Tuscany in central Italy is not only a popular holiday destination, it is also known for good geothermal conditions. Now geologists have discovered the reason for this: beneath Tuscany lies a previously unrecognized reservoir of molten rock. The magma chamber, which was discovered through seismic measurements, contains around 6,000 cubic kilometers of magma. This is similar to well-known supervolcanoes such as the Yellowstone volcano area in the USA or the Taupo supervolcano in New Zealand. The huge magma chamber, located eight to 15 kilometers deep, explains the great geothermal heat beneath Tuscany. However, it is still a mystery why no major eruption has been reported from this supervolcano magma reservoir. The researchers speculate that it is due to the special composition of the magma.
Be it Yellowstone in the USA, the Campi Flegrei near Naples or Toba in Indonesia: supervolcanoes like these are the giants among the volcanic phenomena on our planet. Their magma chambers hold thousands of cubic kilometers of molten rock, their eruptions tear huge craters – calderas – underground and can cover entire continents with ash and other volcanic deposits. In addition to the caldera, which is up to a hundred kilometers in size, the only evidence of their presence is usually gas leaks, geysers and ground deformations – there is no “mountain of fire” like in normal volcanoes. However, when a caldera and typical eruption signs are missing, it can be difficult to identify such supervolcanoes – especially if they have not erupted for a long time.
Slowed waves under Tuscany
This is exactly the case in Tuscany, as a team led by Matteo Lupi from the University of Geneva has discovered. Although the so-called Tuscan Magmatic Province (TMP) is located in this region in western central Italy, no major eruptions, magma chambers or active volcanoes were known to date. Only a few smaller sites of volcanic deposits and rocks provided clues, as well as particularly high heat underground. “We have known for a long time that this region is geothermally active,” explains Lupi. In Larderello, Tuscany, this underground heat powers one of the oldest geothermal power plants in the world. But despite these indications, drilling and seismological measurements have not yet been able to detect any magma in the earth’s crust beneath Tuscany – also because there was a lack of sufficiently precise and dense seismic measuring stations.
Therefore, to study the “underworld” of Tuscany, Lupi and his team installed a network of 60 broadband seismometers in this region in 2020. The sensors recorded not only earthquakes, but also weaker tremors caused by natural processes such as ocean waves, wind or human activities. Using these background vibrations and special analyses, the researchers were able to map the travel times of seismic waves in the upper and middle crust beneath Tuscany more precisely for the first time. In fact, the evaluations showed something unusual: at a depth of around eight to 15 kilometers, the seismic waves were significantly slowed down. “The wave speed was reduced by around 1.25 kilometers per second at a depth of around ten kilometers – this corresponds to a slowdown of 40 percent,” report the geologists. “According to the common assumption, such a slow speed is due to the presence of magma or partial melting.”
Magma volume corresponds to Yellowstone and Co
Specifically, the measurement data showed: There is a huge magma chamber beneath Tuscany – an underground reservoir full of glowing liquid rock. “Our results show that the core area beneath Lardarello has a proportion of liquid melt of more than 80 percent,” write Lupi and his colleagues. “In the outer, more crystallized area, the proportion of melt is still around 20 percent.” Geologists estimate that the magma reservoir beneath Tuscany, which consists of two interconnected zones, contains a total of around 6,000 cubic kilometers of magma. “The estimated magma volume of the Tuscan Igneous Province is of the same order of magnitude as some of the largest igneous systems in the world, including the Yellowstone supervolcano, the Long Valley volcanic region and the Taupo supervolcano,” said the researchers. The heat flow measured underground in Tuscany is also comparable to that of the Yellowstone volcano. Some geophysical features of the Tuscan magma zone, such as seismic activity, gas leaks and fluid flows in the subsurface, are also similar to the active volcanic area of the Campi Flegrei near Naples.
The strange thing, however, is that unlike these well-known supervolcanoes, there are no signs of a major eruption in Tuscany. “The reason why this enormous amount of molten magma never led to an eruption there is puzzling and controversial,” explain Lupi and his team. They suspect that the special geochemical composition of the magma played a role. According to their analyses, the Tuscan rock melt must be very viscous and rather cool. “Such magmas can accumulate in the upper crust and form a viscous barrier that prevents further rise of melts,” they explain. This would explain why Tuscany has geothermal energy and hot fluids underground, but neither a caldera nor deposits from a past supervolcanic eruption. According to the researchers, this special geological structure ensures that there is no fear of an eruption of this magma reservoir in the near future.
Source: Matteo Lupi (University of Geneva) et al., Communications Earth & Environment, doi: 10.1038/s43247-026-03334-0