dramatic volcanic eruptions happen all around the world humans are left in both fear and awe of these fiery powerful and highly dangerous events today there are over 600 active volcanoes on earth with at least 50 eruptions occurring each year but did you know there are many other volcanoes out there that are extinct or dormant what is going on under the surface to cause such a destructive event to answer this question we must first understand what a volcano is [Music] a volcano is essentially a rupture in the earth's crust the earth's rocky outermost layer that lies on top of its hotter inner layers the earth's crust and the upper mantle form the lithosphere the lithosphere is broken up into slabs known as tectonic plates and they fit together like puzzle pieces to make up the surface of the earth most volcanoes are found along these tectonic plate margins either at a destructive or constructive plate margin both of these margins allow gas and molten rock to escape from inside the planet to the surface at a destructive or convergent plate margin an oceanic tectonic plate which is a plate mostly covered by the sea and a continental plate which is mostly covered by land move towards each other the denser oceanic plate sinks under the continental plate melting down into the earth's mantle [Music] this melting releases gases that make their way through the crust above and erupt through the surface at a constructive or divergent plate margin the two plates move apart really slowly less than five centimeters per year but over time this allows extremely hot magma to rise up from deep within the planet magma rises and fills the gaps created by the plates moving away from each other and eventually new crust is formed magma at the margin can sometimes explode through this new crust creating a volcanic eruption many of these constructive margins are found under the ocean if you map the locations of volcanoes you'll see that 75 of the world's active volcanoes are along the margin of the pacific plate volcanologists refer to this as the ring of fire since 1850 approximately 90 percent of the 16 most powerful volcanic eruptions on earth have occurred within the pacific ring of fire making it one of the most dangerous places on earth however there are places on earth where a volcano can form away from tectonic plate boundaries we call these hot spot volcanoes they form when rising magma from deep within the planet melts through the tectonic plate above and begins to build up an example of this is the hawaiian islands which are all made of a mixture of extinct and active volcanoes that have broken through the middle of the pacific plate now that you know where volcanoes form let's learn about their different types there are two main types composite volcanoes and shield volcanoes the majority of volcanoes are composite volcanoes these are commonly found on destructive plate margins and have a distinctive conical shape let's slice a composite volcano in half to learn more about it at the bottom of a composite volcano lies a magma chamber which is like a hot bubbling furnace magma enters the chamber from the earth's mantle and collects here until pressure builds causing gases to expand and water to become steam eventually the pressure becomes so high that an explosion or volcanic eruption occurs most of the magma moves through the main vent and exits the volcano through the crater some magma also makes its way through secondary events and out through secondary cones craters can eject four different types of materials into the atmosphere these include lava which is the name for magma that has reached the surface of the earth pieces of rocks called volcanic bombs thick ash and gas with every eruption the lava that travels out of the crater cools and solidifies along with ash to form alternate layers of ash and lava composite volcanoes are also known as strata volcanoes because strato means layers shield volcanoes on the other hand are usually found on constructive plate margins or at hot spots they have gentle sloping sides caused by runny lava flowing quicker and covering longer distances before it solidifies they usually erupt less violently as compared to composite volcanoes volcanoes reflect the movements of earth's major tectonic plates and are signs of activity occurring under the earth's surface this gives us evidence that the earth is not just a static and rigid rock but one full of dynamic and powerful forces you