What are Stratovolcanoes

“Fuji alone

Left uncovered by

The new leaves.” 

Kyoto poet-painter Buson wrote those lines about Mount Fuji.  A well-known symbol of Japan, Mount Fuji rises over 12,000 feet in a nearly symmetrical cone, snow-capped, gentle slopes rolling away to the countryside.  Mount Fuji is a stratovolcano, also known as a composite volcano or composite cone. 

Stratovolcanoes form where one of the earth’s tectonic plates slide beneath another, most commonly on the Pacific rim known as the “Ring of Fire”.  They are the most explosive of volcanoes, though they erupt infrequently, usually only every several hundred years or so. 

The most famous, and arguably most dangerous, stratovolcano in the world is Mount Vesuvius in Italy.  Mount Vesuvius killed tens of thousands when it erupted and buried Pompeii and Herculaneum in 79 AD.  Other well-known stratovolcanoes are Mount St. Helens which erupted in 1981, Mount Kilimanjaro in Africa, Mount Ranier in Washington state, Mount Redoubt in Alaska, Mount Mayon in the Phillipines, Mount Edzizza in British Columbia and Mount Llaima in Chile.  Despite being within the Pacific’s Ring of Fire, Hawaii’s volcanoes are classified as shield volcanoes not stratovolcanoes.  Many of the world’s most beautiful mountains are stratovolcanoes.  Stratovolcanoes are some of the most symmetrical mountains in the world, standing tall and beautiful, often visited and photographed. 

Mount Fuji shows the classic shape of a stratovolcano, likely what most people would consider a classic-looking volcano.  The gently sloping sides at the base are formed by repeated eruptions and lava flows.  The lava is thick, cooling and hardening quickly.  The summit rises steeply and often has a small crater. 

 The crater at the summit acts as the opening to a central vent.  Lava, volcanic rocks and ash erupt from this central vent, accumulating (and building) on the slopes.  Fissures in the slope of the volcano can also vent lava, though often it hardens within the fissures forming hard rib-like channels which strengthen the volcano.

 If a cross-section of a volcano were to be taken, it would look like layer upon layer of different kinds of rock.  These are formed by the lava flows, tephra, pyroclastic flows, volcanic mudflows and debris flows.  The rock types can vary from basalt to rhyolite (quartz and feldspar that may look like granite).  The airborne material that erupts from a volcano (ash, rock fragments) is called pyroclast.  Once that pyroclast falls to the ground, it is known as tephra.  The largest fragments (boulders and large rocks) remain closest to the summit with the spread of tephra directly related to the size of the material.  Ash can spread for thousands of miles on the prevailing winds.  Oftentimes, pyroclasts are hot enough to fuse together into pyroclastic rock (tuff).  When they flow quickly downhill (commonly up to hurricane speeds) with hot gases, it is a pyroclastic flow, and can be particularly devastating to towns and villages downhill from the flow as it is nearly impossible to outrun a pyroclastic flow.  The majority of deaths from the eruption of Mount Vesuvius in 79 AD were caused by pyroclastic flows and the falling ash.   A volcanic mudflow, also called lahar, is basically pyroclasts and water.  The water typically comes from one of three sources:  1) snow or glaciers melted by the lava, 2) a flood caused by a glacier, lake breakout or heavy rains, or 3) water from a crater lake.  Lahars travel significantly slower than pyroclastic flows but can still move as quickly as a fast-moving stream. 

Occasionally, the magma chamber within a volcano is completely emptied in a violent eruption.  The weight of the cone causes it to collapse into itself, forming a caldera.  Crater Lake in Oregon is an example of a caldera which has filled with water.  Wizard Island in the center formed from other smaller eruptions after the caldera formed, creating the island.  Other water-filled calderas are Lake Toba on Sumatra in Indonesia and Lake Pinatubo on Mount Pinatubo in the Phillipines.   The caldera on Mount  Aso in Japan is one of the largest in the world.  

Stratovolcanoes remain some of the most beautiful geological structures on Earth.  Tourists visit dormant ones by the hundreds of thousands every year, marveling at their beauty and potential danger.  At their summits and calderas, volcanologists study and research how these giants form and erupt, to better understand them for future eruptions.  


 Geology Department – San Diego State University

 USGS Cascades Volcano Observatory