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Shield volcanoes are some of the largest volcanoes in the world. For example, Mauna Kea and Mauna Loa on the big island of Hawaii are examples of shield volcanoes. And so is Olympus Mons on Mars, which towers 27 km above the surrounding plains. But how are shield volcanoes formed?
Shield volcanoes form like any volcanoes. They’re spots on the Earth where magma from inside the Earth has reached the surface, and becomes lava, ash and volcanic gasses. Over the course of many eruptions, a volcano builds up layer by layer until the magma chamber underneath it goes empty and the volcano goes dormant.
The main difference with shield volcanoes is that they’re formed out of lava flows which have a low viscosity. Think of liquids. Water is very runny, and has a low viscosity. Syrup, on the other hand, has a high viscosity and flows more slowly. The shape and nature of a volcano depends on the viscosity of the magma. With shield volcanoes, the lava flows easily for many kilometers, creating the gently sloping sides. Shield volcanoes are much less dangerous than other types of volcanoes; they typically don’t explode, and the lava flows are easy to avoid – if you’re in a car or walking.
The kind of lava that has low viscosity is basaltic lava, which typically erupts at temperatures higher than 950 °C. If flows easily, forming puddles, channels and rivers of molten lava.