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A shield volcano takes its name from its shape, which looks like a warrior's shield seen from the side. It's much wider than it is high. Shield volcanoes are built up of a great many flows of runny lava, each of which is relatively thin, only a few meters (yards) thick. Characteristic features of shield volcanoes are lava tubes, underground channels that fed the flows on the slopes of the volcano.
Olympus Mons on Mars is the tallest volcano known, yet it is actually a gigantic shield volcano built from countless thin lava flows. The volcano on Earth that most resembles Olympus Mons is the entire Big Island of Hawaii. The Big Island is Earth's second largest shield volcano, built from five overlapping individual volcanoes. (The largest shield volcano on Earth is the Tamu Massif, on the floor of the Pacific Ocean; it lies wholly underwater.) Two of these — Mauna Loa and Kilauea — are active today. (Kilauea has been erupting continuously since 1983.)
Starting from the ocean floor and going to its highest point (the inactive summit of Mauna Kea), the Big Island rises about 10,000 meters (33,000 feet). That's impressive! But Olympus Mons is more than twice as high and nearly three times wider than the entire Big Island, including its underwater part. Mars also has smaller shields, called low shields, which are also seen on Earth. They usually have a central vent, occur in clusters, and are not seen in the Southern Highlands. The top of a shield volcano usually has a depressed area. Scientists call this a caldera.
A calders forms when the magma that built the volcano subsides in the vent or pipe that fed the eruptions. With some of its internal support removed, the volcano's summit collapses a little. If you see several partly overlapping calderas (as for example on Olympus Mons), it's a sign that magma has risen and withdrawn several times.
On Earth, a great example of these types of calderas can be found at the Emi Koussi volcano in Chad. If you study the flanks of a large shield volcano, you can see many lava flows, collapse pits, lava tubes , and lava channels  that weave around and cover each other. Building a shield volcano is like drizzling lots of chocolate syrup onto a scoop of ice cream. Untangling the volcano's history, flow by flow, is a big job.
Looking at a shield volcano in Iceland from the side, it's easy to see where shield volcanoes get their name. Notice how shallow the slope of the volcano profile is.  It looks like a shield (think Captain America or a Roman soldier) lying on the ground. (Christian Bickel)
Skjaldbreiður Shield Volcano in Iceland
(Christian Bickel)
Emi Koussi Volcano
Olympus Mons, a shield volcano, is the tallest volcano in the solar system. It rises 27 kilometers (17 miles) high, spreads over 600 km (370 mi) at the base, and is surrounded by a well-defined scarp that is up to 6 km (4 mi) high. Lava flows drape over the scarp in places. Much of the plains around the volcano are covered by ridged and grooved terrain called the aureole. The origin of the aureole is controversial, but may be related to material sliding off of the flanks of an ancestral volcano. The summit
Olympus Mons
(NASA/JPL-Caltech/US Geological Survey)