Geologists have long been fascinated by the boundary between the Earth’s crust and mantle. Although it’s relatively close to the surface, the region is difficult to study with current instruments, adding to its mystique. The interactions between the planet’s outer layers make this boundary a geologically dynamic zone.
Scratching the crust. A recent study has uncovered a new aspect of this interaction. Beneath California’s Sierra Nevada, researchers have found evidence of the mantle “scratching” the Earth’s crust.
Delamination. Geologists theorize that fragments of the lithosphere periodically break away and sink into the mantle, a process called delamination. This phenomenon may explain the stark differences in thickness between oceanic and continental crusts, as well as other key geological features.
Typically, delamination is thought to occur slowly, with heavier crustal rock gradually detaching and sinking into the less dense mantle. However, researchers suggest this process can also happen abruptly, with the mantle appearing to “peel” away the crust.
Seismic waves. To investigate, the team analyzed seismic waves traveling through the Earth’s inner layers to examine factors such as composition and density. The Sierra Nevada’s seismic activity made it an ideal region for this research.
The study combined data from receiver function analysis and the Advanced National Seismic System’s Comprehensive Catalog. Within this catalog, researchers identified a “seismicity band” roughly 25 miles below the surface, where small earthquakes ranging in magnitude from 1.9 to 3.2 were concentrated.
Rupture, not drip. Receiver function analysis revealed differences in the mantle that suggest a distinct layer beneath the Sierra Nevada. This layer becomes less pronounced as it extends northward, supporting the theory that part of the lithosphere in the southern region broke away from the crust millions of years ago.
The study’s authors propose that these small earthquakes indicate the lithosphere’s detachment occurred through rupture rather than a slow drip. The findings are detailed in Geophysical Research Letters.
Strengthening the hypothesis. While the evidence isn’t conclusive, the study adds to the growing body of research suggesting that the Mohorovičić discontinuity (the boundary between the crust and mantle) beneath the Sierra Nevada is gradual rather than abrupt.
Image |Jeremy Bishop (Unsplash)
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