Italy has some of the most famous volcanoes in the world: Vesuvius, Stromboli, and Vulcano all lie in a chain along Italy’s western coast. Scientists have found that these volcanoes are all intricately linked to the subduction of the Ionian Sea beneath southern Italy, Calabria, and Sicily.
An oceanic plate contains rocks that have a lot of water in them (not surprisingly). This water is not just sitting in the pore space of sediments, but it is bound into the crystalline structure of the minerals that make up the oceanic crust, as water.
When the oceanic plate reaches depths of about 100 kilometers during subduction, temperatures and pressures become large enough that the water bound in the minerals becomes unstable and is released into the mantle. Water enters the mantle (where no water was before) and lowers the melting temperature of the mantle rocks, so small amounts of rock begin to melt where an oceanic plate is subducting.
This melt then rises through the crust and generates volcanoes at the surface.
These are known as subduction volcanoes, or arc volcanoes, and every active subduction zone has a chain of volcanoes generated by the addition of water to mantle. For example, in Japan, the Pacific Plate is subducting under Asia; in Chile, the Pacific Plate is subducting under the South American plate; and in the northwest United States, the Juan de Fuca Plate is subducting beneath the North American Plate (creating volcanoes like Mount St. Helens).
Although subduction volcanoes dominate Italy’s west coast, it’s largest and most active volcano is not related to subduction. Mount Etna is located in eastern Sicily and stands over three kilometers (11,000 feet) above the ocean. It is one of the most active volcanoes in the world, spewing ash, lava, and gas nearly as often as Mauna Loa in Hawai’I (which erupts, on average, every 3.5 years).
So how do scientists know that it is not an arc volcano, even though it is so close to a subduction zone?
The chemistry of the lavas.
Geochemists analyze the chemical make-up of lavas erupted all over the world to determine their origin. For example, magnesium and iron are found deep in the mantle while potassium and quartz are only found in the crust. Mount Etna’s lavas are rich in Magnesium and Iron, but also have a lot of potassium.
So where is the lava coming from? We are collaborating with geochemists at the National Institute of Geophysics and Volcanology (INGV) in Catania, Sicily to try to figure out just that. Tomorrow we climb up Mt. Etna to look at its most active caldera (responsible for lava flows in 2008 and explosions earlier this year) to learn about Etna’s history and talk about why this immense volcano is even there.