#partial melting

[mineralsandsomerocks]

Buchite on Scoria matrix

Locality: Emmelberg, Üdersdorf, Daun, Eifel, Rhineland-Palatinate, Germany

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“A compact, vesicular or slaggy metamorphic rock of any composition containing more than 20% vol. of glass, either produced by contact metamorphism in volcanic to subvolcanic settings or generated by combustion metamorphism. An uncommon metamorphic rock type composed mostly of glass, formed by the melting of a sedimentary rock or soil by extreme heat from an adjacent lava flow or scoria eruption, or the burning of an underground coal bed. Buchite is a product of a special type of metamorphism, called pyrometamorphism (contact type, high to very high temperature, low pressure, often connected with coal fires). Buchites are often confused with obsidian, which is a volcanic glass rather than metamorphic glass. The so-called para-obsidian is a buchite with small amounts of mullite and tridymite forming microlites. Rarely, buchites can be colorful and transparent, and faceted as gemstones, as for example a few blue-green gems from the Eifel hills in Germany. The original description of buchite was for partly fused, glassy sandstones associated with basalts (in Germany). Now the term also covers more pelitic rocks. The first described buchite contained relic, cracked quartz with tridymite overgrowths and inclusions, feathery or needle-like clinopyroxene, magnetite, cordierite crystals, tiny crystals of a spinel, small voids, pores interfilled with goethite and brown glass. According to Grapes (2006), buchite also occurs as xenoliths and within contact aureoles.” - https://www.mindat.org/min-50131.html

Basically what this means for this rock, is that there was a scoria eruption (a volcanic eruption with lots of air bubbles in the magma) that came into contact with some sandstone.  The intense heat of the magma was able to melt some of the sand, which then rapidly cooled and became glass.  You can even still see the darker bands beneath the glassy layer, displaying this xenolith’s sedimentary origins!

This is my first time playing with the “Read More” function in order to provide some more information or an explanation, so please let me know if you read it and/or it is something you’d like to see more of in the future!

[earthstory]

A granite is born

At first glance these rocks snapped while wandering around some old mountain roots in the Sierras de Cordoba in Argentina don't seem like much, but they record the moment in high grade metamorphism when old rocks turn into new, with nothing but a few zircons left behind to testify to their previous history. The first photo shows a mass of fine-ish grained 'baby' granite, and floating within it a large rounded chunk (in the rough centre and a few attendant blebs) of the grey and white banded gneiss from which it formed, that was still floating unmelted in the magma when it froze (and by then probably some distance from its source rock). In the second photo the act of birth is even more intimately caught. The chunks of gneiss are picked out with a white outline of fresh quartz rich melt, also frozen in place. These rocks are called migmatites and mark a generational turnover in the rock cycle, when deep crust melts (usually during a continental collision and mountain building moment), rises and freezes, becoming shallower, younger crust, potentially susceptible to uncovering and erosion into sediments, whose burial unto melting depth will start the whole cycle anew...

The first photo is of a boulder in a river bed, the area in the photo is maybe a metre squared, the second boulder is about a metre in the longest dimension.

Loz Image credit: Loz

[capsunm]

Partial Melting and Fractional Crystallization

by Tara Templeman and Katie Van Gilst

Tara Templeman is a SI leader for EPS 101: How Does Earth Work? and a tutor for Biology and Chemistry at CAPS. She is majoring in Earth and Planetary Sciences and Biology with a focus in Museum Studies. Tara will graduate in the Fall of 2014. She hopes to work as a Paleontologist and a Curator after attending graduate school.

Katie Van Gilst is a Mathematics, English, and Environmental Studies tutor at CAPS. She is double majoring in Applied Mathematics and English with a concentration in Creative Writing. When she isn't immersed in her studies, she loves to read fictional novels and go hiking with her friends. She hopes to receive her doctorate in Mathematics and work at the University level.