Why Praslin and La Digue are granite: the geology of the inner islands

Why are Praslin and La Digue made of granite?

Praslin and La Digue are granite because they are the exposed tops of a small fragment of continent, the Seychelles microcontinent, left in the Indian Ocean when larger landmasses broke apart. Seismic and gravity surveys confirm continental crust about 30 to 33 km thick beneath the Seychelles Plateau on which the granitic islands stand (Hammond et al., 2013).

A 2004 article in Eos by Collier and colleagues calls the Seychelles, with its exposures of Precambrian granite, the earliest scientifically recognised microcontinent and arguably the classic example of one. The Seychelles Nation describes the Inner Islands as the oldest mid-oceanic granite islands on earth. Within the group, Silhouette and North Island off Mahé are the exception: they are mainly syenite, dated to roughly 61 to 63 million years ago and linked to the volcanism of the Deccan Traps (Shellnutt et al., 2017).

How old is the granite?

About 750 million years. Ashwal, Demaiffe and Torsvik (2002) describe the Seychelles granitoids as Neoproterozoic, around 750 million years old, and published dates fall close to that figure.

Shellnutt et al. (2017), summarising uranium–lead dating, give 748 to 764 million years for the grey granites of the Mahé Group and 750 to 759 million years for the red to pink granites of the Praslin Group, with Île aux Récifs about 50 million years older at 809 million years. L'Union Estate on La Digue quotes the same round figure, around 750 million years, for the granite monolith on its grounds.

Both papers place the origin of the granite in an Andean-type arc on the margin of Rodinia, a supercontinent that existed at that time. The same crust later formed part of Gondwana, and the Seychelles block is one of the pieces used to reconstruct how Gondwana broke up (Hammond et al., 2013; Gnos et al., 1997).

When did the Seychelles separate from Madagascar and India?

In two stages: from Madagascar in the Late Cretaceous, and from India around 66 to 60 million years ago, at about the time of the Deccan flood basalts. The exact figures differ between studies because they date different events.

Gnos et al. (1997) set out a stepwise break-up of Gondwana in which Madagascar separated from India and the Seychelles between 95 and 84 million years ago, and India separated from the Seychelles at 65 million years ago. Tuck-Martin et al. (2018) place spreading on the Mascarene Ridge, which separated India from Madagascar, in a phase from 89 to 60 million years ago.

For the split from India, Armitage et al. (2011) date the onset of sea-floor spreading between the Laxmi Ridge and the Seychelles to 63.4 million years ago, and the Deccan flood basalts to 65 ± 1 million years. Shellnutt et al. (2017) call the rifting one of the fastest rift-to-drift transitions associated with flood-basalt rifted margins, with the microcontinent turning anticlockwise after it rifted from India about 64 million years ago. Tuck-Martin et al. (2018) instead date the India–Seychelles separation from 60 million years ago, when the Carlsberg Ridge propagated south. Hammond et al. (2013) found a lower crustal layer thinning southwards beneath the plateau, consistent with the Seychelles lying at the edge of the Deccan plume before it separated from India.

Why is the granite on Praslin and La Digue pink?

Because Praslin and La Digue belong to a different granite group from Mahé. Geologists divide the Seychelles granites by colour and chemistry, and the Praslin Group is red to pink.

Ashwal et al. (2002) describe Mahé Group granitoids as grey and Praslin Group granitoids as characteristically red to pink, with higher concentrations of elements including rubidium, uranium and thorium. B. H. Baker's 1963 memoir for the Geological Survey of Kenya already separated a "reddish Praslin–La Digue granite" from the grey granites of Mahé. Jędrusik (2020) describes La Digue's landscape as dominated by very old pink or red granites.

Weathering changes the colour of exposed surfaces too. Describing the Mahé granite, Baker recorded weathered surfaces as buff-grey grading to red-brown, from the formation of the iron minerals limonite and goethite.

How did the granite boulders get their shapes?

Mainly by weathering. Weathering attacks granite blocks fastest at their corners and edges, so they become rounded, and when the rotted rock around them is washed away the blocks are left behind as boulders.

Baker (1963) described granite boulders from a few inches to 30 or 40 feet across on the slopes of Mahé, invariably well rounded by preferential weathering of their corners and edges, and visible in various stages of development within the weathered zone. He recorded granite weathered to reddish soil to depths of more than 30 feet in road cuttings, and loose blocks on slopes where the weathered matrix had been washed away.

On the shore, salt may add to the process. Johnson and Baarli (2005) studied the erosional features of granite rocky shores on Mahé, Praslin, La Digue, Ronde, Grande Sœur and Île Cocos; their listed keywords include tafoni, the cavernous hollows found in weathered rock, and rillenstein. In laboratory tests on tuff and sandstone rather than granite, Sato and Hattanji (2018) found sodium chloride caused the most intensive weathering of the salts they tried under cycles of humidity, and concluded that such weathering would occur extensively on stone surfaces in humid coastal spray zones. A 2021 review by Oguchi and Yu notes there is still no universally accepted theory of how salt weathering works, so the exact cause of any single hollow is open to debate.

Where to see the granite on Praslin, La Digue and nearby islands

On many beaches and coastal paths on both islands, with the best-known formations at Anse Source d'Argent and Anse Cocos on La Digue and Anse Lazio on Praslin.

• Anse Source d'Argent, La Digue. Reached through L'Union Estate, which describes the beach as famous for its granite rock formations. The estate's granite monolith, which it estimates at around 750 million years old, stands where its giant Aldabra tortoises roam; Jędrusik (2020) calls it the largest granite monolith in the Seychelles. A coral reef keeps the water shallow and sheltered (SeyVillas). Entry is charged by the estate: see our Anse Source d'Argent and L'Union Estate pages.
• Anse Cocos, La Digue. Jędrusik (2020) photographs granite on the beach, and SeyVillas describes a sheltered swimming area surrounded by rocks.
• Grand Anse to Petite Anse, La Digue. Jędrusik (2020) describes the path from Grand Anse to Petite Anse and Anse Cocos as running through forest and granite rocks.
• Anse Marron, La Digue. The coastal route from Grand Anse passes Grand l'Anse, where SeyVillas notes large boulders and the remains of a coral reef, then runs across, under and between boulders to a large granite pool. SeyVillas says it is only possible with a guide; Jędrusik (2020) says a guide is preferable.
• Nid d'Aigle, La Digue. The island's high point at 333 m (Jędrusik, 2020), with a flat granite plateau on the path (SeyVillas).
• Anse Lazio, Praslin. SeyVillas describes impressive granite formations at the beach.
• Île Cocos. One of the islands in the Johnson and Baarli granite-shore study, see our Coco Island and Félicité page.

Where do the reefs and white sand come from?

Mostly from coral, not granite. The granite islands stand on a shallow bank floored by coral sand and reefs, and the flat coastal land on La Digue is built of coral sand.

Baker (1963) put the Mahé–Praslin bank at about 16,000 square miles, with water no deeper than 240 feet (about 73 m), floored by coral sands and reefs. He interpreted it as a former land surface bevelled by waves that later subsided, allowing coral to grow on top. Sherlock et al. (2025) describe the continental shelf as only 30 to 60 m below sea level, so the islands were likely joined when sea levels dropped during the Pleistocene. Nussbaum and Wu (1995) link differences between treefrog populations on Mahé, Praslin, La Digue and Silhouette to the islands being cut apart by rising sea about 10,000 years ago. Johnson and Baarli (2005) found Pleistocene limestone, built by coralline red algae and vermetid gastropods, still attached to granite surfaces.

On La Digue, Jędrusik (2020) describes the western lowland, one of the flat coastal plains called plateaux in the Seychelles, as a flat Holocene accumulation of sand of coral origin up to 10 m above sea level, probably a former lagoon bed, covering 164 ha. Its soils formed on a mixture of granite and coral. The west coast has lagoons enclosed by coral reefs; on the east coast there are no lagoons, or only very thin ones, and where reefs are absent large waves and rip currents make swimming harder.

How do the granite inner islands differ from the coral outer islands?

The inner islands are high granite islands on continental crust; the outer islands are low coral islands, and seismic work has found no direct evidence of continental crust beneath most of them.

The Seychelles Nation counts 115 islands, 76 of them coralline, although the same article later splits them into 41 granitic Inner Islands and 74 Outer Islands. It gives the coralline islands an average height of 1.5 m above sea level, seldom more than 9 to 15 m. The granitic islands are high and rugged by comparison: La Digue alone rises to 333 m at Nid d'Aigle (Jędrusik, 2020). Aldabra, which it calls the world's largest raised coral atoll, is one of the outer islands; Boyden et al. (2021) note that islands such as Aldabra are almost entirely composed of Pleistocene reefs. Baker (1963) described the rest of the archipelago as coral atolls and sand cays of comparatively recent formation, including raised reef-rock islands in the west. Hammond et al. (2013) found no direct evidence for continental crust beneath most Seychelles islands away from the plateau, with one exception: Desroches, on the northern Amirante Ridge, where a block of continental material is likely.