Stratovolcanoes are some of the most notable, visible, famous geographic forms in the World. In the United States, such mountains as Rainier and St. Helens, Shasta, Hood, Redoubt, Kilauea and Maui are widely known through photographs. Worldwide, Mt. Fuji, Mt. Kilimanjaro, Mt. Etna, Mt. Ararat and Mt. Popocatepetl are a few well publicized peaks. Their massive size, height and obvious shape set stratovolcanoes completely apart from all other volcanic processes.
The U.S. Geological Survey has an excellent website with extensive information focused on Cascade Mountain volcanoes. Oregon State University maintains Volcano World, a fun resource for looking up specific volcanoes and learning about their features. More than half of the volcanoes on Earth are stratovolcanoes, and they are notoriously dangerous to lifeforms and property.
Stratovolcanoes are massive, cone-shaped, inverted funnels composed of many different materials with at least one, and possibly more, vents down through the mountain that allow molten rock, or magma from beneath the Earth's crust, to rise up through the mountain and become lava. Sometimes referred to as "composite" volcanoes, they are made up of layers, or "strata" (hence stratovolcano), from solidified and molten (1) lava, (2) tephra and (3) ash.
1. Generally, the lava from stratovolcanoes is viscous, or thick and slow moving, so it solidifies quickly when ejected from the interior and exposed to air or water and ice. When the thick lava cools suddenly, it leaves a plug or core in the central vents and then dikes of solid rock fill the side vents or other fissures in the mountain. Often, the solidified lava is basalt, although many stratovolcanoes have higher levels of silica in the magma that forms their lava, so the rock composites will be made up of andesite or dacite. A few lava flows will be rhyolite, which can form glass-like obsidian on the surface. For most stratovolcanoes, the solid lava will produce domes that cap the columns of lava in the vents. Very deep, at the base of the mountain and below, it is possible there may be liquid lava, or even magma, pushing upwards against the layers of other materials.
2. The tephra is composed of material that has been forcefully ejected into the air and then fallen back onto the sides of the volcano and the surrounding area. Tephra can be huge chunks of solid lava or basalt or granite blocks larger than 65 mm (about 2.5 inches). It also contains small blasted pieces of lava, fragments of other rock types, or cinders made from other material that has been superheated and cooled as it falls back to the surface. The smaller materials are collectively known as "lapilli" and range from 2 to 64 mm in size. Volcanic bombs are a type of tephra similar to the blocks. The bombs are also large pieces (more than 65 mm) that were blown into the sky as molten material, which then cool into odd shapes on the way back to the ground or become unusual shapes on impact. There are many, many forms of tephra such as pumice, scoria, etc., which may be part of some stratovolcanoes, but not necessarily others.
3. Fragmented volcanic products less than 2 mm in diameter are called "ash", which often lies at the surface on the flanks of the stratovolcanoes. Ash conglomerates that melt together and form a larger structure are called "tuffs."
All of these extruded and ejected materials become dangerous as a stratovolcano builds over the centuries, or millenia, of eruptions. The thick, viscous lava can form a dome, along with cores and plugs, as well as layers of rock on the sloping sides of the mountain. These heavy materials preclude the escape of gases and hold molten lava and magma down beneath the crust. Pressures build. At some point the stratovolcano will explode, blasting all the various composite materials into the atmosphere. That suddenly makes these beautiful, picture-perfect mountains deadly dangerous to humans and other lifeforms that are near any active stratovolcano.
Even the lighter weight ash materials can become extremely dangerous. Most stratovolcanoes are high enough, more than 2,000 meters (6,500 feet), to retain snowpack, ice and/or glaciers. When the volcano erupts, the hot gases and molten lava melt the frozen material which combines with the ash to generate massive mudflows, often named the Indian term "lahar." A lahar may be a highly fluid liquid, that flows like a stream, or so thick that it's much like concrete. The fast-flowing lahars can do a great deal of damage at considerable distances from the volcano itself.
Although actual caldera-type volcanoes are different features, some stratovolcanoes have formed a caldera at the peak or on the steep flank because of a dome collapse or molten material falling into the cavity created by an explosive eruption. For Oregon's Mt. Mazama, the caldera is more famously known as Crater Lake.
Somewhat similarly, Mt. St. Helens began forming a caldera after the May 18, 1980 eruption. Continued dome-building activity, release of gases, steam venting, seismic rumblings, etc., in the St. Helens caldera allow volcanologists to observe and learn a great deal about this process. Study of the St. Helens activities indicate the possibility of a "super volcano" lying under the visible stratovolcano. It is possible that the mountain is only the tip of an incomprehensibly huge volcanic system. [This new knowledge has led to observations about other volcanic activity around the planet and some very startling theories about super volcanoes and the presence of such geologic structures as the "Yellowstone caldera" (Nat'l Geographic) a sequence of three overlapping explosive remnants in Wyoming, Idaho and Montana.]
One of the most dangerous features of stratovolcanoes is the potential for pyroclasm when a mountain erupts. There are pyroclastic flows and surges which have important differences. A flow is combined gases and the composite materials, mainly high-density tephra, from the stratovolcano that blast out of the vent(s), with temperatures above 100 to more than 800 celsius, and move at incredible speeds (50 kmh or more, up to an estimated 700 kmh) scorching and pummeling everything in the way, with sheer wind forces and particle impacts leveling the landscape.
A surge is lower density, made up of heavier composites, and may be less than 100 celsius (cold surge) or well above that temperature (hot surge). A surge tends to travel regardless of topography, where a flow normally goes down the flanks of the stratovolcano and stays within the forms of valleys and river channels. One of the most dangerous features of pyroclastic blasts is that sulphurous materials may combine with the snow, water or ice and produce sulphuric acid droplets which can devestate an area.
All in all, then, stratovolcanoes are some of the most stunning geologic features on the planet, and they are created by forces of incomprehensible magnitude. These large mountains are formed beneath the Earth where tectonic plates push against each other, generating fissures that allow magma to extrude from below the mantle. Examining plate tectonics, there is a "Ring of Fire" (Universe Today) around the Pacific Rim that holds a majority of the World's volcanic activity and supervolcanoes.
Wherever located, though, supervolcanoes are lessons about the incredible geologic activity of planet Earth and the tremendous forces that take place beneath the crust that we live on.