Ah, New York City.

The home of Broadway, the two-dollar slice… and this! St.

Patrick’s Cathedral. The nearly 150-year-old church is truly something to behold. With its sky-high spires, stained glass windows, and massive pipe organ, it’s a favorite destination for tourists.

I guess the shirt gave me away, huh? Yeah, I’m from California. But I do a great Robert De Niro!

I’ll work on it, okay, Dwayne? But there’s something else amazing about the cathedral. Around the church, ten wells are dug deep within the earth, some as far down as 670 meters.

That’s more than six times deeper than the height of its spires. Why? For energy.

Piped up straight from the earth. Hi! I’m Sage, and this is Crash Course Geology. [THEME MUSIC] When we need energy, where do we turn?

When it comes to our bodies, we eat plants and animals. But when it comes to fueling our cars, heating our houses, or powering our studio lights and video cameras, we need more energy. Like, a lot more.

For the past hundred-fifty-ish years, we’ve mostly powered our world with fossil fuels— things like oil, coal, and natural gas. They’re compounds made of algae, plant, and animal remains from millions of years ago. That’s right.

Million-year-old dead stuff powers my toothbrush, TV, and motor scooter. Catch me yelling this to everyone as I scoot around Long Beach. But how do we get fossil fuels?

Well, when most living things die, they decompose, breaking down into simple compounds. But if they're buried in  just the right conditions,  amazing things start to go down. Take oil, for example.

To start, we’ve got plankton, those tiny life forms that drift around in bodies of water. In still water without a lot of oxygen, it’s hard for them to decompose. So instead, dead plankton accumulate at the bottom, mixing with clay to become “organic ooze.” And Earth starts to work its magic.

As the ooze gets buried deeper and deeper, the pressure squeezes out water, transforming it into rock known as organic shale. And if that rock is buried deep enough, the rising temperatures transform it again, into a waxy, energy-dense substance called kerogen. Under Earth’s surface, things can get pretty hot.

If temps down there climb past 90 degrees Celsius, kerogen breaks down into small oil molecules, known as hydrocarbons. And at even higher temperatures, the oil molecules break down even smaller to form natural gas. The natural gas we use for fuel is mostly made of methane, by the way.

Once they’re formed, oil and natural gas can percolate up toward the surface. Some oil might even burble up in ponds called oil seeps. But other pockets get trapped underground by impermeable layers of rock, and settle into reservoirs, which we can tap for fuel.

Now, coal, on the other hand, forms a bit differently. Nearly all the coal on Earth right now formed about 350 million years ago. At that time, Earth was covered in plants.

A super rainy climate gave us tons of tropical areas with wet forest floors. Those swampy areas were low in oxygen, which meant that living things didn’t decompose as much. At the same time, all the continents were coming together in a geologic mash-up that would eventually create the supercontinent Pangea.

As tectonic plates collided, they raised towering peaks and formed plunging basins: the perfect collection points for layer upon layer of dead plant matter. That intense buildup of plants eventually transformed into peat, an energy-rich substance resembling soil. Peat has long been an energy source for humans.

We’ve used it for cooking and heating for thousands of years. But if peat is buried deep enough, the Earth’s heat and pressure concentrate the plants’ carbon and hydrogen into an even more energy-dense, brittle rock called coal. This coal-creating time was only about sixty million years long— just a small slice of our planet’s 4.6 billion-year-history.

It became known as the Carboniferous period, meaning “coal-bearing.” Since then, conditions have rarely been exactly right to create more. Once we use up our ancient reserves of coal, it’ll be gone, possibly forever. Wow.

I think I’d actually be down to get a lump of coal for Christmas this year. [bells jingle] You’re real?! Dairy free right? Oh my god, thank you. [Chapter 3: The problems with fossil fuels] This coal-nundrum captures one of the big problems with fossil fuels: they’re nonrenewable, which means it’s possible to use them up, or for them to become so hard to extract that it’s not practical to keep using them.

Even though the earth can make more coal, oil, and natural gas, it can’t do it on a human timescale. It would take millions of years to restore what we’ve consumed. And even if they were infinite, there are issues with how fossil fuels are extracted, too.

When more easily accessible reservoirs of oil and natural gas run dry, we have to resort to tactics like fracking, which uses high-pressure water, chemicals, and sand to break up impermeable rock. This can have a variety of harmful effects, from air pollution to habitat degradation and potential health issues for people living nearby. As for coal, mines can cause a whole host of problems, too, like sinkholes, erosion, water contamination, and damage to plant and animal habitats.

That’s not to mention the human cost of environmentally hazardous processes, and the impacts on Earth’s climate. You can learn much more about the effects of fossil fuels in Crash Course Climate and Energy. [Chapter 4: Geothermal energy] So, there are lots of reasons to break up with fossil fuels and find other sources of energy. And Earth might hold one of the keys to powering our future more sustainably: geothermal energy.

Geothermal energy comes from the heat under Earth’s surface. The inside of our planet is super hot. It’s still holding onto heat from the collisions that likely formed it.

Then there’s decaying radioactive elements in the core and mantle, and all the hot magma and volcanic activity in there… that’s a lot of energy stored beneath our feet. No, Dwayne. I’m not saying  that “the floor is lava.” You’re good, dude.

But how do we harness it? We can most easily tap into geothermal heat near volcanic regions, where pockets of molten rock have risen up into the crust. This magma heats up water that collects in cracks and pores in the crust, creating geothermal reservoirs.

The heated water can sometimes rocket through the surface as jets of steam or sizzling geysers like a geologic waterpark. Or it can bubble up like hot tub jets into a hot spring. A lot of volcanic activity happens near the boundaries of tectonic plates, or places where the crust is really thin.

That lets hot mantle rock and blobs of magma sneak up closer to the surface. Like, Iceland is an island that sits right on the mid-ocean ridge, an underwater mountain range where two tectonic plates are pulling apart. This makes it a great place to harness geothermal energy.

And this kind of energy is nothing new. Humans have been using geothermal energy to power their lives for thousands of years. Early hominins in East Africa might have used hot springs to cook their food.

And people in Iceland still do today, like with this traditional “hot spring bread”. You’re telling me I can experience the wonders of the earth and eat delicious bread? Sign me up!

But it didn’t stop there. We started using geothermal energy to power electricity over a hundred years ago at a power plant in Tuscany that’s still operational today. And as of 2023, about a quarter of Iceland’s electricity is powered by geothermal energy.

That leads us to one of the greatest points in favor of geothermal. By running geothermal steam or water through pistons and turbines, we can generate electricity, without producing any emissions, or using up any water. Because of the abundance of heat and volcanic activity under Earth’s surface, geothermal energy is renewable.

And unlike other renewable energies, like solar or wind power, geothermal energy is available all the time, rain or shine. And did I mention the abundance factor? I know I did, but I feel like it’s worth emphasizing!

Some researchers think that just 0.1 percent of the heat produced by the Earth’s core would be enough to meet all of humanity’s energy needs for the next 2 million years. Two! Million!

Years! So… what’s the hold-up? Why aren’t we already running the earth on, you know, the earth?

The biggest challenge is  tapping into all that energy— we can’t exactly dig down to Earth’s core. While geothermal energy can be pretty accessible in volcanic regions, it’s a lot tougher in places where you have to dig deep below the surface. To do it, we have to install heat pumps in deep boreholes in the ground.

The heat pumps send water and other  substances way down into  the crust to capture heat. Then, they transport it up to the surface, where we can use it to cook, heat buildings, or power electricity— even in places as un-volcanic as Ohio or New York City. In fact, heat pumps can give us access to geothermal energy nearly anywhere.

But they’re expensive up-front, and not every location can accommodate the big drills required to put them in place. Add the fact that most folks don’t even know what geothermal energy is, and you can see why there isn’t a heat pump on every corner. So, it’ll take a lot more work  to make the infrastructure  for geothermal energy accessible to everyone.

But in places like Italy, Indonesia, and the Western US, geothermal energy is growing fast. It’s used for air-conditioning in Tuscany, mushroom farming in West Java, and melting snow on roads and sidewalks in Oregon. There are still lots of opportunities to  incorporate this form of  energy into our daily lives.

Maybe someday, you’ll warm your hot chocolate with geothermal energy. Cheers, Dwayne. [Chapter 5: Review and Credits] Sorting out our relationship with energy is a complex problem. As humanity’s energy needs grow, we need to find alternatives to fossil fuels.

And while it’s unlikely that geothermal energy is the solution to all our problems, it could become a key part of a wider effort to live within the capacity of our planet. Geologists today are figuring out how to combine geothermal energy with other renewable energy sources and strategies so we can say farewell to fossil fuels for good. *crash* Next time, we’ll learn about the minerals that power our modern world. See you then.

Thanks for watching this episode of Crash Course Geology which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these beautiful, bubbly people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.