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Despite having sailed over it for thousands of years now, we still know shockingly little about the seafloor. Like, as of 2017, only 6% of the ocean floor had been mapped in detail.

But one thing we do know is that the ocean floor is not a flat, featureless landscape. Just like the world above the water, it’s full of valleys, canyons, mountains, and plains. It is also home to some of the biggest events shaping our planet.

And we would know a whole lot less about all of that without the work of a researcher named. Marie Tharp, who successfully changed our understanding of the ocean and also the entire world. Tharp’s story began in 1948, when she was employed at what’s now the Lamont-Doherty.

Earth Observatory in New York. At the time, her job was to do drafting and computing for graduate students, one of whom was Brian Heezen. Before Tharp joined the group, Heezen had collected extensive data about the floor of the Atlantic Ocean.

But since he was often away at sea, and because regulations didn’t allow women to sail on research vessels, Tharp took over the task of organizing his data. Before long, she was working on Heezen’s project exclusively. And that turned out to be huge for the future of science.

See, Heezen was involved in collecting sonar data from ships, which was used to measure the ocean’s depth. And while sonar is common today, this was, like, cutting-edge work at the time. In the past, depth measurements were made with sounding.

That’s where researchers would lower a rope with a weight on the end over the side of the ship. And then, when the weight hit the bottom, someone would mark the depth, and then they’d pull up the rope. I guess that’s how you’d do it!

There were of course, though, some problems with this. Like, sounding was often inaccurate because the weight would rarely fall straight down. Also, that gives you one data point at a time, which means it was exhausting to map a big area.

So, the advent of sonar drastically improved things. Instead of a rope, the ship would produce sound waves and measure the amount of time it took for the waves to bounce off the seafloor and return to the ship. Then, because we know the speed of sound in water, researchers could calculate the depth of the ocean with some quick math.

Thanks to sonar, Heezen had compiled tons of data. But Tharp figured out what to do with it. She made hand-drawn diagrams that plotted the ships’ paths and their depth measurements.

Then, by stitching them together, she created what were basically topographical maps of the sea floor. And as she did, she found something… unexpected: a cleft in the center of the North Atlantic. Ocean, kilometers wide and hundreds of meters deep.

She thought it looked a lot like a rift valley, a feature first observed on land that forms when two tectonic plates pull apart from each other. Tectonic plates being the giant slabs that make up the Earth’s surface. Except... at the time, the idea that these plates could pull apart and that the continents were moving was not widely-accepted, because it just, like, seemed a little unlikely.

Instead, a lot of people, including Heezen, held up an idea called “expanding Earth,” which said that continents were pulling apart because the actual volume of the Earth was increasing… like we’re a balloon. But then came Tharp’s rift valley — this gigantic cleft that just looked suspiciously like a place where tectonic plates were pulling apart. And today, we know that’s exactly what it is!

We call it the Mid-Atlantic Ridge, and there, the tectonic plates are being shoved apart as hot material from inside the Earth rises to the surface. This ridge ultimately gave us a mechanism for continental drift, and it was a nail in the coffin of the expanding Earth idea. But back when Tharp shared those thoughts with Heezen, he called the idea “girl talk” and asked her to redo her calculations.

And so Tharp did, and she found — like, maybe unsurprisingly —  there was a rift valley still there. Also, to add even more evidence, it turned out that another assistant in the lab was making a map of earthquake epicenters. And the earthquakes were forming in the same spot as Tharp’s rift valley.

Since earthquakes are formed by the movement of tectonic plates, this made it clear that the rift valley was between two tectonic plates and they were pulling apart. And now, continental drift is foundational to our understanding of the planet and how it changes. Like, among many other things, it explains why some of the continents look like they used to be connected — because they were!

And it might have taken us a lot longer to understand that without Tharp’s maps. In the end, Tharp studied the ocean floor until her death in 2006. Along the way, besides like, proving continental drift is a thing, she also made some more incredibly detailed maps of the deep ocean.

And she worked constantly on understanding the Mid-Atlantic-Ridge and other systems like it. So, while we have a lot left to learn about the ocean, Marie Tharp gave us an enormous shove in the right direction. We now have a new appreciation for the world beneath the water — and how rift valleys are changing the world beneath our feet, too.

Speaking of the world beneath our feet… If you’ve grown up thinking the inside of the Earth looks like this, with nice, neat layers, you might want to check out another one of our episodes next. And as always, from the entire team here at SciShow, thank you for watching this episode as well. [♪ OUTRO].