Hotspot volcanoes, those in which magma from mantle plumes reaches the surface, have long been considered clues to mantle mysteries.
Spain’s hot volcanoes the Canary Islands, however, are a magmatic mystery in themselves. The magma from these volcanoes can have very different chemical compositions despite being part of the same volcanic system and in some cases only appearing a few meters apart. But one new study Posted in Geology sheds light on the mystery, showing how the crystals that form when magma rises to the surface can alter the physical properties of magma, such as density and water content, and its final composition.
Finding a great place for magnesium
Teresa ubide, a senior lecturer at the University of Queensland in Australia, and his colleagues analyzed samples of lava flows and dikes on El Hierro, one of the eight islands that make up the Canary Islands. The researchers compared the compositions of supply dikes (which carry magma vertically to the surface) and lava flows fed by the dikes. They discovered that these flows and dikes could have identical or very different compositions. The key is the crystals. If the dam and the flow were rich or poor in crystals, then their compositions were the same; but if the dike was rich in crystals and the lava poor in crystals, then the compositions were vastly different.
“It was then that we realized how important this process of crystal accumulation was,” said Ubide, lead author of the new study.
In El Hierro’s crystal-free samples, the researchers noticed that magnesium oxide levels were typically 5% and up to 8%. When the team expanded its scope from El Hierro to include basalts of oceanic islands from hotspot volcanoes around the world, an abundance of samples had a magnesium oxide level of 5%. This abundance suggests that these are the dominant melt compositions erupting on these volcanoes.
Ubide’s work suggests that lavas with higher magnesium oxide content represent lavas with accumulated magnesium-rich minerals (like the crystal-rich dikes on El Hierro), rather than direct mantle melts. The team also modeled how deep crystallization to form the 5% magnesium oxide melts would affect the physical properties of magma, such as density and water content.
This level of magnesium oxide was found to be a key contributor to a volcanic eruption. “When [the magma] reaches that sweet spot or tipping point of 5% to 8% magnesium, âsaid Ubide,â it can have the ideal properties of reduced density and increased volatile content, so that in carbon dioxide rich systems like basalts in oceanic islands, it can actually burst, like opening a bottle of champagne.
This sweet spot described by Ubide occurs at a depth of 10 to 15 kilometers below the surface, a fact that could prove useful in assessing the risk of volcanic eruptions. Magma accumulating at this depth (near the crust-mantle limit in oceanic environments) could be considered a risk factor for eruption.
Indeed, seismic data showed that at least a week before the eruption on La Palma, the magma was accumulating about 12 kilometers below the surface. It was the same before the last major eruption, ten years ago, according to Ubide. In this case, the magma accumulated at the depth of the sweet spot for 3 months before the eruption.
âIt’s interesting that the knowledge we are building dovetails with what has been observed recently in these volcanoes,â said Ubide. But, she warned, while observing an ideal point build-up could mean risk of a rash, that does not provide information about its timing. âNothing is predicted in volcanology; we make predictions about what’s likely to happen.
Some of the data the team examined came from cast inclusions – cast iron capsules trapped by crystals as they grow. In the inclusions melted on the most primitive crystals, such as olivines, Ubide and his colleagues expected the samples to be rich in magnesium. (A greater presence of magnesium usually indicates crystallization deep within the mantle.)
However, they were surprised to see that these melted inclusions also only had 5% magnesium oxide levels. âThey look pristine, like they’ve come from a great depth in the mantle,â said Ubide, âbut they’re actually a mixture.â
“I find this really quite surprising, quite alarming, because we often use these molten inclusions to tell us what is going on in the very early stages of crystallization, in the deepest magma chambers,” said Margaret hartley, senior lecturer at the University of Manchester in the United Kingdom who did not participate in the study.
Erupting crystals and melts from hotspot volcanoes were considered direct indicators of mantle and mantle melting composition. If not, if the fonts have also been changed as they come up to the surface, she says, âit raises all of these questions. Can we ever get [pristine mantle melts] get out of the top of the volcanoes? If we can’t, can we recalculate it? Can we model it? “
âKate Wheeling (@katewheeling), science writer