Types and Processes Gallery

Shield Volcanoes
Shield volcanoes contrast with stratovolcanoes both in origin and morphology. Their name derives from their low-angle profile, which resembles the personal shields carried by 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.

Alcedo
Alcedo is one of the lowest and smallest of six shield volcanoes on Isabela Island. Seen here from the coast of Fernandina Island to its west, Alcedo has a broad 7-8 km wide summit caldera. Most of the flanks and summit caldera are vegetated, but young lava flows are prominent on the northern flank near the saddle with Darwin volcano. Alcedo is the only Galápagos volcano known to have erupted rhyolite as well as basalt. Only one eruption is known during historical time. An active hydrothermal system is located within the caldera.

Photo by Lee Siebert, 1978 (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).


Darwin
Volcán Darwin, named after the celebrated naturalist, rises above a narrow channel opposite Point Espinosa on the NE tip of Fernandina Island. Darwin volcano contains a symmetrical 5-km-wide summit caldera that is nearly filled by lava flows. The most recent summit activity produced several small lava flows from vents on the east caldera floor and NE and SE caldera rims. Two breached tuff cones on the SW-flank coast, Tagus and Beagle, were a prominent part of Darwin's geological studies in the Galápagos Islands.

Photo by Lee Siebert, 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 Chuck Wood, 1978 (Smithsonian Institution).


Fremrinamur
Fremrinamur central volcano, NNE of Askja and SE of Myvatn lake, is a volcanic system that is being constructed over the Ketildyngja shield volcano (center). Associated fissure systems, including the Sveinar fissure, extend 130 km to the northern coast of Iceland. Iceland's renowned Myvatn lake formed as a result of the eruption of the massive 70-km-long older Laxarhraun lava flow from Ketildyngja shield volcano about 3800 years ago. The latest eruption from Fremrinamur produced the Burfellshraun lava flow about 2500-3000 years ago.

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


Hualalai
Hualalai shield volcano, seen here from the SE at the summit of Mauna Loa, rises to 2523 m. Almost the entire surface of Hualalai is covered by lava flows of Holocene age. Unlike Mauna Loa, Hualalai has been relatively inactive during historical time. It's latest eruption took place in 1800-01, when lava flows from vents on the NW rift zone reached the sea. This eruption was of particular volcanological interest because of the large numbers of olivine-rich nodules that were brought to the surface by the lava flows.

Photo by Lee Siebert, 1987 (Smithsonian Institution).


Mauna Kea
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 Lee Siebert, 1987 (Smithsonian Institution).


Mauna Kea
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 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).


Mauna Loa
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 Lee Siebert, 1987 (Smithsonian Institution).


Mauna Loa
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 Paul Kimberly, 1994 (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 Lee Siebert, 1998 (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).


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.

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


San Quintín Volcanic 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 Jim Luhr, 1990 (Smithsonian Institution).


Wolf
Wolf, the highest volcano of the Galápagos Islands, straddles the equator at the north end of the archipelago's largest island, Isabela. Volcán Wolf shield volcano has steeper slopes than most other Isabela volcanoes. A 5.5 x 7 km caldera, 600 m deep, is located at the volcano's summit. The broad caldera floor is largely covered by fresh, unvegetated lava flows. Prominent unvegetated lava flows drape forested eastern flanks of the volcano to the sea. Wolf's 1797 eruption was the first documented in the Galápagos Islands.

Photo by Lee Siebert, 1978 (Smithsonian Institution).