Geology
Atiu is 11 million years old. It is rising from the sea. This makes Atiu very different from your normal tropical island that starts off as a high volcanic island like Rarotonga, then slowly sinks into the sea becoming like the island of Aitutaki, then an atoll and then a reef over a few million years. Atiu is still rising from the sea, so if you visit us you will be going up in the world. It's guaranteed. But only at a rate of 0.2 mm per year.
Atiu started off with a sub-surface hot spot melting the sea floor, starting an undersea volcano, which grew into a mountain. The sea floor was moving over the hot spot and so, eventually, the hot spot was too far away from the mountain and the volcano died. We were left with a mountain sitting on the sea floor. It was a fizzer. It didn't make it. It was just an undersea mountain and that would have been the end of Atiu, but the sea floor kept moving. It covered a distance of about 1200 km in 8 million years and then it met a long rise in the mantle of the earth. As the sea floor slid up this rise, the mountain was pushed up out of the sea.
Now eight million years does a lot to volcanic rock. It dissolves in seawater at a rate of about a millimetre a century into volcanic clay, mud and slush. That doesn't sound much but over 8 million years that is 80 metres. As Atiu rose, the waves washed the top off the mountain until eventually the waves could not cut fast enough and Atiu rose flat topped to a height of 50 metres above the sea. There it remained for 3 million years, eroding and growing an encircling reef and big lagoon.
Yesterday, it rose again, another 20 metres and the encircling lagoon and reef rose too. Now that reef surrounds Atiu. It is weathered, sharp, jaggered, riddled with coral caves and covered with jungle. We call this encircling region of Atiu, the Makatea.
Makatea is fossil coral. The top fossil coral dates at 120,000 years ago, giving that 'yesterday' a geological meaning. It is interesting that the new reef that is now growing dates from the present to 8,700 years ago and that nowhere on the island can you find coral that is between 8,700 and 120,000 years old.
Scientists and some Australian cave divers that visited Atiu solved this riddle. The scientists discovered that over the last 3 million years the world has been going through a series of ice ages and warm periods. The last warm period was 120,000 years ago and the latest started about 8,700 years ago. The cave divers managed to dive to a depth of 46 metres below sea level and found stalactites and stalagmites in a cave down there. These only grow in air, so the sea level must have dropped a least 46 metres during an ice age. Our reef of 120,000 to 8,700 years ago is beneath the sea.
This makes Atiu a weird island and a fascinating land.
