Gravity Anomaly and Isostatic Compensation

The question describes a scenario where the measured gravity value over an elevated land mass is 310 milligals (mGal) smaller than the normal gravity value. This represents a negative gravity anomaly ($\Delta g$).

• Thickness of land mass, $h = 1.0 \text{ km}$.
• Observed gravity anomaly, $\Delta g = -310 \text{ mGal}$.

A negative gravity anomaly over a region of positive topography (elevated land mass) indicates that there is less mass beneath the surface than expected, or equivalently, that the elevated feature is supported by a volume of less dense material extending downwards.

Isostatic Compensation Concepts

Isostasy describes the state of gravitational equilibrium between the Earth's crust and mantle, such that the crust "floats" at an elevation dependent on its thickness and density. Different states exist:

• Under-compensated: More mass is concentrated than predicted by isostatic models, leading to a positive gravity anomaly.
• Over-compensated: Less mass is present than predicted by isostatic models, leading to a larger negative gravity anomaly than expected.
• Compensated: The mass excess of the elevated feature is balanced by a mass deficit (less dense root) at depth, consistent with isostatic equilibrium. This often results in a negative gravity anomaly over elevated regions.

Analysis of Gravity Anomaly

The observed gravity anomaly is negative ($\Delta g = -310 \text{ mGal}$). This negative value signifies a deficit of mass beneath the elevated land mass.

Isostatic models, particularly the Airy model, predict that elevated regions like mountain ranges are supported by deep crustal roots composed of less dense material. This root system compensates for the excess mass of the topography, leading to a state of gravitational equilibrium.

The presence of a negative gravity anomaly over the elevated land mass is consistent with the existence of such a compensatory root. The magnitude of the anomaly reflects the extent of this compensation.

Therefore, the observation strongly suggests that the land mass is in isostatic equilibrium, meaning it is isostatically compensated.

The statement "The land mass is isostatically compensated" accurately describes this situation.