A volcano is a mound with a hole at the top. Through this hole, molten rock and hot gases erupt or explode out of it. Volcanoes occur mainly along the edges of tectonic plates, especially the “Ring of Fire” around the edge of the Pacific Ocean plate. Plates are slabs of solid rock that float on top of the Earth’s mantle. The mantle is the middle layer of the Earth, surrounding the core. It is made up of melted rock called magma. Around this is the outermost layer of the Earth—the crust. The crust, or plates, is the surface of the Earth, where we all live.
There are more than 1,500 active volcanoes on the planet. Active volcanoes are ones that have erupted in the last 11,700 years, are currently erupting, or could erupt in the future. Inactive volcanoes, or dormant volcanoes, are ones that may have erupted in the distant past and are now showing no activity. There are millions of inactive volcanoes on the planet.
On August 24, 79 CE, Mount Vesuvius in Italy erupted, destroying the city of Pompeii. The volcano released so much ash and debris that it covered the city over 9 feet (3 meters) deep. Early archeologists were able to discover this lost city that remained perfectly preserved, providing insight into ancient lives. This volcanic eruption is one of the most famous examples of the destructive power of volcanoes.
On May 18, 1980, Mount Saint Helens in Washington, United States erupted and became one of the greatest volcanic explosions in North America. Prior to this, the volcano’s last eruption was in 1857. For months, the pressure mounted within the volcano from rising magma. Small eruptions and explosive steam emitting from the volcano foreshadowed what was to come. The eruption was triggered when a 5.1 magnitude earthquake caused a landslide on the volcano’s north face. A cloud of superheated ash and debris reaching temperatures of 660 °F (350 °C) spread out about 15 miles (25 km). At the same time, gas and ash erupted upwards about 16 miles (26 km) high. The ash rained down as far as central Montana. It caused complete darkness over Spokane, Washington, which was 250 miles (400 km) away. Although there has been some activity in the volcano since, it has been minor; however, there could be a similar eruption in our future.
There are 169 active volcanoes in the United States. More than half of these could have a catastrophic eruption similar if not more severe than what we saw in 1980 with Mount Saint Helens. Below are 5 of the most dangerous volcanoes in the United States so far:
Learn more about these volcanoes and others here.
Share this classic science experiment with students to learn firsthand (and safely) how volcanoes work. Watch the video, print out the instructions, gather the materials, and follow the step-by-step guide from NASA’s Jet Propulsion Laboratory.