Magnetism on Rapa Nui
Among the sites most visited for their supposed magnetism are the large oval rock called Te Pito Kura and a section of the road to Anakena, about half a kilometer from the beach. Illusion or reality? What do the scientists say?
by Cristian Moreno Pakarati – Historiador / Historian
Rapa Nui, as all volcanic islands, has a wide variety of magnetic anomalies due to a combination of geological forces. The most important is the amount of ferrous elements in the rocks of the Island, especially in those areas which have bedrock of basalt or andesite. Some of the rocks on the Island contain information for the study of paleomagnetism, which indicate the intensity and direction of the earth’s magnetic field in the period in which they were formed. This is called natural remanent magnetism.
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One of the most well known magnetic sites is a large, oval rock called Te Pito Kura. This rock has a high ethnographic value and was first described by William J. Thomson on an expedition in 1886. He noted that the local inhabitants considered it very significant, since, apparently, it served to divide two coastal tribal districts in the period immediately before the arrival of the Christian missionaries. Older stories indicated that the rock had come from far off of the Island and was brought by the first settlers. Another story, noted by Father Sebastian Englert, tells that, after each birth on the Island, the umbilical cord which joins the mother to the child, would be cut on this rock or a similar one. However, scientific analysis has left it clear that this stone fits within the geological formation of the Island, both for its date and for its chemical composition. Te Pito Kura have a very high 29.60 x 103 SI of magnetic susceptibility, that is, it has facility to be magnetized by an external magnetic field.
Another place that is of interest to visitors is on the road that goes from Hanga Roa to Anakena, about half a kilometer from the beach and marked by a pipi horeko (small stone tower). In between two long uphill segments, there is a sector of about 60 meters (200 feet) which seems to be on a downward slope (or vice-versus, according to the perspective). However, when a vehicle stops and is left in neutral without the brakes on, it seems to be pulled upward, faster and faster, instead of dropping with the road. Many guides will show this to the impressed tourists and tell them that it’s due to magnetism. It really isn’t anything more than another “gravity hill”, where the trick is a product of an optical illusion. Nonetheless, facts and reality often have no effect on social perceptions or cultural importance of sites and it’s most probable that this story of the magnetic road will continue to be told for generations.
The first study of the magnetic fields on Easter Island was made in 1916, when the scientific expedition of the ship “Carnegie”, under the command of Captain James Percy Ault, visited the Island for a few hours, within a project of the Department of Terrestrial Magnetism of the Carnegie Institution for Science. The mission was to sail the oceans and make permanent measurements on the direction and strength of the earth’s magnetic field as a major part of the World Magnetic Survey. The “Carnegie” returned to Easter Island in 1928, at this time staying for six days and bringing more modern equipment for the study of magnetism on the Island. The results were important within the project for collection of magnetic data on a global scale. On this trip, one of the anchors was lost in the bay of Hanga Roa. Some Chilean publications announced that the “Carnegie” had detected large magnetic anomalies on the Island, but the official publications make no mention of that.
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During the 1960s, new studies were held to get information on magnetism prior to the construction of the airport at Mataveri. At that time, the German meteorologist Karl Schanz told the Chilean newspaper “La Nación” that he had found a rock at Mataveri (later dynamited since it was in the way of the airport construction) which contained lines that indicated the position of the sun on the days of the equinoxes and solstices, as well as noting lines on other nearby rocks which marked both geographical north and magnetic north. This finding was later studied by William Liller, but the fact that the original rock had been destroyed and that photos of this “calendar rock” were of such low resolution and without external references made it impossible to confirm the discovery. Schanz noted that his studies were hindered by inconsistencies in the readings on his magnetic instruments, anomalies which he attributed to high concentrations of iron and geomagnetic currents. Dr. Ramón Campbell also noted similar anomalies on the Poike peninsula at the peak of Ma’unga Puakatiki.
As a result of the studies of paleomagnetism which were made by several notable geologists between 1976 and 2002, it has been possible to determine that almost all of the rocks on Rapa Nui show a normal Brunhes polarity, which would indicate that they were laid down within the last 780 thousand years. From this, the dates of geological formation of Rapa Nui have been revised by Vezzolli and Acocella, demonstrating that the last lava flows (Roiho) date to around 110 thousand years ago, replacing theories that the Island was formed nearly 3 million years ago and also that it had volcanic eruptions into the Holocene (from 11,700 years ago to the present).
Recently, it has been shown that some magnetic anomalies are not of geological origin nor have they been generated by natural remanent magnetism, but directly produced on the surface by natural or man-made causes, among which could be: (1) fires, both naturally occurring and those set by humans, which could significantly alter the magnetism in the soil; (2) the presence of magnetotactic bacteria in the soil; and (3) direct action from rain or run-off water carrying sediments and enriching the soil with magnetic minerals.
Following a study in the field, Jörg Fassbinder and his team concluded that these slight anomalies are measurable by the proper equipment, in spite of the natural background magnetism that could skew the results. Taking this into account, the use of a simple cesium magnetometer could allow scientists to detect the presence of archaeological remains which are hidden under the soil without the need to excavate. It would certainly facilitate the work of archaeologists. During their experiments, they have already discovered a large number of buried structures made of basalt (for example, the base rocks of hare pa’enga or boat-shaped houses), including some that apparently were not previously identified. In the future, archaeologists will be able to do their work with a minimal amount of excavation and a lesser impact on the heritage sites of Rapa Nui.
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