1. Stratovolcanoes

Dan Barker March 2009

Tongariro group, North Island, NZ

Stratovolcanoes (sometimes called composite cones)are built by repeated eruptions. Lava flows aresubordinate to pyroclastic debris in the total preservedsection. Reworked debris is also a significant component, promoted by long repose times between eruptions, highelevations, and steep slopes (commonly 15 to 33 degrees). Intrusive rocks may contribute significantly to the totalvolume of the pile. Magma compositions that buildstratovolcanoes span the entire range of silica content andviscosity, perhaps because phreatic and phreatomagmaticexplosions, independent of magma composition, are soimportant in producing the dominant pyroclastic material.The old generalization that stratovolcanoes are moresilica-rich than shield volcanoes is challenged by thefollowing two examples.

Fuji, on the island of Honshu, Japan, is a classic example of a stratovolcano,yet it has only erupted basalt.

Sierra Grande, NM, is a shield volcanobuilt of andesite.

The most notorious stratovolcano is probably Vesuvius, seen here at dawn from the center of Naples.

Seen from the Bay of Naples, Vesuvius is partly surrounded by the older Somma volcano, most visible on the left. The cone of Vesuvius has been built up following the 79 AD eruption that destroyed most of Somma.

Pico de Teide, on Tenerife, Canary Islands, is the highest point in the Atlantic Ocean basin. It is mostly phonolite.

Volcan Osorno, in the Chilean Andes, is (surprise) andesite.

Ixtaccihuatl (left) and still-active Popocatepetl (right) are easily seen from Mexico City.

Eruptions of Etna, Sicily, have been described for at least 3400 years.

Mts Rainier, St Helens, and Adams arepart of the Cascades arc in Washington.

Stromboli, off the north coast of Sicily, has also been active for millennia of recorded history.

Here, new gray ash accumulates on the slopes of Stromboli.

The combination of steep slopes and weak pyroclastic layers makes the flanks of Stromboli prone to collapse.

The following examples in the Cascade arc of the Pacific Northwest show some dangerous stratovolcanoes. They present hazards that may not necessarily follow eruptions, but just the sudden collapse of snow fields and glaciers causing floods and mudslides.

Mt Shasta, CA

Mt Hood, OR

Mt Rainier, WA

Mt Baker, WA

Mt Garibaldi, BC

Mt St Helens, WA, viewed from the south.

Mt St Helens in another unfamiliar view, from the NE. It is difficult to judge the relative size of this volcano (Lewis and Clark thought it was the highest peak in North America).

Just to put sizes in better perspective, here is a view of Shasta with its satellite cone, Shastina, on the right. AND…

Shastina

Shastina (here on the left) has a larger volume than Mt St Helens.

Here is a beautiful little stratovolcano, Augustine, AK, as photographed in 2002.The summit isn't there anymore; it was destroyed by a 2005-2006 eruption.

This is the island Vulcano, from which volcanoes take their name. The active cone, Fossa di Vulcano, is small enough to be ranked as a tuff cone, except for one important fact; it has had many eruptions.

The mostrecent eruptionof Vulcano wasIn 1888-1890. Atthe time, minerswere working in the crater, scraping sulfur and alum from the fumaroles.They are missing and presumed dead.

Dark ash from the 1888-1890 eruptionsis eroding rapidly from Vulcano, and ina few centuries there may be no evidenceof this explosive activity.

As this slide show was being prepared, Mt Redoubt in Alaska was showing signs of an impending eruption. The last was in 1985.