Curiosity Challenge: “How Do Volcanoes Erupt?”

A spectacular eruption in Stromboli, Italy. Image via Places Under the Sun.

“Why Do Volcanoes Have Lava?” -- Alondra Sanchez, 6

“Why Do Volcanoes Erupt?” -- Darcy Baker, 6

“How Do Volcanoes Erupt?” -- Sara Solomon, 9

“What's In a Volcano?” -- Schnaubelt Baronvil, 10

A volcanic eruption is probably the most dramatic event that occurs in nature.  Imagine living next to a mountain that’s normally peaceful and calm.  Then, one day, the ground shakes, the mountain explodes, hot ash rains from the sky, and a wave of hot molten rock comes downhill--right towards you!  The giant clouds of ash belched up by the volcano block out the sun, sometimes for so long that the entire region’s climate changes.  It’s no wonder that most ancient cultures thought that volcanoes were controlled by gods.  We now know a great deal about volcanology--the science behind volcanoes--but we’re still in awe of these mighty features of the earth.

How are Volcanoes formed?

Volcanoes are essentially a gap in the Earth’s crust, where the Earth’s liquid mantle can flow up to the surface.  Earth’s rocky surface is broken up into roughly 17 tectonic plates that rest on top of the hot, churning, molten rock of the mantle.  Since the crust is thinnest at these edges, most volcanoes form at the boundary of these plates, where the plates are either moving away from each other--called a divergent boundary-- or towards each other--a convergent boundary.  (Some volcanoes, such as the islands of Hawaii or the Yellowstone Caldera, don’t form at boundaries, but at a “hotspot” in the middle of a plate, where the crust is particularly thin).  

A divergent boundary, a convergent boundary, and a hot spot, from left to right.
Via the US Geological Survey.

Divergent boundary volcanoes are usually constantly flowing, such as fissure vents at the bottom of the ocean where magma makes its way to the surface, cools, and forms new crust.  (Magma is the term for molten rock below the surface; when it makes its way up to the surface, it’s called lava.)  The volcanoes that erupt violently--the kind you’re probably most excited about--usually happen at convergent boundaries.  Much like if you tried to push two cookies together on top of a table, one plate must be pushed underneath the other.  This plate is submerged back into the hot liquid mantle, where it melts and builds up lots of pressure underneath the top plate.  When that pressure becomes too great for the surface rock to hold back--KABLOOEY!  That’s when a volcano erupts.

The Pacific Ring of Fire, with each red dot representing a volcano.
Via Nature in Japan.

Since the earth’s crust is very, very old, it’s rare that new volcanoes are formed when plates collide.  Most volcanic eruptions are old volcanic vents being re-opened when enough pressure builds up.  Each time these old volcanoes erupt, more rock, ash, and hardened lava is deposited around the vent, building up into the distinctive cone shape most volcanoes have.  Other volcanoes might form wide shields, or a dome shaped like a bubble, or might start out on the ocean floor, then grow until they become an island, such as the islands around the Pacific Ring of Fire.

What does an eruption look like?

Left: an artist's rendering of the 1883 Krakatoa eruption;
Right: a map that shows the original size of the island pre-eruption.
Via Penn State University.

Being right at the scene of the volcano is terrifying: eyewitness accounts of the eruption of Krakatoa--estimated to be the largest sound produced on Earth, ever-- report hot ash and rock raining down from the sky, enough smoke to turn the morning as dark as night, and all trees and houses in the blast zone being set on fire.  Those close enough to see actual lava didn’t live to tell their tale, since the whole island was blasted away.

But the giant explosion and fountains of burning lava that we see in disaster movies aren’t always what eruptions look like, and aren’t always the most dangerous part.  Some can simply be a release of steam and gas, and some can be a very slow-moving lava flow.  The gases released through eruptions are often poisonous and hot enough to burn people and plants.  Ash clouds spewed into the air can block out sunlight, kill crops, and cause acid rain.  The shifts in Earth’s crust that cause eruptions can often also cause earthquakes, landslides, and tsunamis, and later form geysers and hot springs.  Cities that are close to active volcanoes often issue warnings so their occupants are prepared in case of an eruption.

Famous Volcanoes Around the World

The Decade Volcanoes are 16 volcanoes that are large enough and close enough to big cities to warrant close study, ranging from Japan to Central Africa.  They were identified by volcanologists during the “International Decade of Natural Disaster Reduction” declared by the UN in the 1990s.  These studies helped nearby cities develop plans for what to do in the case of eruption.

Ngauruhoe (pronounced "Nah-goo-rah-hoe-ee"), a cinder cone volcano in New Zealand, is probably Hollywood’s most famous volcano, better known by its stage name, “Mount Doom.”  It’s part of the Tongariro volcanic complex, which is highly active today and attracts many hikers.

A warning sign to Tongariro hikers.

Mt. Vesuvius is most famous for burying the nearby Roman city of Pompeii when it erupted in AD 79.  Excavations of the city, in addition to teaching us lots about the damage volcanoes can cause, led to many interesting archaeological discoveries.   

The Izu-Bonin-Mariana Boundary is a highly-studied region because, in addition to being deepest part of the ocean in the world, it’s a convergent plate boundary that has led to the formation of 76 distinct volcanoes!  Submarine volcanoes are fascinating to study because lava cools differently in water than in air, and many bizarre ocean creatures adapt to survive among volcanic gases.

The Mariana Arc--one of the most volcano-dense regions of the world, and most of them underwater!
Via NOAA Ocean Explorer.

Learn more about earth science at the Cambridge Science Festival!

A Walk Through Geologic Time along the Charles River, April 15, 16: 5:30pm & April 17: 11am; 5:30pm (~45 minute tour).  Step back in time and put the earth as we know it in perspective on this unique walking tour, where every inch represents 240,000 years!  Free!

Workshop: Mission to Mars with MIT Society of Women Engineers at the MIT Museum, April 19, 11:00am - 2:00pm.  Studying volcanoes on earth might help us understand the extraterrestrial geology, too!  In this workshop, create a Mars rover, plan your resources, and explore the unique surface of Mars.  Ages 9-12, pre-register at web.mit.edu/museum/programs/festival.html

Free Admission to Harvard Museum of Natural History, April 20, 3-5pm, April 24, 9am-12pm, and April 17, 9am-12pm.  Take a look at volcanic rock, minerals from around the world, and a slew of other natural treasures!  Includes admission to the adjacent Peabody Museum of Archaeology & Ethnology.  Free!

Diving into the Pale Blue Dot at the MIT Museum, April 22, 10am-3pm.  Meet the scientists who delve deeper than any others to learn about the earth’s surface in the most remote parts of the ocean with the Woods Hole Oceanographic Institute and the Harvard/MIT Center for Chemical Innovation.  Free with Museum admission.

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E. Rosser is a science writer and mechanical engineer currently wrapping up a degree at MIT.  A year ago, she backpacked around the volcanoes of New Zealand, and while she was fascinated by the alien landscape the volcanic activity had formed, she’s glad to be away from the constant sulfur smell.

Next to the crater of Nguaruhoe, Jan 2015.