The island of Madagascar is one  of the most biodiverse places on Earth.

Around 5% of the  world’s species live there… on less than half a percent  of the world’s total landmass! The vast majority of those species are endemic.

That means they’re found only in Madagascar. And I’m not just talking about  animals, like lemurs or tomato frogs. It’s the same for plants and fungi, too.

At least for the species that  scientists have the data on, anyway. For many years, experts have cited Madagascar’s isolation as the cause of  all this biological variety. But Madagascar is simply too diverse for just one reason to be behind it all.

Instead, it appears the island’s  secret lies not just in its seclusion, but also in its geological past,  including getting ripped off of not one, but two separate continents. [♪INTRO] Today, Madagascar is the fourth  largest island in the world. With an area of 587,000 square kilometers, it’s roughly on par with  SciShow’s home state of Montana… if Montana also ate its neighbor North Dakota. And across all those square kilometers, the island’s got some pretty  distinctive geography.

It has steep cliffs stretching  along its east coast, and mountains in the center  and north of the island. Meanwhile, its western coast  is flatter and more varied. Ultimately, all these  landscapes are the product of a 170 million year-long game  of geological ping pong.

Madagascar currently sits about 400 kilometers off the east coast of Africa. But 200 million years ago,  the island was a slice of land in the middle of the  giant continent Gondwana, which also included Africa,  India, Antarctica, and Australia. Then, in the mid-Jurassic, between 170 and 140 million years ago, Gondwana split apart.

India, Australia, and Antarctica  moved south together, taking Madagascar with them at the  western edge of a new continent. This continent then experienced its own breakup during the early Cretaceous, taking both Australia and Antarctica out of our story for today. Fast forward to about 90 million years ago.

Deep down in Earth’s mantle, a plume  of hot material started to rise. This created a hotspot under the surface of Greater India, a bit like  what’s under Iceland, now. Molten rock soon poured out of  the crust and over the edge of the ancient continent, helping  to open another ocean rift.

This time, Madagascar was split  from the Indian subcontinent, and got left behind as the rest of the landmass wandered off to crash into the rest of Asia. In both of these rifting events, Madagascar was basically an inactive spectator. A chunk of land getting pushed out of the way as a new ocean basin formed.

For explaining Madagascar’s extreme biodiversity, that’s actually a bit annoying. Biological diversity is often higher  in tectonically dramatic settings, like where mountains are forming.  Madagascar doesn’t fit that picture. And yet, that pair of  massive geological upheavals, and the many lesser processes  that followed each of them, did help shape the island’s  inhabitants in some critical ways.

Let’s start with those two major rifting events. A rift starts when tectonic forces,  like the churning of the mantle, or the tug of distant subducting tectonic plates, stretch one piece of crust in opposite directions. The top layer of that crust is  relatively cold, solid rock, so the main way it accommodates  that stretching is by breaking.

Not unlike how I react to excessive stress All this breakage results  in a series of steep faults, or fractures between separate blocks of rock, which slide huge chunks of rock in all directions. Sideways, up, and down. On a large scale, this faulting makes the crust at the center of a rift thinner.

This is why geologists often  talk about rift ‘valleys’. When the crust becomes thin like this, hot mantle material can flow upward  to try and balance things out. But that ends up pushing on  the sides of the rift, too.

So like a root pushing up through  asphalt, the rifted valley sides each get tilted up and back, creating  steep cliffs called escarpments, and gradually downward  sloping plateaus behind them. And escarpments don’t just  change how a landscape looks. They also change how it works.

The cliff edge becomes a sharp water divide, separating one drainage basin from another. A raindrop at the top of the cliff could find itself traveling along either the short, steep rivers down the face of the escarpment, or a longer, shallower path  down the tilted plateau. Those steep rivers on the escarpment  face have a ton of energy, which helps erode the cliff face, and slowly move the whole  escarpment away from the rift.

It seems this is what’s been  happening on Madagascar. But before we can continue,  all science needs funding. So here’s an ad.

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They mapped all the river networks  along with changes in elevation, and developed a model to explain  the strange combination of features. They concluded the first episode of rifting, the one that separated Madagascar from Africa, created an escarpment on the  western side of the island. The resulting plateau behind the cliff sloped gradually down towards the east.

The water divide was near  to the new western shore, and the short steep rivers  that ran to the sea would have helped the escarpment creep  eastward over millions of years. Then, when the island eventually  rifted from India, the upheaval created another escarpment, this  time on the island’s eastern side. The plateau that had originally  tilted east now tilted back towards the west, and the water divide  jumped to the eastern shore.

This basically reversed all  of the rivers in Madagascar, and the westward-flowing channels now crossed over the old escarpment. But these channels had steeper slopes than before, meaning any erosion they caused happened faster. So the river valleys expanded faster, too.

These reconfigured rivers and their basins then ate away at the old western escarpment, until much of the original topography was lost. Today, all that’s left is the occasional remnant bit of plateau left alone on a flattened plain. Meanwhile, the newer, eastern  escarpment is currently retreating in a much more orderly  fashion maintaining a straight, well-defined cliff edge as it  inches away from the shore.

But this isn’t the end of Madagascar’s story as a piece of rifting collateral damage. Somewhere between 30 and 60  million years ago, large scale movements in Earth’s mantle raised  the north part of the island. And for the last 30 million  years, parts of Africa have been slowly ripping the  continent apart once again.

The East African Rift has  been spreading southward, causing effects that ripple over to Madagascar. For example, it’s made a pulse of  volcanic activity and uplift that’s created mountains like pimples  in the center of the island, as well as chopped up parts of the  central plateau into a mess of faults. All of this has enhanced  the effect of river erosion, changing the topography as well  as where the water divide sits.

In fact, in the south of the  island, the water divide is moving at less than 200 meters per million years. But in the north, it’s up to a breakneck  2.5 kilometers per million years. And next to the central volcano and active faults, it’s closer to 4 kilometers per million years.

If you’re wondering, no. That’s still slower than  your toenails grow. Probably.

So what does all of this geologic history have to do with why the aye-aye exists? Or the avenue of giant baobab? Or any of the island’s  non-photogenic endemic life forms?

When Madagascar split away  from Africa and then India, it would have taken its native  flora and fauna with it. But the majority of these  species, including any dinosaurs, were wiped out at the end of the Cretaceous, around 66 million years ago. It wasn’t a complete fresh  start, but it was pretty close.

The island was soon recolonized  by plant and animal immigrants, who followed wind and water  currents from India and Africa. And once they reached the island, they had to deal with an  ever-evolving escarpment landscape. In 2024, researchers built a computer model to simulate how river erosion would change both Madagascar’s landscape and  plant habitats over time.

In general, their simulated escarpment migrated away from the coast at a constant rate, but developed an irregular shape due to each rivers’ drainage basin  growing at a different rate. As a result, the habitat patches were dynamic, frequently changing their size, shape, appearing and disappearing,  and fragmenting and merging. Unique, isolated habitats would persist  for a few million years at a time, which is exactly what evolution needs to create new species adapted to one specific place.

For instance, water divides, steep cliffs, and large rivers are enough to separate lemurs. And when populations can’t intermingle, their gene pools can each go in  their own evolutionary direction. This creates what’s known as micro-endemism, where species evolve that are  unique and specific to a much more local region than your general,  say, island-level, endemism.

Together, these models of  Madagascar’s geological and ecological history help explain the  island’s high overall biodiversity and how this diversity differs  so much across the island. Today, the low terrain along the western coast, created by the migration of  the older, western escarpment, is home to vast swathes of mangrove forests. Meanwhile, the central highlands are dominated by a mosaic of grasslands and woodlands, which vary locally along with  the remnant escarpment landscape.

And the more recent, eastern  escarpment plays a big role in shaping rainfall across the island, making it lush and humid in the  east, compared to the drier west. This escarpment, with its higher terrain and higher rainfall, is itself  a hotspot for biodiversity. For example, out of the nearly 9,000 species of seed plants mapped across all of Madagascar, more than 70% of them live along or just below the eastern escarpment.

There are also biodiversity differences along the length of the escarpment. In general, researchers have  reported a higher diversity of plants, amphibians, reptiles, and lemurs in the northern part of the  island compared to the south. This is thought to be linked  to the faster escarpment retreat in the north, caused  by that extra uplift over 30 million years ago, which  creates more habitat disruption.

All that geologically recent volcanic and tectonic activity has helped shift the water divide, too. Yet another source of disruption. In fact, researchers have found  evidence that species started diversifying faster starting  around 40 to 30 million years ago.

Looks like a pretty solid link to me, and to people with fancier science degrees getting paid to figure all this out. Escarpment landscapes, and how they evolve, help boost biodiversity around the world. But in Madagascar, the Earth  has basically doubled down.

It gave Madagascar two, turning  it into an island in the process. So the next time you pause to appreciate a picture of some weird Madagascan critter, thank a bunch of rocks. [♪OUTRO]