= Solution
Choose an ocean–continent <convergent margin>. The essential relationship is \b[recycling of <oceanic lithosphere> by <subduction>, together with production and modification of <crust> in a <volcanic arc>].
\Image[/past-exam-of-the-plate-tectonics-course-of-the-university-of-oxford/2023-convergent-margin.png]
{title=An original ocean–continent subduction cross-section, with slab recycling, fluid release and arc crust production}
The incoming <tectonic plate> consists of thin <oceanic crust>, chiefly <basalt> and <gabbro>, above colder <lithospheric mantle>. As that <oceanic lithosphere> approaches the <subduction zone>, bending produces the <subduction trench>. Its cold interior can become denser than the surrounding <mantle>; the resulting <slab pull> helps sustain <subduction>. The descending <slab> carries both <oceanic crust> and <lithospheric mantle> into the interior. Thus the recycled unit is an entire portion of <lithosphere>, rather than just its <crust>.
At the <subduction trench>, some incoming <sediment> is scraped off and stacked by <thrust faults> into an <accretionary prism>. Material can also be added beneath the <overriding plate> by <tectonic underplating>. A <forearc basin> collects <sediment> between the <accretionary prism> and the <volcanic arc>. This is an accreting example: other <convergent margins> lose upper-plate material through <tectonic erosion>, so an <accretionary prism> is not universal.
The cold <slab> undergoes increasing <pressure> and <metamorphism>. Conversion of <basaltic rocks> and <gabbro> towards <eclogite> increases their <density>, while <slab dehydration> releases <water> from hydrated <minerals>. The rising fluid lowers the <solidus> of hot <peridotite> in the <mantle wedge>, causing <flux melting>. The usual source of <arc magma> is therefore the fluid-modified <mantle wedge>; extensive <slab melting> is not required. Thermal-petrological calculations show that the depth and amount of <slab dehydration> depend on the incoming <slab>'s thermal structure.<Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide by Peter E. van Keken and others>[Abstract and depth-dependent water-release models.]{ref}
Buoyant <magma> rises through the <overriding plate>, freezes in <igneous intrusions> or erupts in the <volcanic arc>. <Fractional crystallization>, interaction with existing <continental crust> and repeated intrusion can produce more evolved <arc magmas> and add new <crust>. Some material merely reworks old <crust>, and simultaneous <tectonic erosion> can remove it: <volcanism> alone does not measure net continental growth. <Tectonic shortening>, <folding> and <thrust faults> can thicken and raise the upper plate.
The <Wadati-Benioff zone> marks <earthquakes> along and within the cold descending <slab>. Shallow interface <earthquakes> reflect unstable slip where the plates are coupled; deeper <earthquakes> require mechanisms such as <dehydration embrittlement> and are not simply dry shallow friction operating at every depth. In contrast, the hot <mantle wedge> deforms mainly by <material creep>. Together, the <subduction trench>, <accretionary prism>, <forearc basin>, <volcanic arc> and inclined <seismicity> connect surface features to the processes recycling and creating <crust>.
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