melts, erosion carries rock away) → the crust slowly rises.
• The rise is slow because mantle rock flows back in very slowly.

Two classic models: Airy and Pratt (both 1855)

• Both models came from puzzles found by the Great Trigonometrical Survey of India near the Himalayas.

Feature	Airy (1855)	Pratt (1855)
Crustal density	Same everywhere	Changes from place to place
Crustal thickness	Changes from place to place	All blocks end at the same depth, the level of compensation (the depth where the blocks are in balance)
Why mountains are high	Thicker crust with deep roots	Lighter (less dense) rock columns
Simple picture	Icebergs of different sizes floating in water	Blocks of different metals floating in mercury

Post-glacial rebound: isostasy you can measure

• Post-glacial rebound, also called glacial isostatic adjustment (GIA), means land rising after a heavy ice sheet melts.
• Cause: during the last ice age, thick ice sheets covered Scandinavia and northern North America. Their weight pushed the crust down into the mantle.
• Process: the ice melted and the weight was gone. The crust began to rise back towards balance. Mantle rock flows back very slowly, so the rise is still going on thousands of years later.
• Result: some land still rises today. Land at the edges of the old ice sheet can sink. A "hinge line" (the boundary between rising and sinking land) separates the two [2].

Erosion, uplift and how isostasy is tested

• Erosion keeps mountains standing longer.
• Rivers and glaciers carry rock away from a mountain range, so the range gets lighter.
• Its roots then push it up slowly.

• So the mountain stands much longer than erosion alone would allow.

• Gravity anomalies show whether an area is in balance.

• A gravity anomaly is the difference between the gravity actually measured and the gravity expected.

• A strong anomaly means the area is still adjusting.

• GIA rates tell us about the mantle.

• Scientists use them to estimate how thick and sticky (viscous) the mantle is [2].
• Mantle viscosity matters for understanding the mantle flow that moves the plates [2].

Where it occurs

• Himalayas (India and its neighbours): high mountains held up by deep crustal roots. Surveys here led to the Airy and Pratt models of 1855. Note: the Himalayas are rising because India is pushing into Eurasia, not because of glacial rebound. They still rise by more than 1 cm a year [1].
• Hudson Bay (Canada): the land is rising by about 10 mm a year. This is where the ice was thickest at the last glacial maximum. The data come from about 360 GPS sites in Canada and the USA (study published 2007) [2].
• Great Lakes (Canada–USA border): south of the Great Lakes, the land is sinking by about 1–2 mm a year [2]. The hinge line runs across the lakes. Their northern shores rise and their southern shores sink [2].
• North America as a whole: the land also moves sideways, outward from Hudson Bay [2]. In the past 20,000 years, ice melting and GIA together caused up to several hundred metres of relative sea-level change in parts of North America [2].
• Scandinavia (around the Gulf of Bothnia): this was the area of the old Fennoscandian ice sheet, and it is still rising.

Don't confuse with

• Plate tectonics / orogeny: these are sideways crustal movement and mountain building by squeezing at plate edges. Isostasy is mostly vertical balancing. The Himalayas rise mainly because of plate collision, not isostatic rebound.
• Epeirogeny: broad up-and-down warping (bending) of large areas, which is a form of diastrophism. Isostasy is the balancing principle that explains why the crust settles at a certain height. It is not counted as one of the four forms of diastrophism.
• Airy vs Pratt: Airy = same density, different thickness (mountain roots). Pratt = different density, same base depth (level of compensation). Exams often swap these two.
• Eustatic sea-level change vs isostatic change: eustatic change is the sea itself rising or falling worldwide, for example when ice melts into the ocean. Isostatic change is the land moving up or down. Relative sea level at a coast depends on both.

Prelims Hooks

• Airy (1855) = same crustal density, different thickness, so mountains have deep roots. Pratt (1855) = different density, same level of compensation. Both models grew out of surveys in India.
• Post-glacial rebound examples: Scandinavia and Hudson Bay (Canada). Trap: the Himalayas are rising because of plate collision, not glacial rebound.
• Hudson Bay is rising by about 10 mm a year, and the land south of the Great Lakes is sinking by about 1–2 mm a year. Both are caused by glacial isostatic adjustment [2].
• Hinge line = boundary between rising and sinking land. The northern Great Lakes shores rise and the southern shores sink [2].
• Load added (ice, sediment) → crust sinks. Load removed (ice melt, erosion) → crust rises.
• Gravity anomalies are used to test isostatic balance. A strong anomaly means the area is still adjusting.

Mains Points

• Isostasy and sea-level risk (GS-I/III):
• Land moves up or down while the sea rises.
• Rising land (Hudson Bay, Scandinavia) can cancel part of the sea-level rise. Sinking land (south of the Great Lakes hinge line) makes it worse [2].

• So coastal-risk planning must measure land movement as well as sea level.

• Long life of mountains (GS-I):

• Erosion makes a range lighter → its roots push it up → relief lasts much longer.

• This helps explain why ranges like the Himalayas stay high even though they are worn down all the time. Plate collision adds further uplift [1].

• Understanding the earth's interior (GS-I):

• GIA rates and gravity anomalies help scientists estimate how viscous the mantle is [2].
• Mantle viscosity matters for understanding the mantle flow that drives plate movement, and with it earthquakes and mountain building [2].

Related concepts

• Sources of the earth's internal heat
• Diastrophism
• Orogeny
• Epeirogeny
• Volcanism

Read more

• Endogenic processes: internal energy, diastrophism, volcanism and isostasy