For decades, geophysicists attributed intraplate volcanic hotspots—such as the Hawaiian chain—to deep mantle plumes carrying superheated material directly from the core-mantle boundary. However, skeptics of the plume hypothesis argued that localized upper-mantle processes, such as tectonic plate shearing and shallow stress-induced fracturing, could independently generate volcanism by melting shallow asthenospheric mantle without requiring thermal anomalies from the deep Earth. To counter this shallow-origin model, plume theorists highlighted seismic tomography imaging that revealed narrow, vertically elongated columns of low seismic velocity extending down to the lower mantle, interpreting these anomalies as hot, ascending plume conduits.
Recently, proponents of the shallow-process model offered a counterargument, demonstrating that seismic velocity anomalies in the lower mantle might instead reflect compositional heterogeneity—such as subducted oceanic crust rich in dense minerals—rather than high temperatures. They argued that such mineralogical variations could slow seismic waves without implying upward thermal transport, thereby undermining the primary empirical evidence for deep plumes.
Nevertheless, recent high-resolution geodynamical simulations provide a persuasive rebuttal to this counterargument. By integrating mineral physics data with thermal transport dynamics, the simulations show that purely compositional anomalies would cause distinct shear-wave anisotropy and high seismic attenuation patterns that are absent in the observed Hawaiian seismic profiles. Furthermore, the simulations demonstrate that only an active thermal buoyancy flux can account for both the observed swell elevation of the ocean floor and the precise geochemical ratios of helium isotopes found in hotspot lavas. Consequently, the rebuttal demonstrates that compositional variations alone cannot explain the full suite of geophysical observations.
Based on the passage, which of the following best characterizes how the high-resolution geodynamical simulations address the shallow-process model's counterargument regarding seismic tomography?
- They neutralize the counterargument by demonstrating that compositional anomalies produce specific seismic patterns absent in actual observation and fail to explain key topographic and isotopic evidence.Answer
- BThey concede that lower-mantle seismic velocity anomalies are caused exclusively by subducted oceanic crust, thereby validating the shallow-origin hypothesis.
- CThey definitively disprove all aspects of upper-mantle tectonic shear while condemning proponents of the shallow-process model for unscientific methodology.
- DThey weaken the deep mantle plume hypothesis by proving that helium isotope ratios in hotspot lavas originate entirely in the shallow asthenosphere.
- EThey establish that seismic velocity anomalies can be used as the sole metric for determining ocean floor swell elevation across all intraplate volcanoes.