Holly berriesTrue eclogites might well be my favorite rock – they’re my favori…

Holly berries

True eclogites might well be my favorite rock – they’re my favorite metamorphic rock at least. An eclogite is a metamorphic rock that forms when basaltic igneous rocks, like those that make up the ocean floor, are pushed deep down into the upper mantle or to the lowermost part of the crust. The basalt starts off with minerals like plagioclase and pyroxene, but those minerals transform into reddish garnet and a pale greenish pyroxene called “omphacite”. The resulting rock winds up with a lovely pair of red and green minerals, with the occasional bit of snowy white feldspar in-between, as seen here – almost perfect for Christmas.

Aside from just being a gorgeous rock, this is one of the most important rocks on Earth. When oceanic crust turns into this metamorphic rock, it becomes denser than the rest of the mantle. That means: oceanic crust goes down a subduction zone, turns to eclogite, and the eclogite will sink through the entire mantle, dragging more oceanic crust with it. Oceanic crust naturally turns into this dense rock when it is put in the mantle and this rock naturally sinks, driving the process on Earth today we call plate tectonics.


The garnets (the red dots) in this frame are probably a little less than a cm across – this is a polished slab at a museum in Scotland.
Image credit: https://flic.kr/p/bsFajs