Tahiti

Deep under the sea, hot melted rock rose up. It built volcanoes taller than the water.

Two volcanoes grew side by side. A narrow strip of land joins them.

Tahiti is part of a chain of volcanic islands. The chain formed as the Pacific Plate moved northwest over a fixed hotspot. Tahiti has two old volcanoes: Tahiti-Nui in the northwest and Tahiti-Iti in the southeast, joined by an isthmus.

Tahiti-Nui first grew as a round volcano between 1.4 million and 870,000 years ago. Around 860,000 years ago, both its northern and southern sides collapsed. Soon a second volcano began to grow, and its lava began to fill in the southern gap.

NASA's Earth Observatory places Tahiti in a volcanic chain formed by the northwestward movement of the Pacific Plate over a fixed hotspot. From the island's rock layers, geologists reconstructed Tahiti-Nui's history: a roughly round shield built between 1.4 million and 870,000 years ago; collapse of both the northern and southern flanks sometime around 860,000 years ago, gouging great arcs out of the coastline; and, soon after the northern collapse, a second shield whose lava overtopped the old structure and began filling the southern depression.

Today Tahiti-Nui is fairly symmetrical in outline but asymmetrical in three dimensions, with taller mountains on its northern half.

Lots of rain falls on Tahiti. Over time, the rain carved deep valleys.

Coral reefs grow in the sea around the island.

Heavy tropical rain has carved deep valleys into Tahiti, some with almost straight-up walls as tall as 1,000 meters. The same rain helps thick green plants cover both volcanoes.

Coral reefs fringe the island and are thickest on its southern and western sides. Old reefs now lie deeper underwater, in steps called reef terraces. Cores drilled from them in 2005 show that sea level once rose about 17 meters in just 350 years, when polar ice sheets collapsed.

Heavy tropical rainfall has cut deep valleys, some with nearly vertical walls up to 1,000 m, which complicated geological surveys; it also supports the lush vegetation on Tahiti-Nui and Tahiti-Iti. Reefs fringe the island, thickest on the south and west.

In October and November 2005, IODP Expedition 310 cored 37 boreholes at 22 sites, in water 41.65 to 117.54 m deep, into the old reef terraces seaward of the living barrier reef. ECORD reports that the cores record Meltwater Pulse 1A, between 14,650 and 14,310 years ago: a global sea-level rise of 17 m in 350 years, about 45 mm a year against about 3 mm a year today in ECORD's comparison, which ECORD calls the fastest in Earth's history.

A rare bird called the Tahiti monarch lives only on Tahiti. In 1998, only 25 were known.

People have worked to keep rats away from these birds.

The Tahiti monarch is a forest bird that lives only on Tahiti. Scientists writing in 2017 called it critically endangered. In 1998 it was known from just four valleys, with only 25 birds in total. Another group of 33 birds was found in 2001, high above waterfalls, but only 12 of them were left by 2009.

The main danger is ship rats, which prey on them. Two birds brought from other places, the common myna and the red-vented bulbul, also cause trouble. A recovery program began in 1998. It first worked to control rats, and later these birds too, the scientists reported.

Blanvillain and colleagues (Bird Conservation International, 2017) studied the Tahiti monarch, or O'mamao (Pomarea nigra), a critically endangered forest bird endemic to Tahiti. In 1998 it survived in four valleys of the Paea and Punaauia districts, five to seven birds each; a remote group of 33 above waterfalls in the Maruapo valley, found in 2001, had fallen to 12 by 2009. By 2015 the remaining territories covered just 50 ha in three valleys.

The main threat is predation by ship rats; the introduced common myna and red-vented bulbul prey on nests or disturb pairs. The monarch breeds slowly: only one young per nest was ever seen, and parents keep feeding it for an average of 74 days after it leaves the nest.

On a reef, never touch or stand on the coral. It is alive!

Swim only with a grown-up. If water pulls you out, swim along the beach.

NOAA, the United States ocean agency, asks snorkelers and divers to be careful not to touch, kick or stand on corals, and not to touch other sea animals. The slimy coating on fish protects them from infection and rubs off easily.

NOAA also warns about rip currents. If one catches you, don't fight it: swim parallel to the shore, then back to land at an angle. PlayOwl's advice in the forest: walk quietly and watch birds from the trail.

NOAA Fisheries advises that when snorkeling or diving on coral reefs, people should not touch, kick or stand on corals, and should not touch other animals, because the slimy coating on fish and many invertebrates protects them from infection and is easily rubbed off. These are US guidelines, but the reasons apply to any reef.

NOAA's National Ocean Service notes that rip currents can move at up to eight feet (about 2.4 m) per second; swim parallel to the shore and return at an angle rather than fighting one. In the forest, PlayOwl suggests quiet watching from trails, since the monarch's territories lay in just three valleys in 2015.