The biggest water cycleAs I’m writing this, there are a handful of puffy white clouds in the s

The biggest water cycleAs I’m writing this, there are a handful of puffy white clouds in the sky above me. Clouds are part of the water cycle that we experience every day – water evaporates from the oceans, moves around the atmosphere, comes down as precipitation, and makes its way back towards the ocean through rivers, lakes, and groundwater.That water cycle is a part of our daily lives, but did you know it’s not nearly the biggest water cycle on Earth? Deep beneath our feet there is another water cycle. It’s much slower than the one at the surface and it’s much, much bigger than the one we deal with up here.This gorgeous shot of Halemaumau crater on the Kilauea Volcano shows a tiny sliver of this water cycle, taken during the last series of eruptions. A gas cloud is rising from the crater, leading to the question “where is the gas in those clouds coming from?”That gas had a long voyage. The gas is a mixture of water (steam), a bit of carbon dioxide, a bit of sulfur, and a few other minor ingredients. A few days before this photo was taken, those elements were within the volcano’s plumbing system. Molten rock can actually dissolve small amounts of gas; as the magma approaches the surface, these gases leave the magma and form bubbles just like in a soft drink as the pressure is released. Those bubbles arriving at the surface supply water and the other minor ingredients to the atmosphere.This degassing step is just the final part of the water’s voyage. For water to be in the molten rock it has to be present where the molten rock forms – over a hundred kilometers deep inside the Earth. We occasionally get crystals from that deep carried to the surface by magmas, little bits of the mantle we call xenoliths (foreign rocks), that tell the same story – a tiny bit of water can dissolve in the rocks of the Earth’s mantle. When those rocks melt, the water goes into the melts and is eventually carried up to the surface.Where then does that water come from? Occasionally we get even deeper bits of the mantle brought to the surface within diamonds, where the diamond forms a shell that protects the precious bit of the deep earth. A few years ago, a couple scientists located a xenolith within a diamond grain from about 500 kilometers deep in the Earth, 1/5 the way through the mantle, that was soaked (see here: http://on.fb.me/1M1SPh0). The structure of that grain held as much water as it could – over 1 percent of the mass of the grain was water. If that rock were brought up to the surface, it would melt en route and give off even more water than we see coming out of Kilauea.So, even parts of the deep Earth are pretty wet, but where does that water come from? That diamond inclusion came from about 500 kilometers deep – the same level that, interestingly, we see some subducted plates get trapped at.Subduction zones are driven by water – a subducting oceanic plate has had millions of years to interact with the waters of the ocean and the minerals that make up the plate have profoundly transformed. They’ve reacted with the water and have taken up huge amounts of water into their structure; sometimes more than 10%.When an oceanic plate sinks into the mantle at a subduction zone, some of that water is released rapidly – that water drives the melting that creates volcanic arcs – but not all the water is released. The sinking plate keeps some of the water as it travels and that water is eventually released into the mantle as the plate breaks down. That water is a potential source of deep earthquakes like the one in Japan at the end of May – it could help build up fluid pressures that could drive earthquakes.In other words, over geologic time, the planet recycles water. The earth’s mantle is enormous, so even a tiny bit of water dissolved in the minerals of the mantle represents a huge amount of water. When gas comes out of a volcano like Kilauea, it is tapping this reservoir of water and releasing it to the atmosphere. When a bit of oceanic crust sinks into the mantle, it is restoring that reservoir of water.We don’t know exactly how big the reservoir of water in the mantle is, but based on evidence like that diamond it is probably huge. The mantle must hold at least as much water as the oceans on the surface do today and probably several times more – the mantle might even hold 10x as much water as is found in the oceans today, if the diamond discussed earlier is representative of a large portion of the mantle.Over geologic time, this water cycle dominates the planet. A small bit of water is released at the surface and some amount of water heads back down. This cycle could even vary the volume of the oceans over time – if it gets slightly out of balance and more water goes down subduction zones than comes out of volcanoes, the oceans on the surface could actually shrink for a time until volcanoes catch up. If those types of changes happen, they’d take a huge amount of time to happen – hundreds of millions of years, but they’re actually possible over geologic time.Beneath your feat right now there might be up to 10 oceans worth of water. Want to take a swim? Might be kinda tough, you’d have to swim through 1200-degree rocks, but still neat.-JBBImage credit: http://bit.ly/1KBtfivRead more:http://bbc.in/1xDNWTVhttp://minmag.geoscienceworld.org/content/69/3/229.abstracthttp://www.sciencemag.org/content/255/5050/1391 -- source link
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