The island · Geology
A mountain vanished.Life built an island.
There was once a mountain where this lagoon lies. To understand Tetiaroa, look beneath the water.
Go beneath the surfaceFrom fire to reef
An island, three lives.
Choose a chapter to see the island change.
First, fire.
Eruption by eruption, a mountain rises from the ocean floor. Its summit breaks the surface. Eventually, the Pacific Plate carries it away from its magma supply. The volcano falls silent.
Look across the Society Islands
The story is written in the shoreline.
Tetiaroa’s mountain is hidden. On its neighbours, volcanic slopes, wide lagoons and reef islands show us the landforms behind the story. Look from the peaks to the water’s edge.
Tetiʻaroa
The mountain is out of sight.
Start with Tetiʻaroa: reef and low motu around a lagoon, the volcanic foundation hidden below. Compare its open centre with the mountains that still rise on neighbouring islands.
Annotated aerial view via Tetiaroa Society
Different islands, different histories. These views illustrate the landforms in the story, not a dated sequence of Tetiaroa’s past. Images are shown at different scales.
02 / A shared ocean floor
One island’s weight changes its neighbours.
Tahiti’s mountains are just the part we can see. Most of its volcanic mass lies below the sea, weighing on the oceanic plate. As Tahiti grew, the plate bent downward across a region that includes Tetiaroa. Farther out, it arched upward.
01 / Tahiti
A mountain’s weight
Each eruption adds rock to the growing volcano. Beneath that enormous weight, the oceanic plate slowly bends.
02 / Tetiaroa
Drawn downward
This extra sinking may help explain why Tetiaroa is already an atoll while nearby islands still have mountains.
03 / Makatea
Reef above the sea
Makatea’s limestone stands high above the waves. Upward bending beyond the depression may help explain how an ancient reef became dry land.
A simplified model, not a geographical transect. Distances and bending are exaggerated. Regional flexure is measured; its precise contribution to Tetiaroa’s sinking and Makatea’s uplift remains uncertain.
03 / A landscape under water
The lagoon was once dry land.
During ice ages, water locked in continental ice sheets lowered the ocean. Tetiaroa’s limestone platform stood exposed. Rain worked into the rock, dissolving hollows and leaving ridges between them. The returning sea hid that landscape, but did not erase it.
- 01
The sea retreats
The reef platform emerges as the ocean falls. What looks like open water today becomes land.
- 02
Rain carves the rock
Fresh water dissolves calcium carbonate, opening cavities and leaving an uneven limestone surface.
- 03
The sea returns
Coral grows on some of the submerged high points. Just below the surface, these ridges still shape the routes boats take across the lagoon.
The control reveals a simplified, already-eroded landscape. It is not a dated sea-level record or a depth chart; dissolving this rock took thousands of years.
04 / How a motu takes shape
The ground beneath your feet came from the reef.
Look closely at the ground on a motu: broken coral, rock and sand, carried there by waves. The ocean moves this material, sorts it and piles it up. Deposit by deposit, the remains of marine life become an island.
01
Waves carry it
Powerful swell and cyclone waves lift broken coral, boulders and sand across the reef crest onto the reef flat.
02
The rubble settles
Successive deposits build a ridge. Parts of its base cement together, while loose sand and stones remain above.
03
Plants take root
Vegetation establishes on the deposits. A motu takes shape, with beaches that waves and currents continue to rearrange.
The work does not stop when an island takes shape. Every storm, swell and shifting current can move its shores again.