Questions: Determining Crustal Thickness from Gravity Data

Question 1

A regional gravity survey over a major mountain range shows strongly negative Bouguer anomalies beneath the highest peaks. A student attributes this to the large mass of rock in the mountains themselves. What does the negative anomaly actually indicate?

• A. The mountain rocks are unusually low density compared to surrounding crust
• B. A thick crustal root beneath the mountains — low-density crustal material at depth is replacing what would otherwise be denser mantle rock, producing a mass deficit
• C. A sedimentary basin hidden beneath the mountains, filled with low-density material
• D. The absence of volcanic intrusions that would otherwise add dense material to the crust

Answer: (B)

The student's intuition is backwards. Mountains have a positive topographic mass (extra rock above sea level), which would naively suggest a positive gravity anomaly. But the Bouguer correction removes the effect of topography, and what remains — the Bouguer anomaly — reflects subsurface density variations. The strongly negative residual after Bouguer correction indicates a mass deficit at depth: a thick crustal root where low-density crust (≈2800 kg/m³) extends downward into the mantle, displacing denser mantle material (≈3300 kg/m³). This is exactly the Airy isostasy model — mountains are 'floating' on roots, and those roots are what create the negative Bouguer anomaly.

Question 2

Why does gravity-based crustal thickness estimation produce lower-resolution Moho maps than seismic refraction surveys, even when both cover the same area?

• A. Gravity instruments are inherently less precise than seismometers and introduce more measurement noise
• B. The gravity field smooths out with distance from its source, so sharp lateral changes in Moho depth are blurred and cannot be resolved accurately from surface measurements alone
• C. The Moho density contrast is too small for gravity instruments to detect reliably
• D. Gravity surveys can only be conducted on flat terrain where vehicles can travel

Answer: (B)

Gravity anomalies decrease in amplitude and increase in wavelength as the depth of the source increases — a phenomenon that smears out lateral variations in Moho depth when viewed from the surface. A sharp lateral step in Moho depth (say, from 35 km to 50 km over 20 km horizontally) produces a broad, smooth gravity gradient rather than a sharp edge. Seismic refraction can detect this step clearly because seismic waves travel directly through the structure and arrive with timing that precisely constrains layer depths. Gravity data cannot 'see' sharp boundaries at depth as clearly — the physics of potential fields limits resolution of deep structure.

Question 3

A positive Bouguer anomaly in a continental interior generally indicates that the crust is thinner than average because denser mantle material sits closer to the surface.

• T. True
• F. False

Answer: True

The relationship is symmetric: negative Bouguer anomalies correlate with thick crust (low-density crustal root displacing mantle), and positive anomalies correlate with thin crust (mantle rock at shallower depth, higher density than average). Continental cratons with ancient, thin crust, rift zones where the lithosphere has thinned, and ocean-continent transition zones often show positive or near-zero Bouguer anomalies precisely because the Moho is relatively shallow. Gravity inversion exploits this systematic relationship: measuring the Bouguer anomaly and solving for the Moho depth that would produce it.

Question 4

Gravity data alone can determine absolute crustal thickness without any additional seismic or geological constraints, because the density contrast at the Moho is a fixed physical constant.

• T. True
• F. False

Answer: False

The density contrast at the Moho varies depending on crustal and mantle composition — it is not a universal constant. Typical values range from about 300 to 600 kg/m³, but the exact value affects the calculated Moho depth significantly. A gravity inversion that assumes the wrong density contrast will produce systematically biased crustal thickness estimates. This is why gravity-derived Moho maps require calibration using seismic control points where Moho depth has been independently measured. The best results come from joint interpretation: gravity data provides broad spatial coverage, seismic data anchors the density contrast and absolute depth. Neither alone is as powerful as the combination.

Question 5

Explain the physical relationship between Bouguer gravity anomalies and crustal thickness, specifically what creates the negative anomaly beneath mountain ranges.

Model answer: The key is the density contrast at the Moho — the boundary between crust (approximately 2800 kg/m³) and mantle (approximately 3300 kg/m³). Where crust is thick, a large volume of low-density crustal rock occupies space that would otherwise be filled by denser mantle rock. This creates a mass deficit relative to a reference model where crust has normal thickness. The Bouguer anomaly measures deviations from expected gravity after removing the effect of surface topography, so this deep mass deficit registers as a negative Bouguer anomaly. Beneath mountain ranges, the crust is thickest (isostatic compensation requires a root to support the topographic load), so the mass deficit is greatest and the Bouguer anomaly is most negative. The inverse applies at ocean basins and thin-crust regions: mantle is close to the surface, the mass excess produces positive anomalies.

The gravitational effect of the Moho is large because the density contrast (300–600 kg/m³) persists over a thick horizontal layer. A 10 km variation in Moho depth produces a Bouguer anomaly difference of roughly 50–80 milligals — easily detectable with modern gravimeters. This sensitivity is what makes gravity a powerful tool for crustal mapping despite its resolution limitations.