Quarrying stone for Easter Island statues made soil more fertile for farming

Easter Island’s Polynesian society cultivated crops in soil made especially fertile by the quarrying of rock for massive, humanlike statues, a new study suggests.

Soil analyses indicate that weathering of volcanic sediment created by quarrying enriched the slopes of Easter Island’s major rock quarry with phosphorus and other elements crucial for farming. Microscopic plant remains suggest that food grown in the enriched soil included sweet potatoes, bananas, taro, paper mulberry fruit and probably bottle gourd, say anthropological archaeologist Sarah Sherwood and colleagues.

Starting in roughly 1400, Easter Islanders farmed in this way, even as soil quality deteriorated in many parts of the island, also known as Rapa Nui, due to deforestation and possibly drought, the team reports in the November Journal of Archaeological Science.

The island’s Polynesian society, which got started from around 900 to 1100, is famous for two reasons: for having erected large statues known as moai that were sculpted out of volcanic rock, and for collapsing in the late 1600s after supposedly overusing the land. But previous research has questioned that narrative of societal disintegration. The new study is “one more piece of evidence against the traditional story of Easter Island’s self-inflicted environmental demise,” says Sherwood, of the University of the South in Sewanee, Tenn.

Radiocarbon dating of burned wood and plant fragments found in sediment layers and on two of 21 partially buried statues on the quarry’s slopes identified two main phases of farming at the quarry. During the first phase, visits probably started between 1495 and 1585 and lasted until roughly 1675 to 1710, shortly before Europeans first arrived on the island in 1722. During that time, one of the statues — which has been more intensively studied than the other — was raised, the scientists say.

Cultivation occurred in many parts of Rapa Nui before European contact, says archaeologist Carl Lipo of Binghamton University in New York, who was not involved in the study. Investigators need to determine whether any other sites on the island contained soil as productive as that at the statue quarry, he suggests.

Findings from Sherwood’s group help to show how Rapa Nui was transformed from a palm forest into a cultivated terrain that supported islanders for more than 500 years, Lipo says. Quarry cultivation “adds to growing knowledge of how pre-contact people smartly utilized their landscape,” he says. Related research has found that, as palm forests shrank on Rapa Nui, farmers cultivated yams and other crops using clever techniques such as rock gardens (SN: 12/16/13) that fortified soil quality.

Farming at the quarry by the island’s native population began again in the 1800s and likely lasted into the early 1900s, the researchers found. Other evidence also suggests that Rapa Nui farming continued after European contact (SN: 1/5/15).

What’s more, excavations of the two partially buried statues, led by archaeologist and study coauthor Jo Anne Van Tilburg of UCLA, revealed that each had been placed in a carved pit packed with gravel and boulders to hold it upright. Crescent shapes and other figures carved on statues’ backs, and a carved human head found resting against the base of one statue, suggest that these objects were used in ceremonies of some kind, perhaps intended to promote crop growth. Red pigment pieces and coral found near the statues probably also had ritual uses, the team says.

Researchers traditionally have assumed that builders of the island’s partially buried quarry statues had either planned to move them elsewhere on the island or abandoned them. Designs on the roughly 6.6-meter-tall quarry statues display similarities to those on the only other Rapa Nui statue displaying numerous carved images. That carved figure was previously found at a ceremonial site nearly 20 kilometers west of the quarry.

Although the quarry measures only about 800 to 1,000 meters across, the new soil data show that it was a “little productive gold mine” for farming, says archaeologist Christopher Stevenson of Virginia Commonwealth University in Richmond, who did not participate in the study. Reeds growing in a lake at the base of the quarry would have provided additional phosphorus to the soil, he says.

“The area immediately to the east of the quarry was and is one of the most intensively settled parts of the island, and now that makes much more sense,” Stevenson says.