It is the classic image of a volcano: a tall mountain that looms over the surrounding countryside, growing progressively steeper as one approaches the summit. There, a large crater marks the spot from which past eruptions have issued.
In erupting, the stratovolcano suits the popular imagination more than ever; the same volcano could easily emit a steady stream of lava, or belch forth a cloud of ash and fire.
The stratovolcano takes its name from the layers (or strata) by which the volcano was built over time; it is also known as a composite cone volcano. It did not begin as a mountain, but built itself up over a series of eruptions. Through the natural effects of gravity, each layer solidifies more thickly near the base of the volcano than at the peak, which makes the volcano progressively steeper.
The layers result from more than just the accumulation of eruptions, however. More importantly, it is the fact that both effusive and explosive eruptions have occurred at this volcano that creates discernible layers.
Effusive eruptions are ones in which a steady flow of lava issues forth from the fissure and spreads out as it descends the mountain, eventually cooling into a hard and solid mass.
They occur when the viscosity of the magma is strong enough to keep the liquid rock together as the gases build up for an eruption. Once the crust is breached, the lava spills out in a constant stream, rather than exploding outward in comparatively small fragments.
Explosive eruptions, on the other hand, result from the fragmentation of the magma while the accumulation of gases increase the pressure from below. When the eruption finally occurs, the result is a massive plume of intensely hot gas carrying with it small pieces of magma.
The smallest pieces cool into flakes known as ash; larger pieces form into pumice or pyroclastic bombs that may or may not have solidified by the time that they land.
It is the latter category that gives stratovolcanoes their surface instability. The larger fragments of pyroclastic material tend to fall much closer to the volcano than ash, and such pieces are likely to tumble down the slope after a steady buffeting of wind and rain. And while the bulk of ash travels further from the volcano, much of it still lands upon the volcano’s outer surface.
The pieces of ash that fall so close are still hot upon landing, and stick together to create deposits known as tuff. Most of the pyroclastic layers of stratovolcanoes are comprised of tuff.
The ease with which ash can pile up and solidify into tuff explains the increasing elevation of the volcano with each eruption. A certain amount of it lands around the crater and builds up the pile ever higher. At the same time, tuff contributes to the steepness and instability of the stratovolcano’s surface.
Unlike the solid layer caused by lava, tuff is created by many pieces of volcanic rock stuck together. The erosive activity of wind and rain can easily break apart these deposits, and of course, when they break free they will fall down the slope until they settle somewhere.
Eventually, a new lava layer will spread out over the whole, cementing the pieces in place. As a result, the volcano becomes much narrower as the crater is approached.
This same characteristic leads to the comparatively short lifespans of stratovolcanoes, in geologic terms. The layers of tuff are especially prone to erosion, which means that an extinct stratovolcano can be reduced to almost nothing in only a few million years.
The stratovolcano is dynamic even by volcanic standards. It can rise quickly, it is prone to different kinds of eruptions, and once exhausted, it is also worn away quickly.