Strato Volcanoes

Jeff Jenkins, Geosystems R-7

This page has had hits.

Strato volcanoes comprise the largest percentage (approximately sixty percent) of the Earth's individual volcanoes and most are characterized by eruptions of andesite and dacite-- lavas that are cooler and more viscous than basalt. These more viscous lavas allow gas pressures to build up to high levels, therefore these volcanoes often suffer explosive eruptions. Strato volcanoes are usually about half lava and half pyroclastic material, and the layering of these products gives them their other common name of composite volcanoes. The lava of strato volcanoes occasionally forms 'a'a, but more commonly it barely flows at all, prefering to pile up in the vent to form volcanic domes. Some strato volcanoes are just a collection of domes piled up on each other. Strato volcanoes are commonly found along subduction-related volcanic arcs, and the magma supply rates to strato volcanoes are lower. This is the cause of the cooler and differentiated magma compositions and the reason for the long repose periods between eruptions. Examples of strato volcanoes include Mt. St. Helens, Mt. Rainier, Pinatubo, Mt. Fuji, Merapi, Galeras, and Cotopaxi.

Although they are not as explosive as large silicic caldera complexes, strato volcanoes have caused by far the most casualties of any type of volcano. This is for many reasons. First is that there are so many more strato volcanoes than any of the other types. Additionally, strato volcanoes are steep piles of ash and lava, and domes are often rained heavily on, shaken by earthquakes, or oversteepened by intruding blobs of magma (or all of these). This makes the likelihood of landslides, avalanches, and mudflows very high. Occasionally as well, entire flanks of strato volcanoes collapse in a process termed "sector collapse."

Another very common and deadly hazard at most strato volcanoes is called a lahar. Lahar is an Indonesian word for a mudflow, and most geologists use the term to mean a mudflow on an active volcano. Sometimes the word is reserved only for mudflows that are directly associated with an ongoing eruption (which are therefore usually hot), but that starts to make things confusing. It is probably simplest to just call any mudflow on an volcano a lahar. Lahars are so dangerous because they move quickly, and often times a small eruption or relatively small rainstorm can generate a huge lahar.