Getting to Know: Where Volcanoes Form In 2010, a volcano erupted in Iceland. It spewed a giant ash cloud for weeks and disrupted travel throughout Europe. In January 2011, Krakatau volcano in Indonesia erupted for several days, displacing thousands of people. What caused these volcanoes to erupt? Why do volcanoes form in some areas but not in others? In this concept, you’ll learn more about why volcanoes form in certain areas, the causes of eruptions, and the different types of eruptions that can occur. Do volcanoes only form in Volcanoes tend to form at the boundaries of tectonic plates. certain areas? Volcanoes primarily form at tectonic plate boundaries. Tectonic plates are enormous pieces of Earth’s crust that are slowly moving. As tectonic plates move, the edges of the plates can collide, separate, or slide past each other. Tectonic plates collide at convergent boundaries, and they separate at divergent boundaries. Tectonic plates slide past each other at transform boundaries. Volcanoes tend to form at convergent and divergent plate boundaries—they are not usually associated with transform boundaries. Why do volcanoes tend to form at plate boundaries? When plates move away from each other at divergent plate boundaries, interesting things happen. For example, when plates move apart, molten rock from the Earth’s core ascends and forms new crust. This molten rock can erupt when it reaches Earth’s surface. At convergent plate boundaries, one plate can be pushed under another plate. When this happens, the plate can melt and turn into magma. This molten rock can rise up to the surface, forming a volcano, and causing volcanic eruptions. Misconception 1: Is it true that volcanoes are only on land? That is not correct. There are many volcanoes on the ocean floor. When volcanic eruptions happen on the ocean floor, they can create volcanoes that are completely underwater. In particular, divergent plate boundaries such as the Mid-Atlantic Ridge are characterized by numerous volcanoes. Concept: Where Volcanoes Form © Discovery Education. All rights reserved. Getting to Know 1 Discovery Education is a subsidiary www.discoveryeducation.com of Discovery Communications, LLC. Why does the molten rock rise to the surface? Molten rock rises to the surface because it has a different density than the surrounding rock. Density is a measure of the amount of mass in a given volume. The more dense something is, the more mass it has per unit of volume. Molten rock is less dense than solid rock and will rise above solid rock if there is passage for it to travel through. The molten rock, also called magma, rises to the surface of the Earth through cracks and fissures in Earth’s crust and can erupt in dramatic explosions when it reaches the surface. What’s the difference between magma and lava? Magma and lava are both forms of molten rock. Magma is molten (or melted) rock that is beneath Earth’s surface. Lava is the term for magma that reaches Earth’s surface (usually during a volcanic eruption). Lava and magma can both have the same chemical composition. Why do some volcanoes have violent eruptions? The type of magma in a volcano determines the type of volcanic eruption. All types of magma contain gases. When magma reaches the surface, these gases can expand, causing a violent eruption. However, magma varies in its ability to hold gases. One type of magma, called mafic magma, has high iron content and tends to be less viscous. Gases can escape from this type of lava easily; when the lava reaches the surface, there’s less gas, so the eruption is less violent. Another type of magma, felsic magma, is thicker and traps more gases. That means when felsic magma reaches the surface, there’s a lot of expanding gas and a more violent eruption. In addition to plate boundaries, are there other places where volcanoes can form? Volcanoes can also form at places called hot spots. Hot spots are magma chambers that form at places other than plate boundaries. They can even form in The Hawaiian Islands are located far from the middle of tectonic plates. The islands of Hawaii tectonic plate boundaries.They formed were formed from volcanoes that developed at a from a magma chamber called a hot spot. hot spot. Misconception 2: Is it correct that after a volcano erupts it is considered extinct? It’s not necessarily true that volcanoes are extinct once they have erupted. A volcano is considered extinct when it no longer has a supply of magma. A volcano is active when it does have a supply of magma. It can be quite difficult for scientists to determine when a volcano has lost its magma supply. Some volcanoes can lie dormant for hundreds of years and suddenly erupt! You’ll learn more about hot spots and volcanoes as you explore this concept. Concept: Where Volcanoes Form © Discovery Education. All rights reserved. Getting to Know 2 Discovery Education is a subsidiary www.discoveryeducation.com of Discovery Communications, LLC. .