When did the continents appear?This gorgeous shot is Alaska’s Augustine volcano, one of the ma

When did the continents appear?This gorgeous shot is Alaska’s Augustine volcano, one of the many volcanoes produced by subduction of the Pacific Plate beneath the rocks of Alaska. This lovely cone is just one tiny sliver of the story to tell today – it’s a piece of a continent.Earth is unique for a number of reasons, including obviously the existence of life and Facebook pages devoted to Earth science. Another unique property of Earth is the existence of volcanoes like this; almost all of the volcanism in the solar system differs from this peak. Volcanoes elsewhere in the solar system almost universally erupt basalt, the same rock type that makes up Earth’s ocean crust. A very basic feature of the solar system produces basalt. Almost every rocky planet has a mantle dominated by the minerals olivine and pyroxene, including Earth, and when these rocks melt the first thing they form is basaltic magma.This cone, and most volcanoes at subduction zones, are different. They erupt rocks that have different chemistry, specifically higher silica contents and higher water contents. We call those rocks andesites, dacites, and rhyolites. The continents stand high above the ocean floor because they’re a different composition than the oceans; they’re thicker, less dense, and literally float above the waves on the mantle.The presence of these continents has long been a major mystery for geology. We can tell that their formation has something to do with subduction – we see volcanoes like this at subduction zones, which could produce continent-like crust, but when and how that process started we can’t answer.The Earth didn’t start making continental crust; when it originally formed its crust came out of the mantle and was made of something like basalt. The origin of the continents is therefore tied to the start of subduction and plate tectonics, but our records of those processes went down subduction zones long ago. We have some rocks dating back almost 4 billion years, but those rocks have been heavily metamorphosed and look almost nothing like our continental rocks today. The only way to answer that question, when subduction started and the first continents began growing, is to find something we could measure that gives us a proxy, something that changes in the rocks right when the continents started forming.A just-published paper led by scientists at the University of Bristol argues that they’ve found such a proxy. The periodic table contains nearly 100 distinct elements that occur naturally on Earth and they each behave differently. Each element will go easily into some minerals and not into others, depending on the size and charge of the element and the crystal structure. That fact means processes can change the ratio of one element to another, processes like the initial formation of continental crust.The element Rb, rubidium, sits right below potassium in the periodic table. It’s a very big element and it doesn’t go well into crystal structures, so it’s hard to stick that element in the mantle. If the mantle was just convecting and constantly mixing itself, any rubidium that reached the surface would sink back down pretty easily. On the other hand, if continents began stabilizing, the rubidium would get trapped in them and stay there since the continents don’t sink back into the mantle. A signature of continents stabilizing, therefore, could be rubidium building up in abundance in the rocks at the surface.Rubidium also does one other really useful thing – one of its isotopes is radioactive. That means if we measure the amount of rubidium and its decay product, strontium, in rocks today, we can figure out both how old they are and what the chemistry was like in the original rock, even if the rock has been metamorphosed.The group from Bristol compiled over 13,000 measurements of rock samples done by hundreds of researchers worldwide, covering almost the entire history of Earth, and looked for a signal in rubidium. They found one.Early in Earth’s history, the rubidium content in rocks stays pretty constant relative to strontium. Rubidium doesn’t build up in the rocks, it moves up and down a little but there’s no sign of a rubidium spike…until 3 billion years ago.3 billion years ago the rubidium content of rocks starts going up, and within 1 billion years its average abundance worldwide had doubled. This signal makes sense if it is a sign of stabilizing continents. Every time the mantle melts a bit of rubidium comes up to the surface and if there’s a reservoir that can hold it like a continent, it will stay there.To support this mechanism they look at the thickness of modern continents and find that the rubidium contents in rocks correlate with how thick the crust is in these arcs. In other words, today, the more rubidium erupting in a volcanic arc, the thicker the crust is in the arc. Therefore, an ancient increase in rubidium could reflect increasing crustal thickness as well.There are other estimates for when plate tectonics started; triggering subduction requires the earth to cool enough and then something to crack the crust and start the process, otherwise Earth would have wound up like Mars with no plate tectonics. Some scientists still will think subduction started earlier, others think it started later than this, but the sudden change in rubidium contents 3 billion years ago implies that a new process started right around then, a process consistent with what we’d expect from plate tectonics.Further work will have to explain the mechanism for how this signal was produced; where exactly the rubidium sat in the crust and how plate tectonics was triggered. Like most things in science this isn’t the end of the story, but this data set definitely illustrates a very interesting change in the Earth 3 billion years ago that fits with what we’d expect from plate tectonics and the formation of continents.-JBBImage credit: USGShttps://flic.kr/p/v82tZ6Original paper:http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2466.htmlNews & Views:http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2476.html -- source link

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