Tuesday, August 17, 2010

Article review: “Metamorphism: from Patterns to Processes”

As a member of the Mineralogical Society of America I receive a paper copy of the journal, Elements, each month. Some of there previous issues have focused upon a specific element or a specific mineral. The June 2010 issue (Vol 6, #3), on the other hand, focuses on Fluids in Metamorphism. Needless to say, as a metamorphic petrologist this caught my attention. I’ve just finished reading the article titled Metamorphism: From Patterns to Processes by Bjørn Jamtveit, who is located at the Center for Physics of Geological Processes at the University of Oslo and decided that it is worth mentioning here.

This article is a very good one for underscoring one of the reasons I am fond of metamorphic rocks: they are pretty! He includes a variety of figures to illustrate the types of changes that happen to rocks as a result of metamorphism (change), with a focus on how those changes are facilitated by the presence of fluids. As a teaser for the article, I have copied his figure 1 below. The first photo is of a dark, fine grained basalt (formed by the cooling of lava after it erupted from a volcano), which contains the minerals augite, plagioclase, and olivine, though the individual grains of each are too small to see at this scale. The second photo is of a lovely eclogite, which contains red garnets, green omphacite, and white clinozoisite, all of which are much coarser-grained than minerals in the basalt. Metamorphic processes are responsible for the transformation of basalts into eclogites.

The differences in the crystal structures of augite-plagioclase-olivine vs. garnet-omphacite-clinozoisite mean that the eclogite is a denser rock (~3.5 g/cm3) than is the basalt (2.9 g/cm3), which, the figure caption tells us, means that the transition from one to the other is important for large-scale geodynamic processes, including basin subsidence and subduction.

In addition to illustrating the beauty of metamorphic rocks, the article also touches upon the factors that cause the change, and how long these processes take. I highly recommend it to anyone curious about metamorphism.

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