Shield Volcanoes

Shield volcanoes contrast with stratovolcanoes both in origin and morphology. Their name derives from their low-angle profile, which resembles the broad shields used by Hawaiian warriors. They are formed primarily by the successive accumulation of fluid lava flows, which descend from summit or flank fissure systems. Although shield volcanoes are not as visually dramatic as stratovolcanoes, they are often much larger features. Oceanic shield volcanoes such as those in the Hawaiian Islands can rise as much as 8000 m above the surrounding sea floor and 12,000 m above their actual bases, which have sagged due to the immense mass of the volcano. Their volumes can exceed that of stratovolcanoes by several orders of magnitude. Most shield volcanoes are formed of fluid basaltic lava flows. Shield volcanoes can vary widely in size, and much smaller Icelandic-type shield volcanoes are common in many volcanic regions. These volcanoes can form during single long-term effusive eruptions. A less common type of shield volcano is a pyroclastic shield, whose broad low-angle slopes are formed by accumulation of fragmental material from powerful explosive eruptions.

Photo by Jim Cole (University of Canterbury).

Auckland Field

Rangitoto shield volcano, the youngest volcanic center of New Zealand's Auckland volcanic field, forms a 5.5-km-wide island. The volcano, seen here from the NW, erupted about 600 years ago and is capped by a scoria cone containing a deep crater. The 140 sq km Auckland volcanic field contains more than 50 maars, tuff rings, and scoria cones. Of the 19 eruptions known to have occurred during the past 20,000 years, only Rangitoto has erupted during the Holocene.

Photo by Chris Nye (Alaska Division of Geological and Geophysical Surveys).

Wrangell

Alaska's Mount Wrangell is one of the most voluminous andesitic shield volcanoes in the world and is more than three times the volume of Mount Rainier. The massive volcano has a diameter of 30 km at 2000 m elevation and a volume of about 900 cu km. Eruption of unusually fluid andesitic lavas at high eruption rates produced long lava flows that contributed to its low-angle shield morphology. The small snow-covered peak on the left is Mount Zanetti, a flank vent about the same volume as St. Helens. Wrangell is seen here from Glenallen, 80 km to the west.

Photo by Lee Siebert, 1995 (Smithsonian Institution).

Belknap

Little Belknap (upper left) is an example of a small shield volcano in a continental margin setting. Little Belknap was constructed on the east flank of Belknap volcano and spread fresh-looking lava flows over the McKenzie Pass area of the central Oregon Cascades about 2900 years ago. Collapsed lava tubes that fed the flows diverge radially away from the summit. The summit pinnacle of Mount Washington appears at the right beyond the Little Belknap lava apron.

Photo by Lee Siebert, 1995 (Smithsonian Institution).

Newberry

Massive Newberry shield volcano covers an area of about 1600 sq km about 60 km east of the crest of the Cascade Range in central Oregon. The elongated, low-angle shield volcano covers an area of 60 km in a N-S direction and 30 km E-W. More than 400 cinder cones dot the flanks of Newberry volcano, including Lava Butte cinder cone at the left center of this photo, one of many cones formed around 6100 years ago along the NW rift zone.

Photo by Lee Siebert, 1995 (Smithsonian Institution).

Medicine Lake

The broad 50-km-wide Medicine Lake volcano in the southern Cascade Range of NE California, seen here from the NE, is an example of a large shield volcano in a continental margin setting. Its chemistry is more diverse than Hawaiian shield volcanoes, and it has produced both basaltic lava flows and rhyolitic tephras and obsidian flows during the Holocene. Eruptions have occurred during the past 6000 years from vents within a 7 x 11 km summit caldera and from other vents on its flanks.

Photo by Paul Kimberly, 1994 (Smithsonian Institution).

Mauna Loa

The steep walls of Lua Poholo pit crater, immediately NE of Mokuaweoweo caldera, expose a small portion of the massive pile of thin, overlapping lava flows that have construced the Mauna Loa shield volcano. This view from the NE shows the rim of Mokuaweoweo caldera at the upper right. Lava flows from recent eruptions, including the last eruption of Mauna Loa, in 1984, fill the floor of the pit crater.

Photo by Paul Kimberly, 1994 (Smithsonian Institution).

Photo by Paul Kimberly, 1994 (Smithsonian Institution).

Mauna Loa 2

Mauna Kea, Hawaii's highest volcano, is seen here from the south at the broad Humuulu Saddle between Mauna Kea and Mauna Loa. The fresh lava flow in the foreground was emplaced during an 1843 eruption that originated on the NE rift zone of Mauna Loa. The flow traveled directly north to the Mauna Kea saddle, where it was deflected to the west. The irregular profile of the unvegetated summit region of Mauna Kea shield volcano is produced by a cap of cinder cones and pyroclastic ejecta that is not present at Mauna Loa.

Photo by Don Swanson (U.S. Geological Survey).

Mauna Kea

Hawaii's two largest shield volcanoes, Mauna Loa (in the background to the south) and Mauna Kea, have dramatically differing profiles. Mauna Loa, the world's largest active volcano, has the classic low-angle profile of a shield volcano constructed by repetitive eruptions of thin, overlapping lava flows. Mauna Kea is also a shield volcano formed in the same manner, but its profile has been modified by late-stage explosive eruptions, which constructed a series of cinder cones that cap its summit.

Photo by Lee Siebert, 1987 (Smithsonian Institution).

Mauna Kea 2

Mauna Kea (left) and Mauna Loa (right), both over 4000 m above sea level, are the world's largest active volcanoes, rising nearly 9 km above the sea floor around the island of Hawaii. This aerial view from the NW shows the contrasting morphologies of these two shield volcanoes. In contrast to the smooth profile of Mauna Loa, Mauna Kea's early shield volcano morphology is modified by the late-stage products of explosive eruptions.

Photo by Jim Luhr, 1990 (Smithsonian Institution).

San Quintin Volc Field

Isla San Martín, 6 km off the west coast of Baja California, is the westernmost volcano of the San Quintín volcanic field and the only one that is located offshore. The 2-km-wide island is a small basaltic shield volcano that is capped by scoria cones that rise to 230 m above sea level. Wave erosion has truncated part of the shield volcano, forming the sea cliffs seen at the left on the south side of the island.

Photo by Chuck Wood, 1978 (Smithsonian Institution).

Fernandina

Fernandina volcano displays the classic "overturned soupbowl" profile of Galápagos volcanoes. Steep upper flanks formed by eruptions of lava flows from circumferential fissures around a summit caldera rim contrast with the broad, low-angle lower flanks and horizontal flows around the summit. Scientists from the Smithsonian Institution, U.S. Geological Survey, and the Charles Darwin Research Station conduct measurements on a pahoehoe lava flow near the SE coast. Vast fields of fresh, unvegetated lava flows cover the volcano's flanks.

Photo by Richie Williams, 1981 (U.S. Geological Survey).

Prestahnukur

The classic Icelandic volcano Skjaldbreidur is perhaps the best known of the many small shield volcanoes that were constructed along rift zones where the Mid-Atlantic Ridge rises above sea level. Skjaldbreidur, seen here from the west along route 52, was formed about 9500 years ago during a single long-duration eruption at the southern end of the Prestahnukur volcanic system in central Iceland. The broad, low-angle shield volcano produced 17 cu km of basaltic lava flows and is capped by a small 300-m-wide summit crater.