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courtesy=of photos Stephane Tetreau and volunteer Gonzalez right Mexico’s Popocatépetl, seen here in profile and from the air, has the classic cone and crater of a stratovolcano.

Stratovolcanoes tend to form on coastal land and islands at subduction zones (where continental crust is riding over oceanic crust). These volcanoes don’t erupt that often, but their eruptions are often violent. Strato- means “layer” and refers to the layers of ejected material that blanket the slopes after each blast—lava flows and an assortment of pyroclastic (“broken fire”) debris from fine ash to large rocks. Over time, the layers build up into a steep cone in part because the lava is andesite or dactite, which is viscous—stiff like cookie dough. Viscosity is a measure of the resistance to flowing, and so viscous lava doesn’t travel far. Also, the heaviest debris falls close to the vent.

Photo courtesy of Lyn Topinka/U.S. Geological Survey. Belknap Crater is a shield volcano in the Cascade Mountains of central Oregon.
Photo courtesy of volunteer Jenni James Jayne. Lava from Hawaii’s shield volcanoes extends the island into the ocean.

Another type of mountainous volcano is the shield volcano. An example is Hawaii’s Mauna Loa. These volcanoes are the largest ones on Earth, but they don’t look like it. Instead of being tall, steep, and jagged, these monsters resemble a gently rounded warrior’s shield, laid to rest outer-side-up. The low, wide slopes are deceptive. Beneath them lies a towering mass of lava rock that rises higher off the ocean floor than most land-bound mountains.

Shield volcanoes are low and wide because their lava is runny like honey—less viscous than a stratovolcano’s—and so travels fast and far from the vents. Also, because the eruptions are more effusive than explosive, the blast doesn’t shatter the liquid lava to ashy or rocky bits. Shield volcanoes grow to mammoth size due to frequent eruptions of lava over long periods of time.

Cinder cones, also called scoria cones, are a third type of mountainous volcano. These common, small volcanoes are steep, often near-perfect cones with relatively large and very round, bowl-shaped craters on top. Their tidy cone shape is in part the result of uniform ejection material. The explosion breaks liquid lava into little drops that sometimes shoot up from the vent like sparks in a fountain—called a Strombolian eruption. The small drops cool quickly in midair and form glassy, bubble-filled bits of tephra (airborne pyroclastic debris) that rain down in a circle around the vent. The tephra is classified by size and includes cinders, the volcano’s namesake.

Cinder cones can grow in calderas or on the flanks of larger volcanoes. Hawaii’s shield volcanoes support nearly 100 of these small volcanoes. Cinder cones can also appear, seemingly out of the blue, in volcanic fields, where vents are spread over great distances and give rise to a variety of volcanic forms.

Photo courtesy of volunteer Andrew Cutts. Mount Stromboli, in Sicily, is famous for eruptions that shoot lava high into the air.

Some volcanoes are incognito—too hidden or too big to see.