All About Volcanoes

Podcast Transcript

Volcanoes are some of the most fearsome things in nature.

They are responsible for the largest explosions ever known. They have created entire land masses. They have shaped the Earth’s climate and may have been responsible for mass extinction events during our planet’s history.

Today, as you are listening to this, somewhere on Earth, there are over 40 of them erupting right this second.

Learn more about volcanoes, how they work, and how they are different from each other on this episode of Everything Everywhere Daily.

What a volcano is, is actually easy to define. A volcano is an opening in the Earth’s crust that allows for magma from below the Earth’s surface to come to the surface, expelling ash, gases, and lava.

The difference between magma and lava is that magma refers to molten or semi-molten rock below the surface of the Earth, and lava refers to molten rock on the surface of the Earth.

Defining what a volcano is, is much easier than understanding how they work. Every point on the surface of the Earth has magma below it. Why don’t we have volcanos shooting up everywhere, or at least at random points all over the planet?

The crust of the Earth is made up of a lighter rock than the rock inside the mantle. The average density of continental crust is approximately 2.7 grams per cubic centimeter, and the crust on the ocean floor is approximately 3.0 grams per cubic centimeter.

The magma from inside the Earth is usually only going to be able to break through the crust where there is some sort of event or condition that allows it to break through. Most volcanoes on Earth exist at the boundary of tectonic plates.

Tectonic plates are the large pieces of the Earth’s crust that are put together like a jigsaw puzzle. Very slowly, over time, these plates move, and how they move in relation to other plates depends on how volcanoes are formed.

The type of tectonic boundary that creates the most volcanoes are subduction zones. When two plates collide with each other, one plate will dive under the other one, slowly pulling the entire plate down into the mantle like a conveyer belt. Due to density, subduction zones only really happen with oceanic crust.

This is what is happening along the Ring of Fire in the Pacific Ocean. The Oceanic plates in the Pacific are moving under other tectonic plates. 70% of all the volcanoes in the world are located in the Ring of Fire, which is a giant horseshoe that extends from South America up through Central America, the west coast of North America, the Southern coast of Alaska, and the Aleutian Islands, down Russia’s Kamchatka Peninsula, Japan, the Philippines, Indonesia, Papua New Guinea, and then finally Tonga and other south Pacific Islands.

Other subduction volcanoes include those in Italy, such as Vesuvius and Mount Etna.

The notable volcanoes in the Pacific which are NOT part of the Ring of Fire are those in Hawaii, which I’ll get to in a bit.

The subduction zones are notable for their deep ocean trenches where the subduction actually occurs, as well as the many earthquakes and tsunamis which result from the subduction of the plate.

As the plate is pulled down, it brings with it an enormous amount of water and other lighter materials with it. The water reduces the melting point of the surrounding mantle allowing for the formation of liquid magma, which slowly begins to rise as it is now a lower density than the surrounding solid mantle.

When it comes up through the surface, the result is a volcano.

To summarize the process for subduction zone volcanoes, which again make up most volcanoes, is that as one plate is pulled down under another one, the water and lighter materials that are pulled down with it change the chemistry of the rock in the upper mantle, causing liquified magma to rise up and form volcanoes.

Now if there are subduction zones where a plate is moving underneath another one, then that must be causing a gap somewhere else.

A subduction zone is a convergent plate boundary. The area where plates move apart from each other is known as a divergent plate boundary.

The best-known divergent boundary is the Mid-Atlantic Ridge. This is a mostly undersea mountain range that extends down the middle of the Atlantic Ocean, snaking its way from Iceland until it reaches the Antarctic Plate.

In divergent tectonic boundaries such as these, the process is different from subduction zones.

The plates are literally pulled apart, which allows for magma to come up to the surface, where it is cooled quickly by deep ocean water.

Divergent boundaries usually do not result in visible volcanoes. As magma comes up and cools, it stops until the plates spread apart again. The boundary is usually just a ridge. This is the process by which oceanic crust, which is lost due to subduction, is recycled, and new oceanic crust is created.

The amount of magma that comes to the surface depends on how fast the plates are spreading apart. The Mid-Atlantic Ridge spreads apart about 25 millimeters per year, and the East Pacific Rise spreads at a rate of about 145 millimeters per year.

While most volcanoes are the result of plate tectonic activity, not all of them are. The other means by which volcanoes can be formed are geologic hot spots.

A hot spot is when a plume from inside the Earth’s mantle comes up right through the crust. This is due to either an exceptionally hot mantle plume or perhaps a weakness in the crust at that point.

Examples of hotspot volcanoes include Hawaii, Yellowstone, the Canary Islands, and Iceland.

In the case of Hawaii and the Canaries, there is a chain of volcanic islands. The chain is created by the tectonic plate moving over the hotspot over the span of millions of years.

The best way to visualize it is if you had a conveyor belt and, as it was moving, you had something stationary below it which kept poking up. As the conveyor belt moved, it would create a series of indentations, even though the object below the belt didn’t move at all.

The Hawaiian island that is currently the most volcanically active is the Big Island of Hawaii, which is also the furthest east. The current activity on the Big Island is all on the Eastern side of the island.

The Big Island is the largest, highest, and youngest of all the Hawaiian islands. As you move west, the islands get older, smaller, and shorter. Active vulcanism has ceased on these islands, so there is no more land created. As they are subject to erosion, the older the island, the more erosion it has experience, the smaller it gets.

Iceland is a unique case because it is both a hotspot and it is located on the Mid-Atlantic Rift. Unlike Hawaii, where the plate is moving over the hotspot, in Iceland, the plates are pulling apart right where the hotspot is. The result is not a chain of islands like Hawaii but just an island that is growing ever larger.

In addition to volcanoes being formed in different ways, there are also several different types of volcanoes. Volcanoes are classified into roughly three types.

The first type is the cinder cone. A cinder code ejects rocks that are known as scoria and pyroclastics. These are volcanic rocks that are ejected and cooled in the air. The Parícutin Volcano in Mexico that I covered in a previous episode is a cinder cone.

Cinder cones tend only to erupt once, even if that eruption may take decades to complete. They can often be found on the sides of larger volcanoes, and they are usually the smallest type of volcano.

The next type of volcano is a stratovolcano or composite volcano. These tend to be tall, symmetric volcanoes, and they get their names from the different layers or strata which make up the volcano. They can eject both lava as well as ash.

Examples would include Mount Fuji in Japan, Mount Vesuvius in Italy, Mount Ranier in the US, and Mount Kilimanjaro in Tanzania.

Stratovolcanos, because they can alternate between lava and ash, can be the most deadly and can produce massive explosions. If gases are trapped beneath a magma cap, pressure can build up until it is released catastrophically. Mount Saint Helens and Krakatoa are both examples of this.

The final type of volcano is a shield volcano. These are the largest type of volcanos, and they tend only to eject lava. Unlike a stratovolcano which has very steep slopes, a shield volcano has gentle slopes. It gets its name from a warrior’s shield.

Despite having gentle slopes, these are the largest volcanoes. The best example of this, and the world’s largest volcano, is Mauna Loa in Hawaii. They are created by the continual oozing of liquid lava that flows out.

Shield volcanoes like Mauna Loa can certainly cause a great deal of damage if the lava happens to flow into a populated area, but they are not explosive like stratovolcanoes.

The largest shield volcano known isn’t even on Earth. It is Olympus Mons on Mars. It is 2.5x the height of Mount Everest and it has an area about the size of the state of Arizona.

If you ever get a chance you visit the Big Island of Hawaii, you will be able to identify the two main types of lava which can be found in lava fields: aa and pahoehoe.

Aa is a very sharp jagged type of lava. It tends to form with faster-moving lava that is also a bit cooler.

Pahoehoe is a smoother type of lava that often looks like ropes in the ground. It is formed by slower-moving, hotter lava.

You can often find the two right next to each other, as when the temperature and viscosity of the lava changes, its form will change.

It is also easy to remember which is which. Just remember that aa is the sound you would make if you walked over it barefoot.

According to the Smithsonian Institution’s Global Volcanism Program, which is a catalog of all the volcanoes and volcanic eruptions in the world, there have been 9,901 confirmed eruptions from 859 volcanoes over the last 11,700 years.

Volcanic activity is a rather confusing subject because volcanoes work on geologic time scales, not human time scales.

There is no scientific consensus as to what an “active” volcano is. An active volcano usually means that it is either currently erupting or there is subterranean activating going on, which means that it could erupt in the future.

Stromboli in Italy and Mauna Loa in Hawaii are both active volcanoes.

A dormant volcano doesn’t mean that it has seen the last of its activity. Dormant just means that it isn’t currently showing any signs of activity. However, there have been many volcanoes that were dormant for centuries, or even thousands of years, which became active again.

An extinct volcano is a volcano that is believed will never erupt again as it has exhausted or has been cut off from its supply of magma.

However, even being declared extinct isn’t a guarantee. There have been several major volcanic eruptions from volcanoes previously thought to have been extinct.

The Soufrière Hills volcano on the island of Montserrat in the Caribbean was inactive for all of recorded history on the island. Then in 1995, it erupted, destroying the capital city of Plymouth and causing ? of the population to have to relocate permanently. It has been erupting ever since.

Montserrat doesn’t get a lot of visitors, but if you ever get a chance, I recommend visiting. The Montserrat Volcano Observatory is a great experience, and you can see the rooftops of the ruined city of Plymouth from the sea.

Another supposedly extinct volcano was Mount Pinatubo in the Philippines. Its eruption in 1991 caused global temperatures to drop by 0.5 °C or 0.9 °F for two years.

Historically, volcanoes have been one of the most powerful forces shaping the surface and climate of the Earth. Most of Japan, Indonesia, and other islands were all created through vulcanism.

Prior to the discovery of the Chicxulub crater off of the Yucatan Peninsula in Mexico, it was thought that the Cretaceous–Tertiary extinction event which killed the dinosaurs was caused by a massive volcano known as the Deccan Traps in India. The Deccan Traps probably still did have a huge contributing impact.

Billions of years ago, the Earth may have been entirely covered in ice in an event known as Snowball Earth. The thing which caused the ice to melt would have been millions of years of volcanic activity.

Volcanoes are a deep subject. People can and do devote their entire lives to their study. Many of the volcanoes I’ve mentioned in this episode will be the subject of future episodes, and each of them has a unique history and story.

Volcanoes are impressive and powerful. They can shape the Earth as well as destroy it. If you ever get a chance to see an erupting volcano in person, they are truly an awesome sight to behold.

The Executive Producer of Everything Everywhere Daily is Charles Daniel.

The associate producers are Thor Thomsen and Peter Bennett.

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