How two ancient rifts sculpted a living island
Madagascar’s cliffs, rolling plateaus and winding rivers were not shaped by a single violent event. Instead, the island’s breathtaking landscape took shape through two massive tectonic faults that occurred tens of millions of years apart.
These changes tilted the land, redirected the rivers, and sculpted the island’s dramatic shape: steep cliffs falling into the Indian Ocean to the east and gentle plains stretching toward the Mozambique Channel to the west. Together, these forces have created not only a striking landscape, but also one of the most biologically rich ecosystems on Earth.
Two ancient faults, a remarkable island
Long before Madagascar stood alone in the ocean, it was part of the ancient supercontinent Gondwana. About 170 million years ago, the first major tectonic rupture separated it from Africa. The crust folded upward, forming a massive escarpment in the west, and rivers flowed east toward the Indian Ocean, carving deep valleys into a rising plateau.
The rugged mountains of western Madagascar. The landscape is dominated by isolated peaks, evidence of an ancient plateau deeply incised by large river systems over millions of years. (CREDIT: Romano Clementucci / ETH Zurich)
About 80 million years later, another rift opened, this time between Madagascar, India and the Seychelles. The island tilted again, but in the opposite direction. Land that was once tilted to the east began to tilt to the west, reversing the flow of the river and moving the island’s main watershed to the east. The imposing western escarpment eroded into scattered highlands, while a new steep cliff line rose along the eastern coast.
“The water divide is key to the geography of Madagascar,” said Romano Clementucci, a geologist at ETH Zurich and lead author of the new Science Advances study. “Each time the island tilted, the line separating the east- and west-flowing rivers crossed the island, changing the way water and erosion shaped the land. »
Rivers that reshape the territory
These tilts not only bent the waterways: they also rebuilt the surface of the island. Old river beds were abandoned, new valleys were carved out, and some rivers even reversed their course. The result is striking: steep cliffs and fast rivers to the east, gentle slopes and wide plains to the west.
Using high-resolution satellite images, erosion data and computer models, Clementucci’s team mapped Madagascar’s slow transformation in remarkable detail. They used cosmogenic isotopes like beryllium-10 (^10Be), which accumulate in rocks exposed to cosmic rays, to measure how quickly erosion reshaped the island over millions of years.
The rugged mountains of eastern Madagascar, shaped by a tropical climate and rugged topography. The escarpment has retreated inland since the second rifting event (90 Ma) and today acts as a natural barrier to precipitation, marking the western limit of the eastern tropical rainforests of the island. (CREDIT: Romano Clementucci / ETH Zurich)
Their results show that erosion remains most intense along the eastern escarpment. To the south, the cliffs are retreating at around 170 meters per million years. But in the north – a more tectonically active area – the rate accelerates to almost 3,800 meters per million years. In comparison, the central plateau erodes slowly, only about seven meters per million years, preserving remnants of Madagascar’s ancient surface.
Recreating a moving island
To confirm their findings, the researchers performed computer simulations of Madagascar’s geological past. Each rifting event caused one side of the island to collapse, creating a new scarp that gradually eroded inward. During the second rupture, the tilt reversed and the entire landscape was reshaped again.
The model successfully recreated the modern features of Madagascar: the steep eastern escarpment, the gentle western slope, and the “knickpoints,” or sudden drops in river levels, that we see today. These nooks and crannies are relics of ancient changes in the island’s drainage system.
And the story is not over. Volcanic and tectonic forces continue to reshape Madagascar today, particularly in areas like the Ankaratra Volcanic Field and the Alaotra-Ankay Graben. These active areas are still altering the courses of rivers, sinking parts of the plateau and producing mild earthquakes – signs that the island’s crust is far from calm.
Morphostructural characteristics of Madagascar and topographic scarps. (CREDIT: Scientific Advances)
A landscape that gave birth to life
Madagascar’s incredible biodiversity – from lemurs to chameleons to baobabs – has long been attributed to isolation and climate. Clementucci’s study adds another key factor: geology.
The team discovered a strong link between erosion rates and plant diversity along the eastern escarpment. Where slopes are steeper and rivers change more often, plant species multiply – from around 1,200 in the south to more than 2,000 in the north. Precipitation alone cannot explain the difference. Instead, constant land reshaping appears to have fragmented habitats and caused species to evolve separately.
Essentially, Madagascar’s changing relief acted as a “speciation pump.” When rivers changed course or valleys deepened, populations became isolated and began to evolve on their own. This process has helped produce the island’s astonishing biodiversity, where more than 90% of mammals and reptiles and more than 80% of plants exist nowhere else on Earth.
“Our research shows that ancient tectonic forces rejuvenated the surface of Madagascar,” Clementucci said. “By tilting the island and moving its major rivers and mountains, these forces created fragmented environments where species evolved in isolation, particularly along the island’s eastern escarpment.”
Map of channel slope (ksn), normalized distance (χ), and linear geomorphic features defining the remaining scarps on the edges of the central and northern Madagascar plateaus. (CREDIT: Scientific Advances)
Lessons from a “calm” continent
The history of Madagascar calls into question the assumption that so-called “passive” continental margins – such as those of Brazil, South Africa or Australia – are geologically stable. Even after the gap ends, slow but steady movements can continue to reshape landscapes and influence ecosystems.
This understanding may also explain why other “ancient” islands harbor so much biodiversity. Even subtle geological changes, spanning millions of years, can shape how species form, adapt and survive.
Why it matters
By linking geology and biodiversity, this study shows how deeply life is linked to a changing Earth. The living and non-living parts of our planet evolve together, one shaping the other over time.
For conservationists, the findings emphasize the protection of entire landscapes, not just isolated habitats. The same tectonic and erosive forces that once created diversity could, if disrupted, permanently fracture ecosystems.
As Madagascar continues to shift and wear away, it serves as living proof that the Earth is never truly still: it tilts, breathes, and builds life in the process.
The research results are available online in the journal Science Advances.
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