This episode is brought to you by the Music for Scientists album, now available on all streaming services.

To start listening, check out the link in the description. [♩INTRO]. You might think of plate tectonics as destructive the ultimate force behind earthquakes, tsunamis, and volcanic eruptions.

But the slow movement of our planet's surface does a lot more than shake things up now and then — some scientists think life may never have survived without it. Plate tectonics keeps the climate stable, makes the air breathable, and even gives evolution a little push now and then. But this is a two-way relationship!

Life’s presence might have been vital to forming continents in the first place and keeping them moving ever since. Now, you know what we’re talking about here: plate tectonics. In middle school, they probably taught you that parts of the Earth’s crust drift around and bump into each other atop the next layer, the mantle.

What they probably didn’t tell you, though, is how much life needs that to happen. One way plate tectonics supports life is by keeping the climate relatively stable over time. Tectonics power almost every step of the inorganic carbon cycle.

It all starts with volcanoes, which emit carbon dioxide into the atmosphere, warming the planet. A higher temperature increases the rate of the chemical reactions that break down silicon-containing minerals in continental rock. Geologists call this chemical weathering.

These reactions suck carbon dioxide from the atmosphere, reducing the greenhouse effect and cooling the planet back down. As carbon dioxide in the atmosphere goes down and the planet cools, the rate of those weathering reactions also goes back down. Meaning global temperature stays self-correcting over millions or billions of years.

But that’s not the end of this carbon cycle. Rivers wash the products of this reaction out to sea. That includes both dissolved carbon compounds, and elements like calcium and magnesium.

Those react again -- some of the carbon goes back to being carbon dioxide, but some of it forms carbonate minerals on the seafloor. This is often helped along by marine organisms that use calcium carbonate in their shells. Eventually, it’s the fate of oceanic plates to get shoved beneath a continent a process known as subduction.

When that happens, the carbon is returned to below the crust. Finally, it erupts out of a volcano again, completing the cycle. Now, plate tectonics drives many of these steps the volcanoes pumping carbon dioxide into the atmosphere, the formation of fresh crust for CO2 to react with, and the subduction of carbonates back beneath the surface.

So we can thank plate tectonics for maintaining a stable climate long enough for complex life to evolve. Unfortunately for us, this process takes a long time. This balance between volcanoes and weathering takes place over hundreds of thousands to millions of years.

And today, humans are pumping carbon dioxide into the atmosphere at a rate much faster than this cycle can respond. So sadly, plate tectonics won’t save us from ourselves. But carbon dioxide isn’t the only gas in the atmosphere with a link to the ground beneath our feet.

The initial rise of oxygen could have been triggered by the formation of the continents. Ultimately, life is responsible for the oxygen on Earth, by way of photosynthesis. But oxygen is super reactive, so building it up in the atmosphere is easier said than done.

In simple terms: oceanic plates are made of a rock called basalt, and continental plates are made of granite. Oxygen reacts with things like iron and sulfur that are much more common in basalt, and the magma that produces it, than in granite. In other words, the formation of the continents may have caused a chemical shift in the composition of the crust that made it worse at sucking up oxygen letting it build up in the atmosphere.

On top of that, tectonic plates can enhance photosynthesis by supplying nutrients to the oceans. At least, indirectly, according to a 2018 study. Researchers looked at the amount of phosphorus in rocks over Earth’s history and found that it roughly corresponds to oxygen levels in the atmosphere.

As plates move around, they allow heat to escape, and the mantle underneath cools faster. This causes phosphorus to concentrate in the more melty parts of the mantle, which can be carried to the surface and become part of the crust. That means an increase in the amount of phosphorus can be eroded and carried to the oceans.

And phosphorus is an important nutrient, so this basically fertilizes the phytoplankton in the oceans. That leads to more photosynthesis -- and more oxygen in our atmosphere. But it’s not just phosphorus a study in 2015 analysed over 4000 grains of a mineral called pyrite.

When these grains form in the ocean, they capture some of the trace elements and nutrients that were in the water. So the researchers used these grains from the seafloor to track how nutrients in the ocean have changed over time. Tectonics drives these changes because when lots of mountains are being built, more nutrients are eroded and carried to the oceans.

The researchers found that higher nutrients in the oceans roughly corresponded to more oxygen in the atmosphere. More importantly, they showed that several major evolutionary events happened when more nutrients were carried to the oceans. We’re talking about times like the Cambrian explosion, which was the first appearance of many complex animals in the oceans.

And on the flip side, several mass extinction events occurred when there was less tectonic activity and very few nutrients were flowing from the continents to the oceans. There were definitely other causes for these mass extinctions, but nutrient availability could have contributed. All this suggests that plate tectonics could have been a huge deal for the evolution of life!

With all the ways that tectonics supports life on Earth, many scientists think if we’re looking for life elsewhere, we should probably look for planets with active plates on their surface. Since we’ve started looking, we’ve found thousands of exoplanets out there. The “Goldilocks Zone” is the distance from a star that planets can support liquid water.

It helps narrow down which planets we think might have life. But tectonics might be another critical factor. A 2017 study used a model to calculate that only one third of stars have compositions that would allow tectonically active planets to form around them.

That’s because for plates to stay in constant motion, they have to have the right density compared to the mantle. And these scientists think that not every star has the right mix of elements to make that happen. Even in our solar system, we know our closest neighbors, Mars and Venus, aren’t tectonically active now, and we’re not sure if they were in the past either.

But there is a bit of a problem when looking for factors required for life: we only know of one planet with life on it — this one! That could bias us into thinking that anything special about Earth is also essential for life. Indeed, not all scientists agree that plate tectonics is needed to sustain life on other worlds.

For example, a different 2017 study used a model to simulate the carbon cycle on a stagnant lid planet — one with a crust but no moving plates. They found with the right combination of carbon dioxide and weathering, it could maintain a stable climate for billions of years — even without tectonics. So this is still an active area of research and one we’ll only become more interested in as we look closer at all those exoplanets.

It's not too surprising that our planet’s geology shapes life as we know it. But every relationship is a give and take, and some geologists think that tectonics itself may never have begun if it weren't for life. See, some researchers have pointed out that the earliest evidence of plate tectonics and the earliest evidence of photosynthesis roughly line up in Earth's history.

Both start showing up around 3.8 billion years ago. Of course, this could be a coincidence. That was a very long time ago, and there could be older evidence for either one that we’re not aware of -- or that’s simply lost to time.

But there is one hypothesis, put forth in a 2006 paper, that says the onset of photosynthesis could have suddenly changed everything about how our planet works. Remember that we said oceanic plates are made of basalt, and continental plates are made of granite. Basalt is denser than granite.

That difference in density is an essential part of why we have clearly differentiated continents and oceans -- the less dense rock sits higher on the surface. But we can’t take that granite for granted. When magma from the mantle comes to the surface and cools to form rock, it makes basalt.

We see this at hot spot volcanoes like Hawaii and mid-ocean ridges like the one running down the middle of the Atlantic. In fact, this works on pretty much any terrestrial planet we’ve found basalt on the Moon and Mars. But granite is special — it requires the perfect conditions, and the only place we know of it forming is on Earth.

Granite is only formed as oceanic plates subduct. The two key ingredients are basalt and water. If you just have dry basalt, the magma formed from it will make more basalt.

The water changes everything. It causes the rock to melt at a lower temperature, and different components to turn to magma first. The magma formed from wet basalt contains lots of silica and metals like aluminum.

When it cools, you end up with granite! This is where life comes in. We’ve already talked about chemical weathering.

The presence of life makes it happen faster. Photosynthesis releases oxygen and other chemicals that can react with and wear down rocks on the surface -- think of iron rusting. That means more sediment and clay are carried to the oceans, along with plankton settling to the seafloor.

Life means a massive increase in the amount of stuff in the oceans. Adding all of this to the ocean floor effectively seals in water as an oceanic plate is subducted under a continent. By the time it reaches the depth where the rock melts, the plate won’t have dried out yet, and melting it can make granite.

So without life, oceanic plates dry out too quickly. With life, they keep their water and create the perfect conditions to make more continents. A 2014 model simulation comparing Earth with and without life suggested that living things significantly increased the amount of continental crust.

Which is… amazing. You wouldn’t think that plankton could make a difference on the scale of a whole planet. But here we are -- or so these researchers think!

As living things, we owe a debt of gratitude to the giant tectonic plates beneath our feet, which in turn may have the smallest photosynthesizers in the ocean to thank for their existence. It’s a fantastic example of how seemingly very different pieces of our planet are intricately connected. And when we compare Earth to other planets, it’s a reminder of how unique and possibly rare habitable planets are.

We talked about some provocative ideas today, like whether life on Earth can shape the very rock beneath our feet. The song “The Idea,” from the album Music for Scientists, touches on just how hard it is to come up with those hypotheses, and how, for every correct idea, there are thousands of wrong ones. And we just won’t know until we test them!

The song also has a music video produced by our friends at ThoughtCafe. So if you think you might be interested, we’ve got a link in the description where you can start listening to Music for Scientists right now! [♩OUTRO].