Strombolian eruptions are mildly to moderately explosive eruptions characterized by pulsating or intermitted short-lived, discrete explosions that can go on years, centuries, or millennia. They have a volcanic explosivity index (VEI) of 1–2 or up to 3 for the violent ones. The most explosive eruptions are Plinian and have an VEI of 5-8.
This volcanic eruption style is named after the Strómboli volcano in Lipari, or Aeolian Island, in the northern end of Sicily, Italy, which best characterizes it.
For the past 2400 years, Strómboli volcano has erupted nearly continuously. These nearly continuous, pulsating eruptions are characterized by mild to moderate explosions.
Greek and Roman sailors called it the lighthouse of the Mediterranean due to its remarkable, fireworks-like display as they erupt.
Lastly, Strombolian volcano activities are among the most widely studied, monitored, and documented because they persist and are mildly to moderately violent.
Magma composition
Strombolian eruptions form from mafic or intermediate magmas such as basalt and basalt andesite. These magmas have relatively low viscosities of 10-104 Pascal-seconds and are high in volatiles.
The volatiles present in these magmas are mostly water and carbon dioxide and account for a few percentages of the magma composition.
Strombolian eruption description
They are mildly to moderately explosive eruptions characterized by pulsating, discrete explosions. Each intermittent or discrete explosion will last less than a second to several hours, and those that occur close to each other can appear continuous.
While they are intermittent, they can go on for years or thousands of years.
Their volcanic explosivity index (VEI) is 1-2 or 3 out of 8, and each eruption will produce 103-104 kg of pyroclasts.
A typical eruption will eject red, glowing, or incandescent pyroclasts made of mostly lapilli, cinders, scattered blocks, bombs, and a small amount of ash. It will produce loud bangs and spew thousands to millions of cubic meters of ejecta.
At night, Strombolian eruptions will display a spectacular, worth-watching incandescent trail of pyroclasts making parabolic paths before cascading downward on the volcano flanks.
Strombolian eruption jets move at 50-100 m/s, but some go at supersonic speeds that exceed 400 m/s. These jets or plumes of pyroclasts go tens to hundreds of meters above the surface and will deposit tephra up to a kilometer away.
Very violent ones can, however, have plumes going a few kilometers above the Earth’s surface and will deposit tephra several kilometers from the vent.
Since they deposit tephra close to their vent, Strombolian eruptions create cinder or scoria cones. These cinder or scoria cones are steep, small hills, tens to hundreds of meters high, made of mostly cinders. A repeated eruption can, however, make cinder cones taller.
Lastly, Strombolian eruptions can sometimes produce lava flow, especially towards the end of eruption if the magma degasses. A decrease in the volatile content of erupting magmas can also result in lava flows.
Violent Strombolian eruptions
Violent Strombolian eruptions have a volcanic explosivity of 3, are uncommon, and occur once in a few decades. They form when magma rises faster and more gas bubbles coalesce, producing more intense explosions that last seconds to minutes.
These eruptions produce more sustained eruption columns or volcanic plumes, which are sometimes convective. These plumes go beyond a kilometer above the earth’s surface and contain mostly lapilli and ash. However, they also produce scattered blocks and volcanic bombs.
Finally, a collapse of the ash plume from violent types can cause minor pyroclastic density currents.
How do Strombolian eruptions form?
Strombolian eruptions form from low-viscosity magmas that are high in volatiles. These magmas rise at speeds of about 0.5 m/s and erupt at 1000–1200 °C (1832–192 °F) temperatures.
The formed bubbles will rise faster than the rest of the magma body due to its low viscosity. As they rise, the gas bubbles will also coalesce, forming larger bubbles or slugs.
The large gas bubbles will intermittently arrive near the surface of the erupting magma body and burst, blasting incandescent pyroclasts into the air.
These pyroclasts are mostly made of vesicular lapilli and cinders but will also have a small amount of volcanic bombs, blocks, and ash.
Can they transition?
Yes, changes in magma viscosity, ascend rates, or content of volatiles can transition Strombolian eruptions to Hawaiian or Vulcanian eruptions. Hawaiian type are less explosive, and vulcanian are more explosive.
Higher magma ascent rates, volatile content, and viscosity will favor a transition to vulcanian eruptions, while lower ones will form the Hawaiian kind.
Volcanoes like Tungurahua in Ecuador and Pacaya in Guatemala have previously shown transitions.
Strombolian deposits and pyroclast characteristics
Strombolian eruptions are mild to moderate and involve lower energy. They will poorly fragment pyroclasts, forming moderately vesicular coarse cinders and some glassy scorious lapilli.
These eruptions will also produce a few twisted, fluidal, or spindle-shaped bombs and a small amount of volcanic ash. Lithic materials, including blocks, are less than 5% of the total pyroclasts produced.
Most pyroclasts are 1-10 cm, but their size can range from 0.001 cm to more than a meter.
Additionally, these pyroclasts may sometimes contain crystals. These crystals include phenocrysts, which are about a centimeter in size, or tinier ones like microlites and nanolites.
Microlites and nanolites are common in eruptions high in ash, such as the ones that occur at Mt. Etna in Italy.
Sideromelane and trachyte are basaltic glasses that may also form during Strombolian eruptions.
Sideromelane is a yellow to brownish transparent basaltic glass that forms when ice or water quenches basaltic magma quickly. A slightly slower quenching will form tachylite, a black or brownish opaque basaltic glass. Recycled or ash that falls back into the volcano will form tachylite
During eruptions, the larger pyroclasts, like bombs and lithic blocks, will fall closer to the vent and smaller ones further away.
Lastly, most pyroclasts formed during Strombolian eruptions solidify while in flight in the air, forming unwelded tephra. Larger magma clots or volcanic bombs may, however, hit the ground while still semi-molten, forming cow pancake bombs or blocky bread crusts.
What are the examples of Strombolian eruptions?
These volcanic eruptions are common in all geodynamic settings with low- to moderate-viscosity magma like basalt and andesite.
They can occur on central vents, flanks of both monogenetic and polygenetic volcanoes, or even on calderas.
Examples of monogenetic Strombolian volcanoes are Parícutin and Jorullo in Mexico, Sunset Crater, and Lathrop Wells in the USA.
On the other hand, polygenetic shields and stratovolcanoes like Mt. Etna and Mt. Stromboli in Italy, Ruapehu in New Zealand, and Villarrica in Chile have Strombolian eruptions.
Latera Caldera in Italy, Izu Ōshima in Japan, and Tengger caldera in Indonesia are some of the calderas that erupt like Stromboli.
To summarize, some of the popular Strombolian eruption examples are:
- Pavlof volcano in Alaska, Sunset Crater in Arizona, and Lathrop Wells in Nevada, USA
- Stromboli, Latera Caldera, and Mt. Etna (11974-1993) in Italy
- Mount Erebus in Antarctica
- Fuji, Sakurajima, and Izu Ōshima in Japan
- Sangay volcano and Ruapehu in New Zealand
- Fuego and Pacaya volcano in Guatemala
- Sangay and Tungurahua in Ecuador
- Askja and Heimaey volcanoes in Iceland
More volcanoes that show Strombolian style of eruption are Arenal in Costa Rica, La Palma in the Canary Islands, Yasur in Vanuatu, Barren Island Volcano in India, Manam in Papua New Guinea, Parícutin in Mexico, and Croscat in Spain.
Are Strombolian eruption hazardous?
Not really. Since they are less explosive, Strombolian eruptions are not very dangerous, and people live a few kilometers downslope, including at Mt. Stromboli in Italy.
You can also watch or take pictures of these eruptions from a safe distance. You should, however, keenly observe explosion rhythms, as they can turn more violent.
If they turn violent, they can eject pyroclasts to a greater distance and possibly hit and injure or kill onlookers.
Lastly, collapse of cinder cones can cause tephra to tumble downslope, making them potentially hazardous.