The Elastic Rebound Theory relies on rocks being brittle enough to “snap.” However, at depths above 300 km, the pressure and heat are so high that rocks should flow plastically rather than break. The existence of deep-focus earthquakes suggests a different mechanism, like “phase changes” of minerals. ANSWER: (B) Deep-focus earthquakes (above 300 km)
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Below 300 km, rocks shouldn’t be able to store elastic strain; they should “creep” like warm wax. Seismologists believe deep-focus quakes are caused by “transformational faulting,” where minerals like olivine suddenly collapse into denser forms (spinel or bridgmanite) under extreme pressure. This sudden “implosion” or volume change releases energy as seismic waves. Therefore, while Reid’s theory perfectly explains quakes near the surface, the “Deep-focus” mystery requires a more complex understanding of high-pressure mineral physics and thermodynamics.