Crystal Mush beneath YellowstoneThis is a common texture in igneous rocks – a porphyritic texture. T

Crystal Mush beneath YellowstoneThis is a common texture in igneous rocks – a porphyritic texture. This term refers to igneous rocks that have a population of large crystals surrounded by a fine-grained groundmass or matrix.A porphyritic texture is produced naturally as magmas cool. A magma is a complicated, multi-component solution – a single molten rock can be composed of 10 different abundant elements. When magma cools, some of these elements start forming minerals, crystallizing out of the melt while other elements remain left behind.A porphyritic texture therefore is created when some minerals crystallize at higher temperatures than the rest of the magma, forming large crystals surrounded by a groundmass. These larger crystals are plagioclase floating in a groundmass that contains other elements – the photographer labels the rock as an andesite.This rock type, a porphyry, is relevant to a recently published paper by a group led by scientists at the University of Utah on the plumbing system of the Yellowstone volcano.Yellowstone is a huge volcanic system with a giant magma chamber – when it last had a major eruption 640,000 years ago, it was big enough to cover the entire western United States with ash and filled much of the park with tens of meters of erupted rock.An eruption like that takes a lot of time to build – the previous two large eruptions at Yellowstone were 600,000 and 1.4 million years before that last eruption. To generate a magma chamber big enough to feed one of those eruptions it must take a long time; lots of molten rock must rise up through the Earth’s crust and build up in a magma chamber.The term “magma chamber” probably makes most people picture a big hole in the ground filled with molten rock, but that’s not how the world works. The Earth’s crust is pretty cold, so when molten rock sits in the crust it starts to cool off and crystallize. Most magma chambers aren’t big pockets of molten rock, they’re a mush, filled with a mixture of crystals and molten rock in-between. They’re much more like this rock, a porphyry.Although we can’t see individual crystals until they reach the surface, scientists have tools that can characterize these mixtures; geophysics. Seismic waves, for example, will travel through just about anything. If a melt is present, even a tiny bit of melt in-between larger grains, seismic waves will travel through that rock and interact with the melt. The presence of a tiny bit of melt therefore can be detected using seismic waves, and that brings us to this paper.Sensitive seismic instruments can detect very small amounts of melt, and beneath Yellowstone there is a very large pool of molten rock. According to previous seismic work, a large wedge of rhyolitic magma sits beneath the Caldera at depths of about 10-30 kilometers. This study found something new – beneath that wedge of magma there is another another layer composed mostly of crystals, with melt percolating between grains.Between 25 and 50 kilometers deep beneath Yellowstone, the rocks are partially molten. There is only a tiny bit of melt – the rocks are about 98% solid, 2% melt, but over 25 kilometers of crust that’s still a large amount of melt (some other sites have compared it to enough volume to fill the Grand Canyon about a dozen times). A 3-D rendering of these multiple magma layers can be found here at our blog: http://tmblr.co/Zyv2Js1jU2WW2.That basaltic melt is gradually migrating upwards and feeding the magma chamber. For the last 100,000 years or so, the Yellowstone system has remained close to a steady state – heat arriving at the base is removed rapidly by an extremely active hydrothermal system. This basaltic melt percolating at depth represents the source of that heat.This paper therefore is the first to identify the seismic signal of the melt migration zone beneath Yellowstone. If we were to sample it, it’d be a porphyry like this rock, only with far less melt and crystals made of olivine instead of plagioclase, but even still, this is a good comparison for what is happening beneath Yellowstone.Finally, as with most Yellowstone posts, it’s worth highlighting that this discovery does not imply that the system is at risk of a large eruption any time in the near future. This hydrothermal system has been cranking out heat for 100,000 years and there’s no sign of any major changes on the horizon. Yellowstone is one of the best-monitored, most-fascinating volcanic systems in the world, and this study adds another detail to our understanding of how it works.-JBBImage credit: Richard Droker (Creative Commons Licensed)https://flic.kr/p/6LitawRead more: http://www.sciencemag.org/content/early/2015/04/22/science.aaa5648https://www.sciencenews.org/article/massive-magma-pool-found-deep-below-yellowstone -- source link

More pics about