Eclogite: the Christmas Rock

Eclogite is an absolutely fascinating and beautiful rock that has earned the nickname “Christmas rock” due to its colorful mineral garnet (red) and omphacite (green). What’s so fascinating about eclogite is the way it forms

Eclogite is a high pressure, low temperature metamorphic rock formed from protolithic basalt or grabbro (oceanic crust) that has been dragged down a subduction zone to (or nearly to) the upper mantle. Somehow it is brought back up relatively quickly by more buoyant serpentinized material, otherwise it would retrograde (un-metamorphose basically).

The process was first proposed by W. G. Ernst in 1965, and was an immediate controversy. There still is some controversies surrounding eclogite, but Ernst ideas have been mostly accepted, expanded upon, tested, and extended. I had the great pleasure to have gone on a geology field trip, led by Gary Ernst, to Pacheco Pass, California where eclogite is found in the Franciscan Complex. He is a lovely guy, very approachable, enthusiastic and willing to share.

Here is Ernst’s paper: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/76/8/879/5938/Mineral-Parageneses-in-Franciscan-Metamorphic?redirectedFrom=fulltext

Here is Evelyn Mervine’s great AGU blog on eclogites: https://blogs.agu.org/georneys/2011/07/02/geology-word-of-the-week-e-is-for-eclogite/

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