From our last episode, we know the rock cycle, tectonic cycle, and hydrological cycle working together as the geological cycle are a big part of how landforms are created. They represent the give and take between the internal, or endogenic system, and external, or exogenic system, which work in tandem to create, destroy, and shape the Earth’s crust.

In particular, endogenic systems change the crust by increasing the relief, or the local differences in elevation, and are powered by heat in the Earth’s core from decaying radioactive material. Which is where volcanoes come in. If you grew up in the US like me, you probably played “the floor is lava” [-- and if you haven’t played it, I know what should be happening in your living room later today.] Lava holds some of the intense heat from inside the Earth, so it would be incredibly bad to have it threatening your home.

So looking at a volcano, one of the foremost questions in our minds is probably, “when is it going to erupt?” especially for scientists who study volcanoes, or volcanologists. They look for clues both above and below ground that hint an eruption could happen. Like by examining millennia-old cooled rocks scattered about the volcano from past eruptions.

These rocks show how the chemical make-up of the magma miles beneath the surface has changed and what might have happened before an eruption. We can also look out for earthquakes, gravity and magnetic field changes, or the ground starting to warp as magma or gases rise. As part of monitoring a volcano, we might go on a gas flight, which is a small airplane or drone flight just to learn more about the gases being emitted as the volcano prepares to erupt.

These techniques have helped us successfully predict eruptions and save thousands of lives, like when Pinatubo in the Philippines erupted in 1991. But there’s still more to learn.

Research published in 2021 using satellite data showed that in the years leading up to an eruption, the temperature of the ground around the volcano can increase by up to about one degree Celsius.  So monitoring ground temperature using satellite data could become a new predictive tool and help get more people out of harm’s way. But while predicting volcanic activity is still difficult, we at least know where volcanoes are. Their locations follow a definite pattern… mostly.

Most volcanoes are located above subduction zones where two plates are converging and one dives under the other, and at mid-ocean ridges where the seafloor is spreading. But a few occur within plates. Like the Hawaiian volcanoes, which sit in the middle of the Pacific plate thousands of kilometers from the closest plate boundary.

Some of these mysterious volcanoes seem to be driven by mantle plumes, which are special features that can be hundreds of kilometers across where abnormally hot magma can rise up through the mantle and melt crustal rock into more magma. We’re still figuring out how mantle plumes work, but one common school of thought is that they get their start at the core-mantle boundary. The outer core provides the extra heat that helps the magma get pumped up into the mantle.

It’s like a glob of hot material that sits at the end of a stalk of inside-Earth-stuff with its base in a giant magma chamber, which is like a liquid vat of magma deep within the Earth. And it literally melts a path as it moves upwards to the lithosphere. Near the lithosphere the top of the plume mushrooms and spreads out.

Here the plume temperature can be 250 to 300 degrees Celsius hotter than the rest of the upper mantle, so it melts 10 to 20% of the surrounding rock. If this magma along with the newly melted rock cools beneath the surface, we get giant hunks of igneous rock in the crust called plutons. But if it erupts as lava, we get a hot spot volcano.

Or even a trail of hot spot volcanoes. Most of the time a mantle plume stays fixed while a plate slides slowly over it, which spreads out the volcanic activity. Like beneath Yellowstone National Park in the middle of North America, there’s a hot spot that’s created a 500 mile trail of more than 100 calderas, which are giant craters left after a volcano has collapsed, often after exploding.

They once generated enormous volumes of basaltic lava across Idaho’s Snake River Plain. In the ocean, plates moving over hot spots can produce a chain of volcanic islands or a hot-spot trace. Like the Hawaiian Islands formed as the Pacific plate moved across a hot spot for almost 80 million years, producing a string of volcanic islands, each older than the one after it.

But whether located above hot spots or on plate boundaries, volcanoes are kinda like windows into the center of the Earth which help us understand more about how it all works. Throughout our planet’s history, volcanism even helped to produce the atmosphere by emitting gases stored in the Earth’s interior. And the molten material spewed by volcanoes creates new igneous rocks which make up approximately 90 percent of the Earth’s crust.

Like the oldest of the 8 main Hawaiian islands is Kauaʻi which is about 5 million years old, and Hawaiʻi itself -- which is sometimes called The Big Island -- took less than half a million years to build. But the baby in the hot-spot trace is an underwater mountain called a seamount that’s still about 975 meters from the ocean surface. Its name is Lōʻihi, but it won’t see the Sun for another 10,000 years or more.

So geologically speaking, Hawaiʻi is still growing. The volcano called Kīlauea in Hawaiʻi has been nearly continuously erupting since 1983. But it doesn’t make a lot of headlines because the eruptions here don't typically come with a lot of volcanic activity fireworks, which is actually telling us something about the volcano and the crust being formed. 

The magma from Kīlauea has a specific chemical make-up, which determines how fluid it is or its viscosity.  Magmas which built the Hawaiʻian islands are mafic lavas.  They have a low viscosity and are very fluid or runny and don’t hold a lot of gas. Without the extra gases, the pressure doesn’t build up as much and we get a quiet eruption. This allows the lava to travel long distances and spread out in thin layers, so Kīlauea is aptly named because it’s often thought to mean “spewing” or “much spreading.”

The mafic and basaltic lavas cool to form a dark, basaltic rock which is low in silica and rich in iron and magnesium.  These minerals make the crust heavier and denser, and basaltic lava helps make up the bulk of the ocean floor.

The magma shapes the volcano too. We get a broad rounded dome with gentle slopes that looks like a warrior’s shield, which is why they’re called shield volcanoes.  So the Hawaiian volcanoes are hot spot volcanoes because of where they’re located, but they’re also the most famous of shield volcanoes, because of their type of eruption.

In fact Mauna Loa on the island of Hawaiʻi is the world’s largest active volcano. In ōlelo Hawai'i, Mauna Loa means “long mountain,” which makes sense when we look at its profile with smooth slopes rising to a broad-topped volcano.

Hawaiian volcanoes can rise 4,000 meters above sea level but are actually more than twice that high if we include the portion below sea level. And if we compare this gentle giant to the height of explosive Mount Rainier, which is southeast of Seattle, Washington in the Cascade range of volcanoes in the US, the difference is striking and hints at the fact that they actually have different tectonic origins.

Unlike Mauna Loa over a hot spot, Mt. Rainier was created when the lighter continental crust of the North American plate collided with and subducted the denser oceanic crust of the Juan de Fuca plate. Because hundreds of meters of sediments are often carried along when a plate is subducted and melted into magma, it produces a felsic lava which is 50 to 75 percent light minerals like silica and aluminum and creates continental crust. So the magma from Mt Rainier is thicker than magma from volcanoes like Mauna Loa.

This type of lava is gummy with lots of gases that were dissolved when the magma was under lots of pressure deep in the Earth. When too much gas accumulates within the magma chamber the pressure grows and grows. Then as molten rock rises to the surface, the pressure is lower, so the gases expand in a violent blast.

Pulverised rock, called pyroclastics or tephra, and lava are also ejected from the central vent linking the asthenosphere just below the lithosphere to the surface. It all starts to fall outside the crater, or bowl shaped depression that usually forms at the summit, around the base of the volcano. Alternate layers of tephra and lava, which doesn’t flow very far from the volcano, produce a composite or stratovolcano with a tall cone shape that steepens towards the summit.  So we get the classic, symmetrical volcano of our imaginations, like Mount Mayon in the. Philippines, Mount Fuji in Japan, Mount Shishaldin in Alaska, and Pico de Orizaba in Mexico.

We’ve been mainly talking about terrestrial volcanoes or volcanic activity on land, but 70 percent of volcanic activity happens under water along the mid-oceanic ridges -- which are really all linked together.  We can think of the mid-oceanic ridge -- all 60,000 kilometers of it -- as one vast volcano stitched together like the seams of a baseball snaking along the ocean floor. Some physical geographers even call it the most important feature on the Earth’s surface -- and acoustic data from 9 seabed eruptions along this ridge has opened up a whole new perspective on how the Earth works.

We know that Milankovitch cycles explain how the Earth has switched between glacial and interglacial periods throughout the most recent ice age in Earth’s geological history.  Changes in the wobble and tilt of the Earth on its axis and the “stretch” of the Earth’s orbit from nearly elliptical to nearly circular all combine to determine the pattern.

And now scientists like Maya Tolstoy, a marine geophysicist at Columbia University’s Lamont-Doherty. Earth Observatory think that whether land volcanoes will be more active than underwater volcanoes (or the other way around), and when they will erupt, are linked to and sync up with the Milankovitch cycles and climate changes.

During a glacial period, terrestrial volcanoes don’t erupt as much because the weight of the ice on land puts pressure on the volcanoes. When the ice melts, the pressure lowers and we get more eruptions as all the material locked inside tries to escape. But underwater volcanoes are the opposite.  As water turns to ice and the sea levels drop during a glacial period, the pressure on them eases.

Rate is really important here, and modeling suggests that the faster the sea levels drop, the more eruptions we may get from underwater volcanoes. But correlation doesn't mean causation and the timing here is complicated. Like underwater eruptions seem to start increasing when the sea level hits its lowest point, so after the sea level has been going down for about 80,000 years.

Scientists are still trying to figure out how volcanoes and climate change are related and what the timing means, and there's lots of modeling going on. What’s more astonishing is that underwater volcanoes not only respond to the changes in sea levels associated with glacials and interglacials, but also to short term sea level changes from ocean tides every few weeks. Changes in the shape of Earth's orbit could also be an important timing factor.

When the orbit is more elliptical, the Sun’s gravitational pull changes more rapidly and the Earth is squeezed and unsqueezed like an accordion. And we think this causes more undersea eruptions for a couple reasons: either the squeezing and unsqueezing generates more magma kind of like how a paperclip gets hot when you bend and unbend it over and over, OR the squeezing opens cracks up more and magma can move up more effectively. And as the orbit becomes more circular as part of the 100,000 year cycle, this effect is dampened and there may be fewer undersea eruptions.

And last but not least, all modern, well documented mid-ocean ridge eruptions, have occurred during the first six months of the year. January is when the Earth is closest to the Sun and July is when it's farthest, so that unsqueezing is important.

So even though we talk about the different spheres of the Earth, our knowledge about volcanoes shows us that it really operates as an interconnected system.  And it's amazing that we, as tiny humans, can feel that connectedness and see the energy teeming below the surface of the Earth, when peering down into the crater of an active volcano.

Next time we’ll look at how the rocks that volcanoes help build are eventually broken down and worn away. Many maps and borders represent modern geopolitical divisions that have often been decided without the consultation, permission, or recognition of the land's original inhabitants.

Many geographical place names also don't reflect the Indigenous or Aboriginal peoples languages. So we at Crash Course want to acknowledge these peoples’ traditional and ongoing relationship with that land and all the physical and human geographical elements of it. We encourage you to learn about the history of the place you call home through resources like native-land.ca and by engaging with your local Indigenous and Aboriginal nations through the websites and resources they provide.

Thanks for watching this episode of Crash Course Geography which is filmed at the Team. Sandoval Pierce Studio and was made with the help of all these nice people. If you want to help keep all Crash Course free for everyone, forever, you can join our community on Patreon.