Volcanic Landforms

1. Volcanic Landforms Shield Volcanoes • basaltic lava tends to flow long distances before hardening because of its low viscosity • lava builds up in layers, forming shield volcanoes with broad bases and gently sloping sides • Launa Loa on Hawaii is a shield volcano which rises 4170 m above sea level and has a base 5000 m below sea level (for a total height of 9170 m!) • shield volcanoes tend to be less explosive but basaltic flows can be frequent and copious Cinder Cones • the simplest form of volcano, forms when molten lava is thrown from a vent • as it falls, the lava breaks up into fragments that harden before hitting the ground • fragments accumulate, forming a cone-shaped mound with an oval base • cinder cones are relatively small and form in groups on the sides of larger volcanoes

2. Composite Volcanoes • develop when layers of hardened lava flows and other pyroclastic material (i.e. ejected from a volcano) accumulate around a vent from successive explosive eruptions • the eruption of Mt. St. Helens in 1980 illustrated several such forces and events • quiet since 1857, an earthquake burst a bulge that had been forming on the north face, releasing magma, water, and gas in a massive explosion of superheated ash and stones • hot ash mixed with snow and ice to form a fast-moving mudflow called a lahar • in some areas, debris from lahars and landslides reached a depth of about 46 m

3. Calderas • sometimes magma beneath a volcano is released after the top of the volcano collapses, forming a large, crater-shaped basin called a caldera • many active volcanoes have calderas at their summits, sometimes filled with water to form a lake • the caldera in Yellowstone Park is still active • magma superheats the water which feeds the hot springs and geysers • the eruption that produced the Yellowstone geyser may have been 1000 times more powerful than Mt. St. Helens • ES0901: How Are Volcanoes Related to Plate Tectonics?