Here some eclogites I've seen in the field: Turkey (left) & California (right). But this new one (from Norway) is the first I have a hand sample of. ⚒️
Eclogite is metamorphosed basalt, transformed under conditions of relatively high temperature but VERY high pressure. It forms deep, deep in subduction zones. ⚒️
Eclogite is really dense, & the weight of that heavy tip of the subducted slab may help drive subduction by tugging on the rest of the plate, pulling more oceanic crust into the realm where it can be transformed into eclogite, growing denser, yanking more seafloor into the mantle, ad infinitum. ⚒️
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Deep, deep down (400+ km) that basalt goes over to garnetite. 🙂
Or in any case until running out of oceanic lithosphere to tug.