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The destiny and secrets of our islands

Cascade sur l'île de FATUI VA. - © P. BacchetSpectaculaires côtes de Fatuiva, archipel des Marquises. Fatuiva est une île «jeune» car remontant seulement à 1,6 million d’années. - © P. BacchetPresqu’île de Tahiti avec sa barrière corallienne - © Ben ThouardCône volcanique de l’île de Mehetia, vieille de 25 à 70.000 ans seulement. Elle constitue le somm et d’un point chaud volcanique actif - © P. BacchetCascade de la vallée de Fataua, île de Tahiti. - © P. BacchetPtilope des Tuamotu, espèce endémique des atolls des Tuamotu - © P. BacchetFormation coralienne dans l’archipel des Gambier - © P. Bacchettiare Apetahi, variété endémique des plateaux du Temehani à Raiatea. - © P. BacchetPasse de l’atoll de Manihi et sa riche faune sous-marine. - © P. BacchetVolcan de la ceinture de Feu du Pacifique : le Rabaul en Papouasie-Nouvelle Guinée. - © Taro TaylorVue générale de l’île de Tahiti - © P. BacchetEntre l’atoll et l’île haute : Maupiti, île vieille de 4,4 millions d’années - © P. Bacchet
The destiny and secrets of our islands
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How do the Polynesian islands come into being? What will they become? How is it that they are so unique and beautiful? The answers are in this informative voyage through time and space.

I am lost in the middle of the immense Pacific, and already, by default of my geographic location, I am the thing dreams are made of…I am uninhabited and alone, bordered by a beach of immaculate white sand, having but a few coconut trees and bathed in deliciously warm turquoise water. I then become the place of everyone’s fantasy…of course, you have guessed by now what I am. Don’t tell me you need even more hints, such as Robinson Crusoe, Pitcairn, Rapa Nui as well as Tahiti, Bora Bora, the Tuamotus, the Marquesas. Yes, you figured it out ! I am …an island, a tiny piece of earth encircled with water. There are hundreds of these dustings of the earth poking out of the immense Pacific Ocean. They are usually small, ranging from a few hundred square meters—often called islets—to several dozen square kilometers. We refer to the two large landmasses that make up the north and the south of New Zealand as islands. However, these two large “islands” have origins that differ greatly from the miniscule points on a map, but we won’t discuss that here. As far as Australia, it is not even an island but a continent, which is also out of bounds for our discussion.

The origins of the oceanic islands in the Pacific

How did these tiny bits of confetti show up in the middle of this immense ocean? This question has been around for a long while and some with an overactive imagination believe they are what is left of a vanished continent, such as Atlantis and its so-called mysteries, or remnants of “Mu,” another lost or engulfed continent…you decide! After the work of German scientist Wegener in 1912 and the studies that ensued, it became evident that these theories were far-fetched. It was then understood that the earth’s crust was not static. It was formed of giant plates and that these plates, and even continents, shift. The theory of tectonic plates—the shifting of continents—came into being and has been largely corroborated ever since. Our Pacific islands are closely tied to this theory. Here, we will follow the life of one of these islands—that seemingly appeared in the middle of nowhere—from its birth until its death.

First, it is a good idea to picture the geological situation of the Pacific basin. A solid lithospheric plate, known as the Pacific plate, comprises most of the Pacific floor. This largest plate on earth has been continuously shaping itself for millions of years at the level of a zone called the “mid-ocean ridge” that is situated to the east. It is created through a permanent supply of new basaltic substance. A ridge is a true fracture in the earth’s crust, a chain of underwater volcanoes where basaltic substance rises up from the mantle and pours out from two sides, creating an oceanic crust. We became aware that the farther away one gets from the ridge, the older the Oceanic floor…as much as 180 million years old towards the west of the Pacific! Recent substance shapes the older substance into a type of rug that literally floats on the greatest part of the earth’s mantle called the athenosphere. Within the athenosphere, complex phenomena produce enormous quantities of heat, which fuses the substance. This in turn creates convection movements of the magma, somewhat like what occurs in a pot of water on the stove. Between the pushing that takes place by the supplies of new substance and convection movements, the plate slowly makes its way northeast. Based on the ages of the different zones, it is possible to determine the speed at which it progresses. Depending on the location, the speed varies from 8 to 18 cm per year (3-7 inches), sometimes creating what are called transform faults. Hence, the Pacific Ocean has a tendency to get larger. However, since the earth is not expandable, the oceanic plate disappears from one side of the Pacific all the way to the west through sliding under continental plates in the subduction zones, such as the east coast of Japan or New Zealand’s north island. Unfortunately for these regions of the planet, this sliding does not occur without friction. Sometimes they create earthquakes, tsunamis, and volcanic eruptions. This is the Pacific Ring of Fire that has the most active volcanoes on earth.

During very ancient times

However, let us return to our islands! In current-day French Polynesia, the rising seabed shows a several hundred-meter bulge in the earth’s crust (the ocean floor has risen almost

600 m/2000 ft). This type of immense “blister” results from a considerable accumulation of magma. It is the super-bulge of the South Pacific on which lay numerous islands and undersea mountains. Most often, these islands are spread in succession over several hundred kilometers and make up the archipelagoes of the Cook and Austral Islands, Society Islands, the Tuamotus, the Marquesas and so on. For a long time, these archipelagoes have attracted the attention of geologists because all these islands are aligned in a neighborly direction. Once geologists were able to date the rocks on the islands, they realized that the more they ventured northwest, the older the islands. As such, Mehetia island south of the Society Islands is only 25,000 to 70,000 years old; whereas Maupiti on the other side of the archipelago dates to about 4.4 million years. Between these two extremes, the ages are staggered. Tahiti is 1 million years old, Huahine is 2.6 million, and Bora Bora is 3.4 million. The same phenomenon exists in the Marquesas. Fatu Hiva to the south of the Marquesan archipelago is only 1.6 million years old; whereas Nuku Hiva is 4.2 million and Eiao to the extreme north of the Marquesas is the oldest at 5.5 million years old.     

I am born at the bottom of the ocean…

Geologists have concentrated their research on the extreme southeastern point of these alignments, and through conducting underwater research, they discovered the summit of young volcanoes still beneath the surface. In 1963, J. Tuzo Wilson, the Canadian Geophysicist, came up with his hypothesis about the existences of hotspots, which are areas with a fixed location which the lithospheric plate passes slowly over. From time to time, due to mechanisms we still know little about, a plume of magma perforates the lithospheric plate and flows onto the ocean floor. This causes the birth of an undersea volcano. Due to the thick layer of water that sits on top of it, this underwater eruption goes by undetected from the surface. Eruption after eruption, the magmatic mass grows into a volcanic cone. It takes several hundred thousand years for it to grow 4 or 5000 meters (13,000-16,000 ft), and for its summit to emerge out of the ocean to create an island. The study of different archipelagoes shows that the lifespan of the island averages about a dozen million years. These volcanoes, 9,000m – 12,000m above the ocean floor (29,500 – 39,000 ft), rival in size with the largest continental volcanoes.

French Polynesia has six known hotspots of which only three are active: Mehetia’s active volcano south of Tahiti (the last eruption was in 1986); Pitcairn to the southeast of the Gambier Islands, and the undersea volcano McDonald (1988) located to the extreme south of the Austral Archipelago. The tip of the volcano lays at only several dozen meters under the surface of the water. Pumice eruptions have already taken place. No doubt, that in a certain number of years, this volcano will emerge. This spectacle will be similar to the small island that recently emerged from the depths outside Japan. As the eruptions continue, birth of the island occurs through sheaves of molten rock and steam. The seawater vaporizes upon contact with lava at 2,000 °C (3,632°F), which creates huge white plumes. This is the marriage of fire and water. These aerial eruptions take place at the same time as various projections, such as volcanic bombs and gases. This is how an island is born. This island is fragile despite the fact its birthing takes place under frightening chaos that brings together fire, rocks, and water. Swells can quickly destroy the pyroclastic substances. However, if the emissions are abundant, and if the lava flow consists of strong, solid basaltic matter, then there is a chance that the newborn can resist the natural elements.

The Marquesan hotspot, just like the ones by Rurutu Island in the Australs and Rarotonga, doesn’t have any recent volcanic activity on record. Of course, there are other hotspots in the Pacific. Hawaii is a perfect example. Mauna Kea volcano rises up at more than 10,000 meters (33,000 ft) above sea level and is one of the most significant volcanoes on earth.

A young island is but a chaotic mix of black, raw volcanic rock beaten by the wind and ocean swells that must commence the slow process of development. Bit by bit, the coastal shores take form. The ocean rips entire sections of volcanic slag, hurtling them down the underwater slopes of the volcano into the abyss.

Life takes form

Bit by bit, life progressively settles on which is for the moment, nothing but a sterile mineral world. This isolated rock in the middle of the ocean presents an excellent stopover place for certain sea birds to nest. They transport spores of ferns and grains of grass that have clung to their legs and mixed with their excrement to the island. If some of the grains don’t make it, then others will find the moisture and the nutrients necessary for their germination and development in microcracks, which result from the chemical weathering of the basalt. The first ferns, the first lichens, and the first grasses will colonize the island over time, creating perfect nesting conditions for some birds. The droppings from these birds enrich the soil. Sea currents beating against the coast may carry larger seeds. They also bring lizards, ants, spiders, and other insects clinging to debris from another island or even a faraway continent. Wind, birds and sea currents play a critical role in dispersal and colonization. Larger plants, such as trees, will start to grow, creating a richly diverse environment for animals and vegetation. A forest will develop, providing food and shelter to its hosts. This situation will endure millions of years. Due to their long isolation and following an array of mutations, plants and animals will become slightly altered from their original species. Species found only on a certain little island will become endemic. The number of endemic species in the Pacific islands is particularly significant. A natural balance will start to form.

Sea currents bring cup coral larvae from under the surface of the water that affix themselves to the highest slopes of the volcanic cone. They colonize and eventually build a coral reef. The tiny polyps that build these structures hide algae inside their matter that need light and oxygen. Consequently, they prefer to be close to the surface and benefit from water that is usually salty and rich in oxygen. They will not develop within the paths of fresh water charged with organic and mineral particles stripped from the island. These paths will become passes.

And then it is time for humans…

Then one day, aboard double pirogues, the first humans set foot on its shores. The Polynesians arrived, bringing with them new types of vegetation—some for food, others to build houses and make tools and clothing. They inadvertently brought other types of plants that found excellent conditions to thrive and therefore invaded the entire island. The natural balance fell at risk. 

These human beings also brought animals, either voluntarily or not, such as pigs, dogs, rats, mice, spiders, and other insects. These creatures quickly settled throughout the tiniest corners of the island in an attempt to regain their freedom and find their ideal environments.

Other than humans, they did not have many predators. Century after century, humans started to alter the landscape to suit them through chopping down trees, burning land, putting in agricultural terraces and irrigation ditches, etc. Later on, Europeans brought more species of animals and vegetables. Soon after, in the name of progress and economical development powerful engines replaced pickaxes and shovels. People built tracks, then roads, works of art, countless more urban buildings, and created various facilities. Our tiny island has indeed changed since its birth 4 or 5 million years before. However, this upheaval is but a stage of its life. It still has another 5 million years or so to go…

Unchangeable?

Over the course of one or two generations, the island appears to be unchanged. The reality is entirely the opposite, because the island shifts. Importantly, it moves relentlessly towards its end. First, natural phenomena, such as erosion, alter the landscape. Running water modifies and decomposes even the hardest rocks. Moreover, volcanic rocks are subject to chemical reactions and disintegrate. Small vegetal roots infiltrate the tiniest cracks that they stretch open after time, plus blocks of rock split off during volcanic flows. Sometimes, these huge masses come loose. Day after day, the island loses altitude and size. Particularly during the past few years due to global warming and its consequences, there is a slow rise in sea levels that is evident through observing the change in coastlines and a loss of the islands’ surface area. Tides, swells and cyclones all compete to destroy the edifice. Block by block, particle-by-particle, mechanical actions caused by streaming water, swells, and sometimes human interventions, destroy the island.

Slow disappearance

This destruction is significant, but there are more serious and inescapable issues. Through getting farther from the ridge, the oceanic plate gets cold, which provokes a thickening. A few kilometers from the ridge’s level, its thickness will attain 100 or even 200 km (60-125 mi) before subduction occurs, which involves sliding under the continental plate. Its mass increases even more, exerting significant pressure on the asthenosphere. As a result, when it progresses to the west, the Pacific plate sinks, which is an occurrence known as subsidence. This creates a significant depression, for while the ridge is at a depth of only 2,500 meters beneath the sea (8,200 ft); the depths from east to west increase to 6, 8 or even 12, 000 meters (20,000 or 26,000 or even 40,000 ft) from the location of the gulf where the plate slides underneath the continental plate. Islands shifted by the movement of the plate also sink in.

The coral reefs that surround them, desperate for light, survive through growing towards the surface, a true crown of coral encircling the island’s agony. Soon, only the island’s summits that escaped erosion will stick out of the water. This is what we see happening in Bora Bora and Maupiti. In a few million years, these last summits will have disappeared. Subsidence will have eventually dragged them away. Only rings of coral will remain. The high islands will have transformed into atolls, such as those in the Tuamotu Archipelago. However, even atolls will follow their path toward the west and join the sinking of the plate that carries it to its death. Only a sandbank will remain, then an underwater mountain, called a guyot, that several million years later will also be dragged away by the ocean floor, only to be swallowed by subduction. The final remains of our island will disappear, engulfed by magma and swept up by convection movements. Fortunately, we will not be around to witness this, and tourists can be reassured that they still have several thousand years to visit our beautiful tropical islands. 

Michel CHARLEUX

The destiny and secrets of our islands
The destiny and secrets of our islands
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How do the Polynesian islands come into being? What will they become? How is it that they are so unique and beautiful? The answers are in this informative voyage through time and space.
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Welcome Tahiti
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