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Geology Created Madagascar's Weird Animals
Geology Created Madagascar's Weird Animals
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The island of Madagascar is one of the most biodiverse places
0:03
on Earth. Around 5% of the world’s species live there…
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on less than half a percent of the world’s total landmass!
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The vast majority of those species are endemic.
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That means they’re found only in Madagascar.
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And I’m not just talking about animals, like lemurs or tomato frogs.
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It’s the same for plants and fungi, too.
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At least for the species that scientists have the data on, anyway.
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For many years, experts have cited Madagascar’s
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isolation as the cause of all this biological variety.
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But Madagascar is simply too diverse for
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just one reason to be behind it all.
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Instead, it appears the island’s secret lies not just in its seclusion,
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but also in its geological past, including getting ripped off of not one,
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but two separate continents.
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[♪INTRO]
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Today, Madagascar is the fourth largest island in the world.
0:52
With an area of 587,000 square kilometers,
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it’s roughly on par with SciShow’s home state of Montana…
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if Montana also ate its neighbor North Dakota.
1:01
And across all those square kilometers,
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the island’s got some pretty distinctive geography.
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It has steep cliffs stretching along its east coast,
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and mountains in the center and north of the island.
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Meanwhile, its western coast is flatter and more varied.
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Ultimately, all these landscapes are the product of a
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170 million year-long game of geological ping pong.
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Madagascar currently sits about
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400 kilometers off the east coast of Africa.
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But 200 million years ago, the island was a slice of
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land in the middle of the giant continent Gondwana,
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which also included Africa, India, Antarctica, and Australia.
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Then, in the mid-Jurassic,
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between 170 and 140 million years ago,
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Gondwana split apart.
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India, Australia, and Antarctica moved south together, taking
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Madagascar with them at the western edge of a new continent.
1:46
This continent then experienced its own breakup
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during the early Cretaceous, taking both Australia
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and Antarctica out of our story for today.
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Fast forward to about 90 million years ago.
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Deep down in Earth’s mantle, a plume of hot material started to rise.
2:00
This created a hotspot under the surface of
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Greater India, a bit like what’s under Iceland, now.
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Molten rock soon poured out of the crust and over the edge of
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the ancient continent, helping to open another ocean rift.
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This time, Madagascar was split from the Indian subcontinent,
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and got left behind as the rest of the landmass
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wandered off to crash into the rest of Asia.
2:20
In both of these rifting events, Madagascar
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was basically an inactive spectator.
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A chunk of land getting pushed out of the
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way as a new ocean basin formed.
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For explaining Madagascar’s extreme biodiversity,
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that’s actually a bit annoying.
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Biological diversity is often higher in tectonically dramatic settings,
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like where mountains are forming. Madagascar doesn’t fit that picture.
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And yet, that pair of massive geological upheavals,
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and the many lesser processes that followed each of them,
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did help shape the island’s inhabitants in some critical ways.
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Let’s start with those two major rifting events.
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A rift starts when tectonic forces, like the churning of the mantle,
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or the tug of distant subducting tectonic plates,
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stretch one piece of crust in opposite directions.
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The top layer of that crust is relatively cold, solid rock,
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so the main way it accommodates that stretching is by breaking.
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Not unlike how I react to excessive stress
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All this breakage results in a series of steep faults,
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or fractures between separate blocks of rock,
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which slide huge chunks of rock in all directions.
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Sideways, up, and down.
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On a large scale, this faulting makes
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the crust at the center of a rift thinner.
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This is why geologists often talk about rift ‘valleys’.
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When the crust becomes thin like this,
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hot mantle material can flow upward to try and balance things out.
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But that ends up pushing on the sides of the rift, too.
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So like a root pushing up through asphalt, the rifted valley sides
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each get tilted up and back, creating steep cliffs called escarpments,
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and gradually downward sloping plateaus behind them.
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And escarpments don’t just change how a landscape looks.
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They also change how it works.
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The cliff edge becomes a sharp water divide,
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separating one drainage basin from another.
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A raindrop at the top of the cliff could find
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itself traveling along either the short,
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steep rivers down the face of the escarpment,
4:01
or a longer, shallower path down the tilted plateau.
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Those steep rivers on the escarpment face have a ton of energy,
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which helps erode the cliff face,
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and slowly move the whole escarpment away from the rift.
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It seems this is what’s been happening on Madagascar.
4:16
But before we can continue, all science needs funding.
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5:09
In 2025, a team of researchers from Switzerland
5:12
studied Madagascar’s geomorphology in detail.
5:15
They mapped all the river networks along with changes in elevation,
5:19
and developed a model to explain the strange combination of features.
5:22
They concluded the first episode of rifting,
5:24
the one that separated Madagascar from Africa,
5:27
created an escarpment on the western side of the island.
5:30
The resulting plateau behind the cliff
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sloped gradually down towards the east.
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The water divide was near to the new western shore,
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and the short steep rivers that ran to the sea would have
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helped the escarpment creep eastward over millions of years.
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Then, when the island eventually rifted from India, the upheaval
5:46
created another escarpment, this time on the island’s eastern side.
5:50
The plateau that had originally tilted east now tilted back towards
5:54
the west, and the water divide jumped to the eastern shore.
5:58
This basically reversed all of the rivers in Madagascar,
6:00
and the westward-flowing channels
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now crossed over the old escarpment.
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But these channels had steeper slopes than before,
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meaning any erosion they caused happened faster.
6:10
So the river valleys expanded faster, too.
6:12
These reconfigured rivers and their basins
6:14
then ate away at the old western escarpment,
6:17
until much of the original topography was lost.
6:20
Today, all that’s left is the occasional remnant
6:22
bit of plateau left alone on a flattened plain.
6:25
Meanwhile, the newer, eastern escarpment is currently
6:28
retreating in a much more orderly fashion maintaining a straight,
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well-defined cliff edge as it inches away from the shore.
6:35
But this isn’t the end of Madagascar’s
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story as a piece of rifting collateral damage.
6:40
Somewhere between 30 and 60 million years ago, large scale
6:43
movements in Earth’s mantle raised the north part of the island.
6:47
And for the last 30 million years, parts of Africa have
6:50
been slowly ripping the continent apart once again.
6:53
The East African Rift has been spreading southward,
6:55
causing effects that ripple over to Madagascar.
6:58
For example, it’s made a pulse of volcanic activity and uplift that’s
7:02
created mountains like pimples in the center of the island,
7:05
as well as chopped up parts of the central plateau into a mess of faults.
7:09
All of this has enhanced the effect of river erosion,
7:12
changing the topography as well as where the water divide sits.
7:16
In fact, in the south of the island, the water divide is
7:18
moving at less than 200 meters per million years.
7:21
But in the north, it’s up to a breakneck 2.5 kilometers per million years.
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And next to the central volcano and active faults,
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it’s closer to 4 kilometers per million years.
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If you’re wondering, no.
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That’s still slower than your toenails grow. Probably.
7:36
So what does all of this geologic history
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have to do with why the aye-aye exists?
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Or the avenue of giant baobab?
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Or any of the island’s non-photogenic endemic life forms?
7:46
When Madagascar split away from Africa and then India,
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it would have taken its native flora and fauna with it.
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But the majority of these species, including any dinosaurs,
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were wiped out at the end of the Cretaceous,
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around 66 million years ago.
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It wasn’t a complete fresh start, but it was pretty close.
8:02
The island was soon recolonized by plant and animal immigrants,
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who followed wind and water currents from India and Africa.
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And once they reached the island,
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they had to deal with an ever-evolving escarpment landscape.
8:14
In 2024, researchers built a computer model
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to simulate how river erosion would change both
8:19
Madagascar’s landscape and plant habitats over time.
8:23
In general, their simulated escarpment
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migrated away from the coast at a constant rate,
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but developed an irregular shape due to each
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rivers’ drainage basin growing at a different rate.
8:33
As a result, the habitat patches were dynamic,
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frequently changing their size, shape,
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appearing and disappearing, and fragmenting and merging.
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Unique, isolated habitats would persist for a few million years at a time,
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which is exactly what evolution needs to create
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new species adapted to one specific place.
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For instance, water divides, steep cliffs,
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and large rivers are enough to separate lemurs.
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And when populations can’t intermingle,
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their gene pools can each go in their own evolutionary direction.
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This creates what’s known as micro-endemism,
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where species evolve that are unique and specific to a much more
9:06
local region than your general, say, island-level, endemism.
9:10
Together, these models of Madagascar’s geological and
9:12
ecological history help explain the island’s high overall biodiversity
9:17
and how this diversity differs so much across the island.
9:20
Today, the low terrain along the western coast,
9:23
created by the migration of the older, western escarpment,
9:27
is home to vast swathes of mangrove forests.
9:30
Meanwhile, the central highlands are dominated
9:32
by a mosaic of grasslands and woodlands,
9:35
which vary locally along with the remnant escarpment landscape.
9:39
And the more recent, eastern escarpment plays a big role in
9:42
shaping rainfall across the island,
9:44
making it lush and humid in the east, compared to the drier west.
9:47
This escarpment, with its higher terrain
9:49
and higher rainfall, is itself a hotspot for biodiversity.
9:53
For example, out of the nearly 9,000 species of
9:56
seed plants mapped across all of Madagascar,
9:59
more than 70% of them live along
10:01
or just below the eastern escarpment.
10:04
There are also biodiversity differences
10:06
along the length of the escarpment.
10:08
In general, researchers have reported a higher diversity of
10:11
plants, amphibians, reptiles, and lemurs in
10:14
the northern part of the island compared to the south.
10:16
This is thought to be linked to the faster escarpment
10:19
retreat in the north, caused by that extra uplift over
10:22
30 million years ago, which creates more habitat disruption.
10:26
All that geologically recent volcanic and tectonic
10:28
activity has helped shift the water divide, too.
10:31
Yet another source of disruption.
10:33
In fact, researchers have found evidence that species started
10:35
diversifying faster starting around 40 to 30 million years ago.
10:40
Looks like a pretty solid link to me,
10:42
and to people with fancier science degrees
10:44
getting paid to figure all this out.
10:46
Escarpment landscapes, and how they evolve,
10:48
help boost biodiversity around the world.
10:50
But in Madagascar, the Earth has basically doubled down.
10:53
It gave Madagascar two, turning it into an island in the process.
10:57
So the next time you pause to appreciate
10:58
a picture of some weird Madagascan critter,
11:01
thank a bunch of rocks.
11:08
[♪OUTRO]
