Huge reservoirs of magma stored deep inside the Earth's crust are key to explaining volcanic super-eruptions and why they are so rare, researchers claim.
While the amount of magma stored in the upper layer of the Earth's crust determines the frequency and magnitude of eruptions, an international team of scientists say it is the deeper reserves that are responsible for feeding into them, potentially creating the conditions for some of the world's most powerful eruptions.
Co-author of the study Dr Wim Degruyter, from Cardiff University's School of Earth and Ocean Sciences, said: "Our current understanding tells us that hot magma can be injected from the Earth's lower crust into colder surroundings near the surface.
"At this point, the magma can either erupt or cool down to such a point that the magma solidifies and an eruption does not occur.
"Up until now, this theory hasn't been able to explain how the magma can maintain its heat in these near-surface reservoirs and thus produce extremely powerful eruptions.
"Our study has shown that the key to this is much larger reservoirs deeper below the surface that are able to slowly increase the temperature in the upper part of the crust such that it becomes more amenable to the storage of magma.
"When the crust has become fully mature, giant reservoirs are able to form in the upper crust and thus we see extremely powerful eruptions."
Small eruptions that discharge less than one cubic kilometre of material can occur frequently, from daily to yearly, but the largest eruptions that erupt hundreds of cubic kilometres of material are infrequent, with hundreds of thousands of years between them.
Dr Degruyter and his colleagues say these super eruptions are triggered by a slow and steady drip feed of magma from large reservoirs deep within the Earth's crust into smaller reservoirs closer to the surface.
By conducting numerical simulations of this process, the research team showed that these large reservoirs are crucial to generating the largest volcanic eruptions on Earth.
The team also showed that these large reservoirs can take millions of years to form, which is why such eruptions happen so rarely.
The study is published in the journal Nature Geoscience.
