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Name: ____________________ Volcanoes – Mountains from Magma The Earth’s surface is full of volcanoes. Some of them erupt frequently and somewhat quietly, and you can actually watch the lava flow out of them from a safe distance. Others erupt only once every few hundred years – but when they do, it is wise to be as far away as possible. Shield Volcanoes Shield volcanoes do not occur at the edges of plates. Instead, they can be found anywhere on a plate – even rising out of the ocean floor! These volcanoes are formed above hot spots in the mantle, where the magma collects in enormous pools. The hot magma eventually melts the rock above it, and pours out through the hole onto the Earth’s surface as lava. The lava that comes out of a shield volcano is usually very runny, and hardens into a type of rock called basalt. This lava is so fluid that it is very rare for a shield volcano to erupt explosively. The lava simply pours out of the volcano like a river and then hardens. Because of this, shield volcanoes build up very gradually and have gently sliding slopes. When a shield volcano forms on the ocean floor, the lava pouring out hardens more quickly than it would on land. More lava pours on top, forming a volcanic cone. The cone may build up until it rises above sea level, and forms an island. This is how Mauna Loa, a shield volcano on the island of Hawaii, was formed. Stratovolcanoes Stratovolcanoes are the type of volcano that you usually hear about in the news. These volcanoes erupt extremely explosively, blowing ash and rock many kilometers up into the atmosphere, along with an enormous amount of steam and gas. These volcanoes form where two plates collide, with one of the plates sliding on top of the other. As the light rocks on the subducting plate heat up and melt, the magma rises up against the plate above and melts through it. This forms a hole so the magma can eventually escape. While shield volcanoes have very liquid magma, the magma in a stratovolcano is thick and sticky. Because the magma is created from melted crust, and the lower plate often contains water, the lava from stratovolcanoes also contains steam underneath of the enormous pressure. It is this combination of thick magma and steam that causes stratovolcanoes to erupt so violently and explosively. As the lava rises, the high-pressure steam escapes, carrying lava and ash with it. Mount St. Helens is one of the most famous stratovolcanoes. Eruptions where the Plates Part As you have already seen, lava also flows up through the crust and onto the Earth’s surface where plates are moving apart, usually on the ocean floor where the crust is thinnest. Here, the lava cools quickly and forms basalt (rock that is hardened lava) in a long ridge on each side of the crack in the ocean floor. Some of these ridges rise high enough to reach the surface of the ocean, creating islands such as Iceland. These islands are initially caused by eruptions, but there are sometimes no volcanoes created because the lava simply hardens into rock overtop of its escape point. Name: ____________________ Volcanoes – Mountains from Magma 1. Which type of volcano has very liquid, runny magma? 2. Where are stratovolcanoes found? How are they created? 3. Where can eruptions occur without forming volcanoes?