As the last light of day hits the western slopes of Piton des Neiges, the crumbly summit of this behemoth volcano takes on the glowing orange of the sunset. The shadow of the mountain – at 10,072ft, the high point of the Indian Ocean – is cast over my shoulder; projected onto the sea of clouds trapped beneath the serrated ridgelines below, blocking off the wild interior of Réunion Island that we’ve spent the last week hiking through.

La Réunion is a French department, its closest neighbour being Mauritius, 140 miles west. It’s not a huge place, measuring 39 miles long and 28 miles wide, but, as the local moniker of l’île intense (‘the intense island’) suggests, this is a land of extremities.

Beyond the shark-infested waters and coastal cities, where maloya music blares from samosa stalls, sit three deep volcanic cirques – Mafate, Cilaos and Salazie. These natural amphitheatres surround Piton des Neiges; dense and lush, with sharp ridgelines spread like dropped razor blades. Each is enclosed by colossal rock walls that formed from the collapse of that great volcano, which erupted from the sea three million years ago, laying the island’s foundations. Piton des Neiges is dormant these days, but to its southwest is Piton de la Fournaise, one of the most active volcanoes in the world, which still erupts annually.

The GR-R2, or ‘Grande Traversée of Réunion’, runs through all of this intrigue – 80 miles from Saint-Denis in the north to Le Baril, where huge waves meet weathered cliffs, in the south. With my all-knowing guide Frédéric Fontaine, I set out to hike the route.

It does not take long to get out of Saint-Denis and into the green immensity of Mafate, the least accessible of Réunion’s cirques. There are no roads in Mafate, but there is a small population of 700, dotted around aptly named îlets, villages hidden on high plateaus, surrounded by a sea of thick vegetation. Many of these ‘islets’ were first formed as Maroons, the hideouts of runaway slaves who fled sugar plantations in the 1600s.

There is not a single flat yard in Mafate. The near-vertical mountain walls that block you off from the world are the same ones you climb and descend. The heat makes walking slow and sweaty, but almighty views of this unruly landscape are ample reward.

The hilltop gîtes seem to materialise out of nowhere. Their isolation is well summed up by a joke told to me by Emmanuel, who runs a gîte in the tiny îlet of Aurère. “I am single because to get to a nightclub, I have to hike for days,” he explained over our rougaille saucisse (spicy sausage) dinner. “So by the time I get there, I am too tired to dance.”

On the trail, Fred plucks guava strawberries “for power” and cuts passionfruit from trees. Stonechats are a constant companion, and Réunion harriers circle above. When the wind blows, Fred stretches his arms out to catch it, as though he too could take flight.

After three days in Mafate, we leave the cirque via the Col du Taïbit and enter Cilaos, at the heart of which is a mountain metropolis of the same name (population 5,215).

Having not seen a road in days, the commune appears suddenly, like a rock Atlantis. Dirt bikers wheely down wide streets, past Creole houses and art-deco architecture.

The ascent of Piton des Neiges begins from here, at 3,983ft. We twist and turn up forest trails for hours to reach the Caverne Dufour refuge above the clouds, then ditch our bags and trek on, ascending the volcano. The view from the summit stuns us into silence; sprawling clouds caged in cirques and city lights blinking on the coast.

The days that follow run through messy, mossy forest, full of curling ferns and the twisted trunks of tamarinds. The rain catches us in the farmlands and relents in the Plain de Sables, where the route runs straight as a bolt through a moonscape.

After a blood-orange sunrise on day 12, we arrive at ‘the Furnace’, Piton de la Fournaise. The sprawling Enclos Fouqué caldera that surrounds it is a jumble of dried magma; some old, hard, grippy and grey; some black as night, jagged and only a few years old. When the lava erupts and runs to the coast (as it did in 2007) buildings are destroyed, new beaches are formed and the topography of Réunion is redrawn.

Fred leads us directly up the slopes to the volcanic rim, over jet-black rocks that swirl like a Van Gogh sky. The crater is immense, more than half a mile across, crumbling within.

Our hike continues, ending at Cap Mechant, where mega waves bombard the cliffs. This wild isle is never the same for a second. It is a canvas on which Earth’s forces are still at work; with wavy blue, impasto green and burning red and black brush strokes.

Réunion is an ever-changing, incredible, half-formed thing – a dynamic, living island.