Strombolian eruptions are relatively low-level volcanic eruptions, named after the Sicilian volcano Stromboli, where such eruptions consist of ejection of incandescent cinder, lapilli and lava bombs to altitudes of tens to hundreds of meters. They are small to medium in volume, with sporadic violence.
They are defined as "...Mildly explosive at discrete but fairly regular intervals of seconds to minutes..."
The tephra typically glows red when leaving the vent, but its surface cools and assumes a dark to black color and may significantly solidify before impact. The tephra accumulates in the vicinity of the vent, forming a cinder cone. Cinder is the most common product, the amount of volcanic ash is typically rather minor.
Instead the gas coalesces into bubbles, called gas slugs, that grow large enough to rise through the magma column, bursting near the top due to the decrease in pressure and throwing magma into the air. Each episode thus releases volcanic gases, sometimes as frequently as a few minutes apart. Gas slugs can form as deep as 3 kilometers, making them difficult to predict.
Strombolian eruptive activity can be very long-lasting because the conduit system is not strongly affected by the eruptive activity, so that the eruptive system can repeatedly reset itself. For example, the Parícutin volcano erupted continuously between 1943-1952, Mount Erebus, Antarctica has produced Strombolian eruptions for at least many decades, and Stromboli itself has been producing Strombolian eruptions for several thousand years.
See also 
- explanation with photos on academic site
- comparison of volcanic blast types
- USGS Photo Glossary
- Modelling of Volcano for CG/CFD Papers with abstracts, images, and PDFs of modelling of Strombolian (explosive) volcano for computer graphics (CG) and computational fluid dynamics (CFD). The main entrance page is here.