Remember the rock with alternating bands of green and blue from the last post. The blue in this rock is the blue schist that I talked about in that post, but what about the green? The green is actually a different type of rock known as eclogite.
The term eclogite, like blueschist, can be used in two ways 1) a metamorphosed basalt that contains garnet and clinopyroxene and reaches very high pressures or 2) any rock type metamorphosed to the same metamorphic conditions. Eclogites are sometimes referred to as “Christmas tree rocks” because they are green rocks (clinopyroxene makes it green) that usually contain large reddish garnets (pyrope or almandine). Eclogites are basically what you get when you take a blueschist and increase the pressure. Sometimes, the pressure is so high that you can form coesite- essentially, the mineral quartz with a different crystal structure.
Eclogites are interesting because they form either in the mantle or at the base of the earth’s crust, so we get a peek at mantle rocks when we find them. Eclogites are rare on the surface because they easily undergo what geologists call “retrograde metamorphism” which means that once you change the conditions, it’s not likely to stay an eclogite for long unless you move it to the surface quickly. The easiest way to do this is by rapid uplift or exhumation of the rocks, but pieces (xenoliths) can be found in some igneous rocks that rapidly brought them to the surface (e.g. kimberlites). Just to complicate things even more, the rocks on this particular beach may actually be pieces of eclogite that have undergone “retrograde metamorphism” to become blueschist, preserving just some of the eclogite. Geologists can determine if something like this happened by putting very thin slices of the rocks under a microscope and looking for evidence that the minerals have reacted.
Sorry about all the geology terms, geologists love jargon!
Speaking Metamorphically 