Stromboli is one of the four active volcanoes in Italy, located on a small island in the Tyrrhenian Sea, north of Sicily, within the Aeolian Islands volcanic arc. It has been in a state of near-constant, mild activity for at least 2,000 years, making it one of the most consistently erupting volcanoes on Earth. Its frequent nighttime explosions of glowing material earned it the nickname, the “Lighthouse of the Mediterranean.” The volcano’s total height is over 3,000 meters when measured from the seafloor, though it rises 924 meters above sea level.
Stromboli’s Structural Classification
Structurally, Stromboli is classified as a stratovolcano, also commonly referred to as a composite volcano. This classification refers to the physical build and conical shape of the mountain itself, which develops over thousands of years from successive eruptions. It is a large, steep-sided volcanic edifice that extends deep beneath the sea surface. Its high elevation and symmetrical, steep profile are typical of this structural category.
Anatomy of a Stratovolcano
Stratovolcanoes are characterized by their conical shape and steep profile, which contrasts with the broad, gently sloping forms of shield volcanoes. They are built up from alternating layers, or strata, of hardened lava flows, volcanic ash, tephra, and volcanic bombs. The term “composite” reflects this layered construction of different materials. The magma that forms these volcanoes is typically more viscous, which prevents it from flowing great distances before cooling. This higher viscosity allows gas pressure to build up significantly, which contributes to the more explosive eruption styles often associated with stratovolcanoes. Lavas associated with this structure are often intermediate in composition, such as andesite, but Stromboli’s persistent activity involves basaltic to basaltic-andesite magma.
Defining Strombolian Eruptions
While Stromboli is a stratovolcano, it is most famous for the distinct eruptive behavior named after it: Strombolian activity. This style is characterized by mild, rhythmic, and discrete explosive bursts, which typically have a Volcanic Explosivity Index (VEI) of 1 or 2. These explosions occur at regular, short intervals, often every few minutes to an hour. The activity involves the ejection of incandescent material, such as glowing cinders, lapilli, and volcanic bombs, which arc through the air to heights of tens to a few hundreds of meters.
The mechanism behind this rhythmic behavior involves the regular release of gas from moderately viscous, basaltic magma within an open conduit. Gas bubbles, or slugs, rise and accumulate beneath a crust of cooling magma near the vent. When the pressure from the coalesced gas bubble exceeds the strength of the overlying magma, the bubble bursts, propelling the rock fragments into the atmosphere. This process is cyclical and does not usually produce a sustained, tall column of ash, but rather firework-like rooster-tails of glowing pyroclasts. The consistency of this activity is why Stromboli is classified as an open-conduit system.