Isostasy, the condition of equilibrium between floating landmasses and the denser rock beneath them, can best be explained by:

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The question asks to explain isostasy, defined as the equilibrium between floating crust and the denser material beneath it.

Option provided: "greater crustal thickness extending below sea level and a low density root system caused parts of the crust to stand much higher"

First, consider what isostasy predicts: crustal blocks are supported by buoyancy, so thicker sections of crust (or denser roots) extend downward to balance the weight of the topography. Two classic models are often cited:
- Airy isostasy: thicker crust under mountains yields deeper, denser roots that compensate the added mass of high topography.
- Pratt isostasy: lateral variations in density allow blocks of equal mass but different density to float at different elevations, without necessarily requiring deeper roots.

Now evaluate the statement in the option:
- It asserts a 
  - greater crustal thickness extending below sea level, which aligns with the idea that thicker crust has a deeper root to balance higher topography. This part could be seen as consistent with Airy-type thinking.
  - and a 
  - "low density root system" that causes parts of the crust to stand higher. Here the key issue is the density sign: an isostatic root that supports high topography is typically denser than the surrounding mantle, not low density. A low-density root would be lighter than the surrounding material, which would not provide the downward pull needed to balance extra elevation. This makes the statement inconsistent with standard isostatic reasoning.

Additionally, if the crust is thicker below a region, the compensating mass is typically a denser root (in Airy-type models), not an abnormally low-density root. The idea of a low-density root leading to uplifting contradicts the mass-balance premise of isostasy, where additional weight (thicker crust) needs an accompanying downward pull (dense root) to achieve equilibrium.

Given isostasy involves balancing the weight of the crust with the buoyant rise in the mantle, the combination of a thicker crust and a low-density root does not coherently explain how equilibrium is achieved. The more coherent explanation would be thicker crust with a relatively dense root extending downward to compensate the increased mass.

In summary, while the notion of greater crustal thickness relates to isostatic compensation, the accompanying claim of a low-density root producing uplift is inconsistent with the core mechanism of isostasy and would misrepresent how the root material should contrast in density to balance the crustal mass. If there were multiple options, one would expect a statement along the lines of thicker crust with a deeper, relatively dense root (Airy) or varying density with equal total mass (Pratt), rather than a low-density root causing uplift.

Geography CC 1 Sec ON1 (31167) Quiz #9: Units 30-32

Isostasy, the condition of equilibrium between floating landmasses and the denser rock beneath them, can best be explained by:

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