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.