Читать книгу Earth Materials онлайн

45 страница из 155

Convergent plate boundaries

subductionsubducted platedowngoing slaboverriding platesubduction zonedestructive plate boundaries

ssss1 Convergent plate boundary, showing trench‐arc system, inclined seismic zone and subduction of oceanic lithosphere.

trench‐arc systemsTrenches

inclined seismic (Wadati‐Benioff) zone

What is the ultimate fate of subducted slabs? Earthquakes occur in subducted slabs to a depth of 660 km, so we know they reach the base of the asthenosphere transition zone. Earthquake records suggest that some slabs flatten out as they reach this boundary indicating that they may not penetrate into the lower mantle. Seismic tomography, which images three‐dimensional variations in seismic wave velocity within the mantle, has shed some light on this question, while raising many others. A consensus has emerged (Grand 2002; Hutko et al. 2006) that some subducted slabs become dense enough to sink all the way to the core–mantle boundary where they contribute material to the D″ layer. These slab remnants may ultimately be involved in the formation of mantle plumes, as proposed by Jeanloz (1993).