Date unknown · The volcano forms
Tetiaroa begins as a volcano.
Magma rises through the Pacific Plate above the Society hotspot. Repeated eruptions build a volcano from the deep ocean floor until its summit emerges as an island.
Tetiaroa's volcanic foundation is now completely underwater, so scientists cannot date its emergence directly. Its location in the Society chain suggests that it formed near the time Moorea was forming, but the exact date is unknown.
Geophysical estimates put the submerged volcanic edifice at about 1,300 km³.
After the island forms · Volcanism stops
Plate movement shuts down the volcano.
The Pacific Plate carries the volcano northwest, away from the hotspot that supplied its magma. Once that connection is lost, eruptions stop.
The same movement created the age pattern across the Society Islands: younger volcanoes lie toward the active southeast, while older islands and atolls lie farther northwest.
The plate travels roughly 10 cm northwest each year.
As the volcano ages · Land shrinks; reef grows
The volcano gets lower while the reef grows upward.
Rain, waves and gravity erode the volcanic island. As the plate carries it away from the elevated hotspot region, the seafloor slowly subsides. Living coral around the shore continues to grow near sunlight and sea level.
The volcanic land becomes smaller, but the reef keeps building upward. The widening stretch of water between the land and the outer reef becomes a lagoon.
Living coral communities continue to renew the reef's ocean-facing edge.
As Tahiti grows · The plate bends
Tahiti's weight may have pushed Tetiaroa lower.
The available evidence suggests that Tahiti grew after Tetiaroa and became a much larger volcano. Its weight bent the oceanic plate into a broad depression that includes Tetiaroa.
Geophysical measurements show regional bending under the Society Islands. They do not reveal exactly how much Tetiaroa sank because of Tahiti rather than normal volcanic subsidence. Tahiti's contribution is therefore a well-supported reconstruction, not a direct measurement at Tetiaroa.
Geophysical estimates put Tahiti's volcanic volume at roughly ten times Tetiaroa's.
As volcanic land disappears · Motu form
Waves turn broken reef into islands.
Large waves, especially during cyclones, carry coral boulders, rubble and sand over the reef crest and deposit them on the shallow reef flat.
These deposits build up, cement together and support vegetation, forming motu. After the volcanic summit sinks below sea level, the reef encloses a lagoon and the motu sit on its rim: Tetiaroa is now an atoll. The twelve motu seen today are younger and continue to change.
Motu are made mainly from reef-derived carbonate material, not exposed volcanic rock.
About 20,500 years ago · Sea level falls
Ice-age sea-level fall exposes the reef platform.
At the Last Glacial Maximum, so much water is stored in continental ice that global mean sea level falls by about 125–130 metres. Tetiaroa's reef platform is left above the sea.
Rainwater dissolves the exposed carbonate rock. Holes and caverns enlarge, leaving limestone ridges and pinnacles between them. This eroded terrain is called karst.
The lowest global mean sea level occurred around 20,500 years ago.
From about 19,000 years ago · The lagoon returns
Rising seas flood the karst and restore the lagoon.
As the ice sheets melt, global sea level rises. Seawater floods Tetiaroa's eroded reef platform, and coral begins growing again on the reef margin and on high points inside the lagoon.
Some of the old limestone ridges and pinnacles remain just below the surface. They can be mapped and seen today, and they still affect where boats can travel through the lagoon.
Modern coral growth caps parts of the submerged relief inside the lagoon.
