Stratovolcanoes (composite volcanoes) are steep-sided, conical volcanoes. They are formed by many layers of hardened lava flows, pyroclastic flows and lahar flows. Stratovocanoes are most often formed on continental land, and have gentle slopes at the base that gradually rise until they reach their steepest point near the summit. At the summit area, stratovolcanoes develop a bowl-shaped crater with a centered vent. The shape of a stratovolcano may vary based on the eruptive materials and composition.
The composition of a stratovolcano may vary from basaltic lava rock to rhyolite, which is more felsic, meaning it has higher silica content; therefore, it is much thicker than basaltic lavas. However, stratovolcanoes are most commonly andesitic, which is an intermediate lava type between basaltic and rhyolitic lava. Many marine stratovolcanoes are more basaltic than stratovolcanoes found on continental lands. Mt. Etna and Mt Fuji are composed of basaltic lava flows; Mt Rainier is composed of andesitic lava, whereas, Mt. St. Helens in Washington is of intermediate Silica content type of lava which varies in between andesitic and dacitic lava. Extended felsic lava flows are not very common; however, felsic lava, on occasions has displaced itself for an approximate 10 km (6 miles).
How they are formed
Stratovolcanoes are formed by stratified layers made of alternating lava flows, volcanic mudflows, pyroclastic flows and tephra. During their process of formation, the higher silica content in lava ejections causes lava to flow sluggishly and solidify faster than low viscosity lava. In every eruption, the andesitic composition of lava provides a dense stratified layer over which successive lava layers will adhere. Over time, this process creates a steeped-cone shaped stratovolcano. The ejected lava accumulates more at the rims of the volcanic vent than at the further boundaries of the vent. Over time, a stratovolcano develops a gentle slope at the base, which increases gradually until reaching the summit.
Where they are found
Stratovolcanoes usually form at the point of convergence of two tectonic plates, where one tectonic plate submerges underneath another plate at a subduction zone. Instances of stratovolcanoes can be found throughout the world, although, they are particularly more abundant in the Ring of Fire, a region of high volcanic activity in the Pacific Ocean. The Ring of Fire forms a circular ring along the north Pacific, extending south along the coast of the American continent all the way to south America, then along the south Pacific and encompassing the New Zealand, New Guinea, the Philippine Islands and the Aleutian Islands.
Stratovolcanoes are characterized by explosive Pinian eruptions, which produce powerful convecting plumes of ash, rising more than 45 km (28 miles) into the sky. These dramatic eruptions are often associated with pyroclastic flows, which consist of toxic gases and extremely hot volcanic fragments. Pyroclastic flows descend down the slopes of a volcano, reaching high speeds. Unlike shield volcanoes, stratovolcanoes explode between intervals of hundreds of years. Most stratovolcanoes could be 100,000 years old; however, some others, such as Mt. Rainier, could be over 1 million years of age.
The eruptions of volcanoes most of the times cause tremendous destruction. Pyroclastic flows sometimes can move at speeds of 100-200 km/h (60-120 miles/h). Lahars (mixtures of rock and debris) can flow down the slopes of a volcano at speeds of 100 km/h (60 miles/h). Moreover, an eruption can explode with more power than that of an atomic bomb. Mt. St. Helens which is a stratovolcano is one of the most active volcanoes, and one of the most monitored. According to pbs.org, the blast produced by Mt. St. Helens in 1980 was 400 times more powerful than that produced by an atomic bomb.