GPS measurements show that Scandinavia is rising several millimeters per year, thousands of years after the last ice sheets melted. What does this indicate about Earth's mantle?
ANew magma is being injected under Scandinavia from a deep mantle plume, actively pushing the crust upward
BThe lithosphere under Scandinavia is being stretched by tectonic extension, thinning the crust and raising it
CThe mantle behaves as a viscous fluid over geological timescales — isostatic rebound is still in progress because mantle material flows slowly back to fill the space vacated by the ice's weight
DThe ice sheets are still partially present at depth, gradually releasing pressure as they melt
This is postglacial rebound — one of the most direct lines of evidence for isostasy. When ice sheets loaded the crust, the mantle flowed outward. When the ice melted, the pressure was released, but the viscous mantle flows back slowly — over thousands to tens of thousands of years. Scandinavia is still rising because the mantle has not yet reached isostatic equilibrium after the last glacial maximum. The rate of rebound can be used to constrain mantle viscosity.
Question 2 Multiple Choice
The Himalayas have a crustal root extending to approximately 70 km depth, compared to the global average of about 35 km. Under the Airy isostasy model, why does this root exist?
AThe Himalayas formed recently and have not yet had time to erode down to the average crustal thickness
BThe crust beneath the Himalayas is denser than average, causing it to sink deeper into the mantle like a heavy block of wood
CThe extra height of the mountains above sea level is compensated by extra crustal thickness below — the low-density root displaces dense mantle rock to maintain equal pressure at the compensation depth
DOceanic crust from the Indian Ocean was subducted under the continent and added to the base of the crust
Airy isostasy models the crust as floating blocks of uniform density but variable thickness. For the total weight per unit area to be equal at the compensation depth (pressure balance), a thick, tall column (the Himalayas) needs a correspondingly deep root of low-density crust displacing denser mantle rock. The buoyancy from this root holds the mountains up. The analogy is an iceberg: the part above water (the mountains) is supported by the part below (the root).
Question 3 True / False
The Airy and Pratt models of isostasy are mutually exclusive — a given topographic feature is expected to be explained by one or the other, but not both simultaneously.
TTrue
FFalse
Answer: False
Both mechanisms operate in nature, often simultaneously. The Andes have thick crustal roots consistent with Airy isostasy. Mid-ocean ridges are elevated partly because young, hot oceanic lithosphere is less dense than old, cold lithosphere — a Pratt-type effect. Real geologic features are complex, and gravity anomaly analysis is used to determine what combination of thickness variation and density variation best explains observed topography and gravity in a given region.
Question 4 True / False
When a large volcanic island forms on oceanic crust, elastic lithospheric flexure causes not only subsidence directly beneath the island, but also a peripheral bulge (forebulge) and a moat-like depression in the surrounding seafloor.
TTrue
FFalse
Answer: True
The lithosphere is not infinitely rigid, but it has finite elastic strength that distributes loads laterally. When a volcanic island loads the crust, the plate bends downward beneath the load and flexes upward in a ring around it — the forebulge. This ring of uplifted seafloor and the surrounding depression (moat) are diagnostic signatures of flexural isostasy. The Hawaiian Islands are surrounded by exactly this pattern. The flexural wavelength (how wide the deformation extends) depends on the elastic thickness of the lithosphere.
Question 5 Short Answer
Explain why mountains have deep crustal roots under the Airy isostasy model. What physical quantity is being balanced, and how does the root achieve that balance?
Think about your answer, then reveal below.
Model answer: Pressure is being balanced at a compensation depth in the mantle. Every vertical column of rock — from the surface down to the compensation depth — must exert the same pressure at that depth. A mountain adds extra mass above sea level, which would create excess pressure unless it is offset. The Airy model compensates by replacing dense mantle rock beneath the mountain with a thick root of lower-density crust — the root displaces mantle, reducing the total mass in the column and restoring pressure balance. The mountain floats like an iceberg: the extra height above the surface is supported by extra depth below.
This is fundamentally a buoyancy argument: low-density crust floating on denser mantle, with more crust needed to support a taller edifice. The compensation is not instantaneous — it occurs over millions of years as the ductile mantle flows. This is why geologically young mountain belts are often not in full isostatic equilibrium, and why free-air and Bouguer gravity anomalies are used to measure how far a region departs from the isostatic ideal.