Eclogites are my favourite sort of rocks. They are beautiful, interesting and exotic. Sometimes they even contain (very very small) diamonds. What more could you want from a rock? Most relevantly, they tell us a lot about conditions deep inside the earth, for example providing insights into deep earthquakes.
Eclogites are rocks of mafic composition that have been buried deeply but not heated too much – they have been metamorphosed to eclogite facies.
Rocks reach these conditions within subduction zones, places where oceanic crust dives back into the mantle from which it was born. I describe the incredible journey oceanic crust goes on in a pair of posts. The first traces crust from its creation in a mid-ocean ridge down to 250km depth. Often oceanic crust doesn’t stop there but sinks all the way to the core-mantle boundary, undergoing manymany changes on the way.
Some eclogite does return to the surface, thank goodness. How does it do this? One reason for oceanic crust to come back up may be that it contains more water than normal.