Polynesia-The famous statues of Rapa Nui, or Easter Island, are best known for their deep-set eyes and long ears. They also sport impressive multi-tonne hats made from a different rock type.

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Quite how these pukao, as they are known, were transported and placed atop the statues has long been a puzzle.

But now American archaeologists believe they have a clearer understanding. The giant hats were moved with minimal effort and resources using a ramp and rope technique, they say.

“The fact that they successfully assembled these monuments is a clear signal of the engineering prowess of the prehistoric Rapanui people,” said Sean Hixon, lead author and graduate student in anthropology, at Penn State.

The researchers’ investigations indicate the pukao were rolled across miles of rugged terrain and earthen ramps to reach the top of the ancestor heads, called Moai. The largest of these colossal red hats has a diameter of over 2m and weighs nearly 11 tonnes.

The researchers motion-mapped overlapping digital photographs to capture surface details of each pukao. By applying filters to their models, they began to identify clues that are shared among pukao which show the marks of wear and tear in how they were moved.

The team’s findings, published in the Journal of Archaeological Science, argue that similar notches, grooves and indentations suggest the Rapanui people built ramps in front of forward-leaning Moai. They then rolled the hats till they reached the top, before finally tipping the Moai upright. Islanders are likely to have wrapped the pukao with ropes to roll and tip them. This “parbuckling” method, often used today to right capsized ships, would have made moving the stones physically feasible and required smaller groups of individuals to be involved.

“Even in the case of the most massive pukao that was brought to the top of the tallest statue, 10 people likely could have rolled it up a ramp with reasonable dimensions,” Mr Hixon explained.

The large cylindrical pukaos are made of red volcanic rock called scoria, while the Moai heads are carved from volcanic tuff. The materials were excavated from craters at opposites sides of the island.

In many cases, the giant pukaos were rolled into position as far as 13km away from the quarry where they were sourced. Red scoria is also porous and therefore less dense than tuff, making it easier to raise into position.

Some of the cylinders were seemingly abandoned during the transport and have been left in the upright wheel positions in which they remain today. The researchers found that the wear and tear markings on these examples also give further proof of transport by rolling.