Zámořská Francie

To download the full resolution and other files go to: earthobservatory.nasa.gov/IOTD/view.php?id=78218&src=...

In August 1768, Captain James Cook, naturalist Joseph Banks, and a shipload of sailors set sail from England to Tahiti to observe the Transit of Venus. Camped out on Point Venus, they witnessed the event on June 3, 1769. Cook sketched the transit, but Banks had surprisingly little to say about it. Perhaps he was distracted by the wonders of the island itself.

The Enhanced Thematic Mapper Plus on the Landsat 7 satellite captured this natural-color image of Tahiti on July 11, 2001. This island is part of a volcanic chain formed by the northwestward movement of the Pacific Plate over a fixed hotspot. Tahiti consists of two old volcanoes—Tahiti-Nui in the northwest and Tahiti-Iti in the southeast—linked by an isthmus.

Through studies of its rock layers, geologists have figured out the likely history of Tahiti-Nui. Today it is roughly round, and it was roughly round when it first formed as a volcanic shield between 1.4 million and 870,000 years ago. But between then and now, it had a different contour. Both the northern and southern flanks of Tahiti-Nui collapsed sometime around 860,000 years ago, gouging massive arcs out of the island’s perimeter. Soon after the northern flank collapse, a second shield volcano began forming. Volcanic material on the northern side of Tahiti-Nui eventually overtopped the original volcanic structure and started filling in the southern depression.

Although Tahiti-Nui now has a fairly symmetrical contour, it has an asymmetrical three-dimensional shape. Mountains are taller on the northern half of the island.

Yet something else besides volcanic activity has shaped Tahiti: rain. Heavy tropical rains have carved deep valleys, some with nearly vertical walls up to 1,000 meters (3,000 feet) tall. The angled sunlight in this image brightens some slopes, while leaving others in shadow. Tahiti’s sharp relief has complicated the geological surveys because they cause so much erosion. But the same rains have also promoted the growth of the lush plants that carpet both Tahiti-Nui and Tahiti-Iti.

Though the Transit of Venus was the stated objective of the British expedition, Banks was likely more interested in Tahiti’s plants. Specimens he collected from Tahiti, New Zealand, South America, Australia, and Java accounted for roughly 1,300 new species, and his famed collected is now stored at the Natural History Museum in London.

Complementing the rich life on land is marine life around Tahiti’s perimeter. Coral reefs fringe the island, and the reefs are thickest on the southern and western sides. Reefs frequently form along the submerged slopes of volcanic islands.

NASA Earth Observatory image by Jesse Allen and Robert Simmon, using Landsat data from the U.S. Geological Survey. Caption by Michon Scott.

The Earth Observatory's mission is to share with the public the images, stories, and discoveries about climate and the environment that emerge from NASA research, including its satellite missions, in-the-field research, and climate models.

Like us on Facebook

Follow us on Twitter

With its 14 spectral bands from the visible to the thermal infrared wavelength region and its high spatial resolution of about 50 to 300 feet (15 to 90 meters), ASTER images Earth to map and monitor the changing surface of our planet. ASTER is one of five Earth-observing instruments launched Dec. 18, 1999, on Terra. The instrument was built by Japan's Ministry of Economy, Trade and Industry. A joint U.S./Japan science team is responsible for validation and calibration of the instrument and data products.

The broad spectral coverage and high spectral resolution of ASTER provides scientists in numerous disciplines with critical information for surface mapping and monitoring of dynamic conditions and temporal change. Example applications are monitoring glacial advances and retreats; monitoring potentially active volcanoes; identifying crop stress; determining cloud morphology and physical properties; wetlands evaluation; thermal pollution monitoring; coral reef degradation; surface temperature mapping of soils and geology; and measuring surface heat balance.

The U.S. science team is located at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate, Washington.

Cloud-free skies allowed the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite a clear view of Grande Terre on September 24, 2012. Coral reefs, appearing iridescent blue-green through the seawater, surround the mostly vegetation-coated island.

New Caledonia has a hot and humid climate, but conditions vary between the coasts of Grande Terre. Trade winds from the southeast bring relatively cool, moist air to the eastern part of the island, where rich forests fill the coastal strip east of the mountains. In the west lie open, arid plains. Between the east and west coasts, a mountain range forms the island’s spine. The tallest point is Mont Panié with an elevation of 1,628 meters (5,341 feet) above sea level.

The British explorer James Cook gave New Caledonia its name because its mountains reminded him of Scotland (once named Caledonia). In the years since his expedition, geologists have teased New Caledonia’s history from clues in the rock record. New Caledona was once a part of Gondwana, an ancient supercontinent that broke apart tens of millions of years ago. New Caledonia drifted northeastward and finally settled in its current position about 50 million years ago. Between 45 and 33 million years ago, tectonic plates colliding with each other uplifted the region, including part of the sea floor. Once elevated, New Caledonia began to erode, thanks to the action of wind and water.

New Caledonia’s rocks are unusually rich in iron, magnesium, and nickel; in fact, the island is home to about a quarter of the world’s known nickel resources. Meanwhile the soils are poor in nitrogen, calcium, phosphorus, and potassium, and plant life has had to adapt. About 75 percent of New Caledonia’s plant species are found only the island, placing New Caledonia third, after Hawaii and New Zealand, for endemic plant species.