Composite Volcanoes: Often Snow-Capped Cones Built by Many Eruptions, Rainier to Redoubt

Composite volcanoes are probably the most complex type of volcano, and can be the most picturesque. A classic one is a cone with a concave profile, steeper near the top, rising high above the mountains around it, often with a snow-covered summit. This National Park Service article explains how they work, and where in the National Park System they stand.

Iliamna Volcano in Lake Clark National Park (AVO/USGS, R. G. McGimsey).

What they are

Composite cones are large, many thousands of feet or meters tall, made of lava flows, pyroclastic deposits and mudflow (lahar) deposits, plus lava domes. They stay active for tens to hundreds of thousands of years and erupt from time to time. They are also called stratovolcanoes, though some geologists avoid that name because these volcanoes are typically not stratified and not made of orderly layers.

They are the most complex because they erupt many times, with lavas of many compositions and many kinds of eruption. Most have a main vent at the summit, and some have several. An active one usually has a shallow magma chamber 3 to 6 miles (5 to 10 km) down. They are built up by successive eruptions of domes, lava flows and pyroclastic flows, but can also lose their summits in large blasts, such as the May 1980 explosion and landslide at Mount St. Helens. Like all mountains, they erode, and areas of hydrothermally altered rock and loose pyroclastic deposits make them prone to landslides, rock avalanches, lahars and debris flows.

In the National Park System

At least eight units of the system contain composite volcanoes:

• Mount Rainier is active and the tallest peak in the Pacific Northwest. It is considered the most dangerous volcano in the Lower 48, with destructive lahars the greatest geologic hazard.
• Lassen Volcanic National Park holds Brokeoff Volcano (Mount Tehama), a deeply eroded composite volcano of lava domes, lava flows and pyroclastic deposits between 590,000 and 385,000 years old.
• Lake Clark National Park has two active composite volcanoes, Redoubt and Iliamna.
• Katmai National Park has several, eight of them active. The most recent eruptions in the park were from Trident Volcano between 1953 and 1974.
• Aniakchak National Monument has Vent Mountain and Half Cone, which formed within Aniakchak Caldera after the caldera-forming eruption 3,700 years ago.

The others on the NPS list are Crater Lake, Wrangell-St. Elias and Yellowstone.

Left: Brokeoff Volcano (Mount Tehama) in Lassen Volcanic National Park, California. Right: its projected profile, which would have risen to 11,000 ft (3,300 m) (U.S. Geological Survey photographs by Michael A. Clynne).

Vent Mountain, a small composite volcano in Aniakchak Caldera, made of andesitic and dacitic lava and scoria (USGS photo by Game McGimsey).

How they are built

Left: a schematic of a composite volcano. Right: the diagram with feature labels (modified from a USGS illustration).

Composite volcanoes usually erupt a range of magmas from basalt to rhyolite, though intermediate (andesitic) and dacitic ones are most common. Mount Rainier has less variety than many, with mostly andesitic lava flows and lahar deposits.

Eruptions can be phreatic, phreatomagmatic or magmatic. Phreatic and phreatomagmatic steam eruptions are especially common on snow-covered or glaciated peaks and may come before later magmatic eruptions. Magmatic ones range from gentle outpourings of lava to extremely explosive and reach up to 5 on the Volcanic Explosivity Index; Vulcanian, Sub-Plinian and Plinian eruptions are typical. Larger caldera-forming eruptions can end a composite volcano's life, as when Mount Mazama erupted 7,700 years ago and left the caldera that became Crater Lake.

Where they occur

The Pacific Ring of Fire. Active volcanoes sit above subduction zones at convergent plate boundaries (image from the NPS page on composite volcanoes).

Composite volcanoes are usually found above subduction zones, as along the Pacific "Ring of Fire". The High Cascade volcanoes of Washington, Oregon and California formed as the Juan de Fuca plate slid under the North American plate; Mount Rainier and Mount St. Helens are among the active ones. Subduction of the Pacific plate under the Alaska Peninsula and Aleutian Islands has produced the greatest concentration of active composite volcanoes in North America, including those in Lake Clark and Katmai.

Features on a composite volcano

• Craters and vents. A summit crater, usually blasted out by explosive eruptions, may hold fumaroles, a crater lake, glacial ice or lava domes, such as the dome that grew during Redoubt's 2009 eruption. Rainier's summit crater is filled with glacial ice, pocketed with ice caves melted by active fumaroles. The main vent can move over time, producing complex volcanoes such as Trident in Katmai, and flank vents may build cinder cones, maars, tuff rings or domes.
• Fumaroles. These vents release gases and vapors where shallow magma or hot deposits give off gas, or where groundwater or melted ice is heated to steam. Lassen has exceptional examples at Sulphur Works; Rainier has active ones near its summit, and so do Trident, Mount Griggs and Fourpeaked in Katmai.
• Domes. Lava domes of silicic (dacitic to rhyolitic) lava often grow in summit craters and on flanks. Magma that intrudes shallowly without erupting, a cryptodome, can make a flank bulge.
• Lahars. These volcanic mudflows can occur during eruptions or be set off by heavy rain, unusually rapid melting of snow or ice, landslides, ash falling on snow or glaciers, or subglacial eruptions.
• Pyroclastic flows. Fast-moving hot currents of pumice, ash, blocks and gas that can start when an eruption column collapses, when an explosive eruption boils over or when a lava dome collapses. Their deposits are important parts of most composite volcanoes.

A fumarole at Sulphur Works, Lassen Volcanic National Park (NPS photo).

A dome growing in the summit crater of Redoubt Volcano, Lake Clark National Park, in May 2009 (USGS Alaska Volcano Observatory).

Hazards

Composite volcanoes bring many geohazards, in eruption and in dormancy. Mount Rainier is the most dangerous volcano in the continental United States because it is prone to lahars and close to a large population center, the Seattle area.

Mount Rainier over the Puyallup Valley, near Orting, Washington (USGS photo by Ed Ruttledge).

Sources

• National Park Service, Composite Volcanoes (Stratovolcanoes), NPS Geologic Resources Division; photographs credited to the USGS, AVO and NPS; rewritten in hubnx's own words.