#metamorphicmonday

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Garnet pulled out of the ground in Northern Territory, Australia. We have a site in Montana where I was pulling out stuff just like these.

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Folded light and dark bands in a metamorphic rock that was heated to conditions where it just started melting (called a migmatite) from Poland.

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That’s one single garnet crystal

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physicsfun Acasta Gneiss: this polished sphere is a sample of the oldest known rock on Earth- dated at 4.03 billion years old based on multiple radiometric dating techniques. This rock is only found in the Northwest Territories of Canada and was collected by Yellowknife resident Mark Brown. 

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Metamorphism at Great Depths - Eclogites

Eclogites form at temperatures exceeding 500°C and pressures over 12 kbar. These high pressures can only be attained once a rock has been buried to a minimum of 50 km below the surface of the Earth. Eclogites most commonly form in subduction zones, where oceanic crust is forced deep into the upper mantle at convergent boundaries between two tectonic plates. The downgoing oceanic crust is referred to as a ‘slab’. As Eclogites form at extremely high pressures, they are much denser than their protoliths with densities around 3.4 – 3.5 g/cm^3. This densification of the slab material as it descends, a process known as ‘slab pull’, is the primary force pulling the slab into the mantle and one of the driving forces of plate tectonics.

While geologists believe eclogites to be common in subducting slabs, they are a rare find on the surface of the Earth. The vast majority of eclogites never return to the surface, but are buried ever deeper into the mantle until they melt due to the high temperatures. It takes a very uncommon series of events to exhume a subducted oceanic slab from 50+ km below and expose it on the surface. Several notable eclogite localities around the world are in Greenland, Norway, Italy, China, Newfoundland, and France.

Eclogites are of interest to geologists as they provide clues to the nature of subduction and the physical characteristics of slab material. An interesting example of this comes from a 2012 paper published in Geology (http://bit.ly/2q3QqxM- paywall), about eclogites found in the Italian Alps. These eclogites, which were buried to 80 km below the surface, show evidence for multiple generations of fracturing and ductile deformation. The authors suggest that the fracturing results from earthquakes occurring at depths of 70 – 80 km, within the subducting slabs. The presence of deep, intra-slab earthquakes have been detected by seismic networks, but this locality gives researchers one of the first physical representations as to the nature of these events.

-CD

Sources http://www.sandatlas.org/eclogite/ http://bit.ly/2roxpKF Best, M.G. 2012. Igneous and Metamorphic Petrology 2nd Ed. Blackwell Publishing. Image Credit http://bit.ly/2rClbv7

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Magnificent Kyanite