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Professor Dougal Jerram

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The Largest Volcano on Earth .....Really??

Posted: 09/09/2013 10:32

When it comes to anything these days it's all about what's the biggest, best, fastest etc. That's what makes the headlines, and that's exactly the same for scientific discoveries. But what about the detail behind the hot air? It has been announced that the 'World's largest volcano' has been found in the pacific, the Tamu Massif. A big claim, but what does it actually mean?

Well for one the structure that has been identified is indeed volcanic, and it is big with a footprint of roughly 120,000 square miles, but one single volcano as you might imagine it? Well actually no! This is because it is rather shallow in its construction, made of low viscosity basaltic rocks, building up what is known as shield volcano like those at Hawaii. In this case though the overall structure is a little more complex, and less clear. Invariably as the structures get bigger, there are several vents, or eruption sites that occur through time at different points along the volcanic construction. These are found as smaller volcanoes, volcanic cones or as fissures which extend along cracks in the crust. So the overall structure becomes more a pock marked cake of volcanic material that can build up over several millions of years. As things get even larger, they get harder to erupt as single eruptions from single volcanoes, due to the scale and structure of the volcanic beast in question. In their biggest manifestation on earth, volcanoes produce what is known as flood basalts or large igneous provinces, which produce some of the biggest eruptions that the Earth has ever seen.

What is volcanically big? Well if we look outside our planet which is complicated due to its plate tectonics moving and shaking things around, then the classic comparison for big volcanoes is that of Olympus-mons, on mars. Weighing in at some 624 km (374 mi) in diameter by 25 km (16 mi) high, this is a real monster, but what is clear from its shape and form is that is a relatively simple shield like volcanic structure with a few craters towards its top.

When it comes to the largest ever eruptions on Earth, these are up to ~8000 km3 in volume, found in the Parana-Etendeka flood basalt province, and have been broken up due to plate tectonics and the separation of Africa from South America. This makes the understanding of the actual 'volcano' that feed the eruptions very difficult. So the attention turns to what is the single largest volcano on earth, and this is most difficult to define. Do you look for the biggest single construct with a central vent area? Do you look for large related volcanic constructs that may have many smaller eruption sites but broadly relate to the same eruptive episode? How do we look into Earth's clearly violent volcanic past and understand the volcanoes that have been responsible for many 1000's of km3 of erupted material and have affected the Earth's climate ?
The basic reality is that that there has been some good new imaging with very limited sampling of a large volcanic feature in the Pacific Ocean. Like similar studies on large oceanic plateaus e.g. the Ontong Java Plateau, the amount of sampled material and information (for something the size of France in the Ontong Java case), is limited by the size of the object and the difficulty in sampling it. Particularly the difficulty in being able to fund any exploration that drills deeper than a few tens/hundreds of metres.

Given a comparison to onshore volcanics of a similar size, where significant sampling and data are present, studies reveal large volcanic eruptions from several feeding centres over broad region, building up massive amounts of material in relatively short, ~few million years, geological history. As Prof. Mike Garcia (University of Hawaii), put it on discovery news 'The Tamu Massif could be more like the Columbia River Plateau, which also formed from eruptions of lava that flowed out far and wide and covered a huge area -- but on a continent rather than the sea floor' (Note: the Columbia River Plateau is one of the smaller of the world large igneous provinces). The Tamu Massif is clearly a larger volcanic structure, and has probably formed by relatively large outpourings of basaltic lavas from a series of closely related centres. Certainly worthy of further exploration and study, but there may be bigger volcanoes out there to fry!!

 
 
 

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