Why is the Seven Coloured Earth coloured?
The Seven Coloured Earth is coloured because old basalt lava has weathered in place into clay, and the iron oxides and hydroxides formed in that clay stain it red and brown, with aluminium oxides adding purple. The ridges are rills cut by rain into ground left bare after the forest was cleared, not dunes of volcanic ash. Two popular claims, that mixed colours separate again and that the dunes never erode, have no published scientific support.
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Why the Seven Coloured Earth is coloured
The Seven Coloured Earth is coloured by minerals that formed when basalt, the dark lava rock that built Mauritius, weathered in place into clay in a warm, wet climate. The bare ground is about 7,500 m² inside dense forest, in reddish, brown, grey and purple shades, and the colours come from the weathering of old basalt and the secondary iron oxides and hydroxides that formed in it.
Iron gives the reds, the browns and a dark anthracite grey, and aluminium oxides give the blues and purples. The chemistry behind it is hydrolysis, the breakdown of minerals through reaction with water, which in this hot, humid setting turned the clay into what soil scientists call ferralitic soil.
How the Seven Coloured Earth was formed
The Seven Coloured Earth was formed in three stages, as lava built the ground, weathering rotted the rock into clay and erosion stripped the surface once the forest had gone.
Lava flows
Basalt lava flows laid down the rock at Chamarel. The geopark dates them to about 1.7 to 3.5 million years ago and links them to the Réunion hotspot, which places them in the Intermediate Series of Mauritian lavas, dated to 3.5 to 1.7 million years ago. An older range of 3.5 to 7 million years is also on record.
Weathering into clay
Rain, humidity and chemical weathering then turned the basalt into mineral-rich clay where it lay. A recent geotechnical study around the dunes, described by the geopark, drilled three boreholes up to 10 m deep and found a thin layer of soil, roots and small stones over 6 to 10 m of completely altered basalt, with altered volcanic breccia, rock fragments set in clay, beneath it.
Clearing and erosion
The colours were exposed when the cover came off. The bare ground and its rills are the result of deforestation and sheet erosion, in other words of people changing the land surface. The usual account is that the area was cleared for crops and a severe storm then removed the topsoil, and no record dates the clearing. Rain has carved the ridges ever since.
Is the Seven Coloured Earth volcanic ash?
The Seven Coloured Earth is not volcanic ash and was not coloured by an eruption. Both ideas are common misconceptions, and the colours trace to basalt that weathered after the lava had cooled. The ash idea persists because the site was long described as eroded tuff, a rock made of volcanic ash, a description that confuses erosion with the weathering of rock in place.
A second popular explanation, repeated even on the geopark’s own blog, says the colours come from molten rock cooling at different rates. They do not. The colour lies in minerals formed by weathering, long after the rock had set. Similar features occur in Papua New Guinea and can be expected wherever high tropical rainfall meets volcanic bedrock, so Chamarel is unusual rather than unique.
Do the colours separate again if you mix them?
No published study supports the popular claim that the coloured earths separate back into layers after being mixed. Every version of the claim traces back to travel copy and the geopark’s website rather than to research, the fullest version concedes that the causes of the separation are “yet to be fully clarified”, the local heritage record says only that it “has been said” to happen, and the geopark’s current geological account does not mention it at all.
That does not prove the claim false, but it means the most repeated fact about the Seven Coloured Earth has never been shown to be true. The small test tubes of coloured earth sold in curio shops around Chamarel are souvenirs, not experiments, and prove nothing either way. Collecting earth from the dunes to test the idea is strictly prohibited.
Does the Seven Coloured Earth erode?
The Seven Coloured Earth does erode, and the ridges people come to see are themselves the work of erosion. The claim that the dunes never wear away despite heavy rain comes from popular articles and has no evidence behind it. The rills are well developed and were formed by sheet erosion, rain has carved the patterns into the hillside, and the geopark itself states that the site continues to evolve through erosion, rainfall and climate.
The geopark expects the dunes to keep changing in the years ahead and makes no promise that a new colour will appear. That is the scientific case for the rules on site. Everyone keeps to the walkway and the perimeter, and nobody walks on the earth or takes samples, because the bare surface is easily eroded material that is still being shaped.
Mauritius’s volcanic origin, and whether any volcano is active
Mauritius is a volcanic island built by the Réunion hotspot 8 to 10 million years ago, and none of its volcanoes has erupted in recorded history. The hotspot itself lies today under Réunion, where Piton de la Fournaise is one of the most active volcanoes in the world. The oldest rocks on Mauritius are about 10 million years old, followed by an Old Series that continued until about 5 million years ago and an Intermediate Series from 3.5 to 1.7 million years ago, the period the geopark gives for the Chamarel basalt.
The youngest lava flows on Mauritius, olivine flood basalts, date from between about 700,000 and 20,000 years ago. Trou aux Cerfs in Curepipe, the best-known crater, about 300 to 350 m across and 80 m deep, last erupted about 700,000 years ago and is dormant. Closer to Chamarel, the sacred lake of Grand Bassin, or Ganga Talao, fills a crater at about 550 m.
How long the coloured earth has been studied
The coloured earth at Chamarel has been on record since 1879, when it was first documented at a session of the Royal Society of Arts and Sciences of Mauritius on 26 November 1879. Visitors have come since the 1960s. The first peer-reviewed study, by Sheth, Johnson and Ollier in the Journal of African Earth Sciences, appeared online in August 2009 and in print in April 2010, and a paper by Newsome and Johnson in the journal Geoheritage, published online in December 2012, named the site as an attraction with much scope for showing its educational value. The geopark also cites a recent geotechnical study, with no named authors and no published report.
Questions people ask
Why is the Seven Coloured Earth coloured?
The Seven Coloured Earth is coloured by minerals formed as old basalt weathered in place into clay. The reddish, brown, grey and purple shades come from the weathering of basalt and the iron oxides and hydroxides formed in it, and aluminium oxides give the purples. The ridges were then cut by rain after the forest that covered the site was cleared.
How was the Seven Coloured Earth formed?
The Seven Coloured Earth formed in three stages. Basalt lava flows, dated by the geopark to about 1.7 to 3.5 million years ago, laid down the rock. Warm, wet weather then rotted it into 6 to 10 m of clay, the depth found by boreholes around the dunes. Finally, clearing of the forest let rain strip the surface and carve the rills seen today.
Is the Seven Coloured Earth made of volcanic ash?
The Seven Coloured Earth is not made of volcanic ash. The ash explanation, and the idea that an eruption formed it, are common misconceptions. The material is basalt lava that has weathered in place into clay, coloured by iron and aluminium minerals, and the ash theory confuses erosion with weathering.
Do the colours of the Seven Coloured Earth separate when mixed?
No peer-reviewed study supports the claim that the colours of the Seven Coloured Earth separate again after being mixed. The story traces back to travel copy and the geopark’s website rather than to research, its causes are admitted to be yet to be clarified, and the geopark’s current geological account does not mention it. Collecting earth from the dunes to test it is strictly prohibited.
Is there an active volcano in Mauritius?
Mauritius has no volcano that has erupted in recorded history. The youngest lava flows on Mauritius are dated to between about 700,000 and 20,000 years ago, and Trou aux Cerfs in Curepipe, the best-known crater, last erupted about 700,000 years ago and is dormant. The hotspot that built Mauritius lies today under Réunion, whose Piton de la Fournaise is one of the world’s most active volcanoes.
How old is the Seven Coloured Earth?
The basalt of the Seven Coloured Earth is about 1.7 to 3.5 million years old in the geopark’s dating, with 3.5 to 7 million years also on record. The bare, coloured surface is far younger, because its exposure followed forest clearance, and no record dates that clearing. The site was first recorded in 1879 and has drawn visitors since the 1960s.
Are there really seven colours at the Seven Coloured Earth?
The number seven is a name more than a measurement. The geopark lists red, brown, violet, green, blue, purple and yellow; on the ground the clear shades are reddish, brown, grey and purple. How many colours you see at the Seven Coloured Earth depends mostly on the light, because direct sun after rain brings out differences that grey skies flatten.
See the bands for yourself
The visit page covers the ticket, the walkway round the dunes and the light that shows the colours best.