A CT scan for a volcano
Scientists have created a detailed picture of the plumbing system beneath Mount Vesuvius using a technique called seismic tomography. This method works much like a medical CT scan but uses seismic waves to see through rock. By recording the travel times of waves from micro-earthquakes and controlled explosions, researchers can map out areas where the waves speed up or slow down. These velocity changes reveal the structure deep inside the volcano.
A major research effort, the TomoVes project, found a high-velocity body extending vertically from about 400 meters below the crater down to at least 3 kilometers. This is interpreted as the solidified remnants of magma from past eruptions, forming a rigid central conduit. Deeper still, seismic models show evidence of a low-velocity zone at a depth of about 8 to 10 kilometers. This zone, covering an area of at least 400 square kilometers, is thought to be an extensive, sill-like magma chamber where partially molten rock is currently stored. This deep reservoir is large enough to fuel future explosive eruptions. There is no evidence for a large magma chamber at depths shallower than 8 km.
The volcano's breath
While Vesuvius has been in a quiet, or quiescent, state since its last eruption in 1944, it is not extinct. The volcano constantly "breathes," releasing a steady stream of gases from fumaroles (vents) inside its crater and through the surrounding soil. Monitoring these gas emissions shows the processes happening in the magma deep below.
The primary gas measured is carbon dioxide (CO2). Studies have recorded a diffuse CO2 flux from the summit area of between 122 and 139 tons per day. This steady degassing is a result of magma sitting in the deep reservoir and slowly releasing its volatile components. The low solubility of CO2 allows it to separate from the magma and rise to the surface, a process that also helps cool the magma left in the shallower conduit. The temperatures of the fumaroles at the crater rim are relatively low, between 62°C and 95°C, indicating the gases interact with a shallow hydrothermal system on their way to the surface. Spikes in the amount and composition of these gases could signal a new injection of magma into the system, making gas monitoring an tool for forecasting activity.