2. Shield volcanoes Dan Barker March 2009

Shield volcanoes form when repeated eruptions from the same magma conduit system build piles of overlapping lava flows. Profiles are upwardly convex. The height of a shield volcano is commonly only 5% of the basal diameter, and slopes range from less than 10° to 25°. The lavas of shield volcanoes are typically basaltic, but unusually low-viscosity (high temperature?) andesite and trachyte make up some shield volcanoes. Lavas dominate over pyroclastic deposits in the preserved section, but much ash could have blown away.

Thin lava flows make up this section of a 13,000 yr-old shield volcano, Asbyrgi,Iceland.

Stratigraphy of the Rana Kau shield volcano, Rapa Nui

Mauna Loa, on the Big Island of Hawaii, is the largest volcano on Earth (Mars has bigger ones).

Viewed from neighboring Mauna Kea, multiple lava flows are visible on Mauna Loa's flank.

Mauna Kea, viewed from Hilo, has a more lumpy appearance because of many scoria cones that formed after the shield-building activity was over (Mauna Kea is older than Mauna Loa).

The flanks of West Maui volcano are notably steeper than those of its neighbors.

Haleakala, also on Maui, has a more gentle profile.

Like many other large shield volcanoes, Haleakala has a rift zone marked by small craters and scoria cones. Rift zones mark the courses of dikes that radiate from the summits along topographic divides on the shields.

Eruptions can occur either in a summit caldera or on the flanks, where they are fed by magma moving laterally through dikes. Studies of eroded shield volcanoes show that intrusive rock (dikes and sills) can make up to a third of a shield's volume. In spite of gentle slopes, shield volcanoes are gravitationally unstable, and large- scale failure of flanks has recently been recognized through study of the sea floor around the Hawaiian Islands.

Holei Pali, a fault scarp on the south flank of Kilauea, where parts of the shield are slumping off. Lava flows run down the scarp.

In the Galapagos Islands, shield volcanoes such as Fernandina tend to have steeper slopes close to the top, like inverted soup plates. The cause for this difference is still debated by volcanologists.

At first glance, Hekla in Iceland looks more like an overturned boat than a warrior's shield.That is because it has grown from many eruptions within a long fissure.

Another view of Hekla shows a more convincing shield profile. Note the broad apron of ash in the foreground; it was blown away before it could contribute to the pile to make a stratovolcano.

Trolladyngja is the largest shield volcano in Iceland, even though Hekla is more impressive.

Mauna Ulu ("Growing Mountain") is a shield that grew in the 1970s on the flank of Kilauea.

Longonot, Kenya, is a trachyte shield volcano in the East African Rift.

Newberry volcano, OR, is a large shield volcano capped by rhyolite.

Appearances can be deceiving. Mt Washington in Oregon is a shield volcano, although glaciation has removed much of it.

The 732nd most popular tourist destination in Idaho, this is a small basalt shield on the Snake River Plain.