‘Coral houses’ are dotted throughout the Pacific. Now scientists know exactly when they were built
- Written by: James L. Flexner, Senior Lecturer in Historical Archaeology and Heritage, University of Sydney
The Mangareva Islands are about 1,600 kilometres southeast of Tahiti in French Polynesia. They get their name (which means “floating mountains”) from the way the sea spray breaking on the surrounding coral atolls, or motu, causes the ancient volcanic peaks to appear as if they are floating above the waves.
Today, the islands are home to about 2,000 people, many of whom work on the pearl farms in the idyllic turquoise lagoon. Dotted across the islands are the remains of dozens of remarkable pieces of architecture: homes built from coral.
As part of a larger project studying the transformations of everyday life in 19th-century Mangareva, my archaeology research team has documented dozens of these coral houses, including on the islands of Aukena, Akamaru, Mangareva and Taravai.
Now, in a new paper published in the journal Antiquity, we have established the first precise construction timeline for these coral houses.
The results reveal new patterns in how Pacific societies shaped their built environment after European contact – and how that colonial legacy continues to shape life today.
Colonisation changed community life in the Pacific
French Catholic missionaries set up an outpost in Mangareva starting in 1834.
In addition to learning the habits of prayer, attending religious services and reading the bible, Mangarevan people also changed their day-to-day lives. Among the many changes were a complete transformation of people’s domestic spaces.
Traditional buildings of wood and thatch were replaced within a few decades by a new kind of stone cottage.
The missionaries often recorded specific dates for their constructions, above all the cathedral in Rikitea, churches throughout the islands, and the main Catholic schools.
However, for the largest category of buildings from this time, houses, we usually don’t have any information about construction dates, who built them, and who lived there.
A precise dating method
During fieldwork in October 2024, I noticed that one of the coral blocks that had fallen from the wall of the ruined house we were excavating had branch corals that looked very fresh, almost like they were just cut from the living reef.
We used an advanced technique known as uranium-thorium (U-Th) dating to understand the age of these branch corals – and the structures built from them.
Unlike the more well-known radiocarbon dating, where the error ranges are measured in decades, U-Th dates are super precise, narrowing down the date when the corals died, leaving behind the hard exoskeleton, to within a few years.
Also unlike radiocarbon, which isn’t very reliable for materials less than about 400 years old, U-Th works right up until the present.
We took a “control” sample from a building with known dates, the 1850s boys’ school from Aukena, as well as samples from an additional eight houses, plus a coral watch tower.
We also sampled a branch coral from a pit layer in the same house where I first noticed the “fresh” looking branches from the coral blocks.
At the time, we were thinking that the pit held the remains of a feast held just before the house was built. Overlapping dates in our U-Th results confirmed this hypothesis.
Mysteries of ‘old coral’
After testing the samples, we were surprised to notice several dates that were older than expected.
Some of the corals apparently died before the 1830s when missionaries arrived. Some even pre-dated European contact in the 1790s.
A similar problem is known from radiocarbon dating, called the “old wood” problem where the date of the death of an organism might be centuries or even decades before the event an archaeologist is hoping to date. Did we have an “old corals” problem here?
There are two potential explanations.
An archaeologist visiting Mangareva in the 1930s noted piles of coral rubble he believed were the remains of marae, once sacred structures that were overthrown during the missionary period. This raised the possibility that this ancient coral was repurposed for new buildings.
Another possibility for this kind of coral, from the scientific genus Acropora, is that some branches die off away from the area of active growth on the reef over a period of years or decades but retain their “fresh” look.
This might be the more likely scenario, as our “too old” dates were years or decades, but not centuries, too early. But we also can’t completely rule out the marae theory.
We still have a lot to learn about how people used coral for buildings in the past, and possibly to learn about how coral reefs rebounded, or not, after decades of human exploitation. This last point could be important for thinking more carefully about our own relationships to coral reefs in the present.
Authors: James L. Flexner, Senior Lecturer in Historical Archaeology and Heritage, University of Sydney
