From Fiery Giants

Could ancient volcano eruptions have helped make Earth inhabitable?


The blue planet wasn't always so green. For billions of years, Earth was a mass of hot rock, and it lacked one fairly essential component: breathable air; but ancient volcano eruptions may have changed all that.
Earth’s transition from fiery rock ball to lush green planet is a strange and unpredictable tale. Rather than being a gradual evolution towards an oxygen-rich environment, Earth’s atmosphere appears to have changed in sudden and dramatic spikes. Why, you may ask? Well, intrepid geologists are on the case, and they believe that the answer may lie with volcanoes and colossal eruptions that changed the face of planet Earth.
Oxygen is an essential component of life on Earth. It’s one of the things that distinguishes us from all other planets in the solar system, and without it we would all be pretty uncomfortable; but it wasn’t always this way. Scientists believe that Earth may have started out a lot like Venus, hot and atmospherically dense. The two planets are the same size and began with similar proximity to the sun, but they evolved very differently. So how did we get to be so special?
Dr Oliver Shorttle is an igneous geochemist at the University of Cambridge. He explains how we can use rocks to uncover the mystery of Earth’s evolution. “The history of the world is written in the rocks”, Oliver explains. “Each layer in a rock sample can give us information about the climate in which it was formed. And if we look at these rocks as a whole, then it’s like reading a historical record of the way in which the environment has developed over billions of years.”
On I18, researchers can map elements in complex samples, follow chemical reactions, study real systems such as mineral samples returned from space, environmental samples and materials in hostile environments.
On I18, researchers can map elements in complex samples, follow chemical reactions, study real systems such as mineral samples returned from space, environmental samples and materials in hostile environments.
And it’s the rock record that gives us the major clue about how we got to be such a lively, oxygen-rich planet. Abrupt and striking changes in the rock layers indicate that oxygen levels shot up without warning – this happened repeatedly, over billions of years. This pattern would certainly fit with the behaviour of volcano eruptions, but how could these dirty and destructive events have given us clean air?
When a volcano erupts, it injects ash and volatile gases into the atmosphere, and this impacts on the surrounding landscape and on carbon-levels in the air. But very early in Earth’s history, when there was much less oxygen in our atmosphere, the injection of these volatile gases into the air would have had a much greater effect. Oliver explains: “With little else to dilute the mix, these gases would have hugely altered the surrounding atmosphere. They would have set off all sorts of chain reactions in the environment, and it’s likely that these reactions eventually led to the introduction of O2 into our atmosphere.”
But to further investigate this theory, Oliver needed to get his hands on a sample from the front line. Volcanoes emerge because the Earth’s plates are constantly moving. When two plates meet, one plate is dragged underneath the other. As bits of the Earth’s crust are pulled down into the mantle, they leave a tear in the Earth’s surface, and it’s from this tear that volcanoes emerge. The mid-Atlantic ridge is an underwater stretch of volcanoes that have sprung up along the tear between two plates, all the way from the Antarctic to the Arctic. At one point, the volcanoes actually peep above the surface of the water, forming the inaptly-named country of Iceland. 
Iceland provides a fascinating insight into volcanoes, and its rock samples are like catnip for geochemists. Scientists have even gone to the trouble of using submarines to gather samples from the Icelandic coast. Oliver explains why it’s worth the effort: “If you pick up a chunk of rock in England, you could easily be looking at something that’s 5 or even several hundred million years old; but if you gather your samples from Iceland, then some of the signals in those rocks are much older than that.” And that’s why those volcanic rocks are so useful to scientists, because they’re truly ancient: “The volcanic rocks now on the surface in Iceland are carrying the chemical fingerprint of seafloor that was going down into the Earth’s mantle 2 billion years ago. That means your sample can reveal information about climate history going back billions of years.”
With his precious samples in hand, Oliver is using one of Diamond’s spectroscopy beamlines, I18, to uncover their secrets. Using a technique called ‘X-ray absorption spectroscopy’, Oliver is able to identify the elemental make-up and characterise the chemistry of his samples based on how they absorb X-rays. With this information, he can determine the levels of oxygen in the sample; what’s more, as the samples are from recently surfaced volcanoes, he can also work out the levels of oxygen inside the Earth’s mantle.
Oliver hopes that this information will shed new light on the history and evolution of the Earth’s atmosphere, revealing the part that volcanic activity had to play in creating the air we breathe today. If Oliver and other geochemists are right, then we may owe everything – from daffodils, to hot air balloons, to the creation of life itself – to these fiery giants. 

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