The Cook Islands are the result of volcanic activity and coralgrowth in the centre of the South Pacific Basin. The islands of Rarotonga, Mangaia, Atiu, Mauke, Mitiaro and Aitutaki are the emergent peaks of extinct volcanoes. Manuae, Palmerston, Penrhyn, Manihiki, Rakahanga, Pukapuka, and Suwarrow are atolls, that is coral reefs around a lagoon on the top of submerged volcanoes. Aitutaki is part volcanic, part atoll; it is also called a near-atoll. Takutea and Nassau are sand cays on a coral reef foundation.

The islands of Mauke, Mitiaro, Atiu, Takutea, Manuae, the Eclipse Sea-mount (between Manuae and Aitutaki), Aitutaki, and after a small depression Palmerston form a linear volcanic chain with Wnw trend. This chain, a continuation of the Austral Islands chain, is probably due to a major fracture in the crust of the earth.

Rarotonga and Mangaia are on the crest of a separate arch surrounding the Mauke -Aitutaki chain. Rarotonga is probably due to crustal responses to the continued sinking of the main Mauke-Aitutaki group.

The volcanic activity of the Mangaia, Atiu, Mauke and Mitiaro volcanoes was followed by subsidence: the peaks sank below sea level, and barrier reefs grew around the submerged peaks. Later the peaks and surrounding reefs rose slowly above the surface of the ocean, or the level of the sea dropped beneath the peaks and coral reefs. A new coral reef grew around them below sea level. The raised coral reef surrounding the emergent peaks is called “makatea”. The four islands are raised coral or makatea-type islands.

The atolls of the Northern Group, except Penrhyn, rise from the periphery of a broad, shallow platform, which is termed the Manihiki Plateau.

This plateau, 2,500–3,000 meters below sea level, and covering an area of 600,000 km2, is thought to be one of the micro-continents of long ago (2). It is sited along the axis of the Darwin Rise, and was probably levelled in the Cretaceous (136 to 65 million years ago).

Manihiki-Rakahanga are on the Nne-trending eastern margin of this plateau, and Pukapuka, Nassau, and Suwarrow are on the Wnw-trending southern margin. Penrhyn lies between the Manihiki Plateau and the Line Islands, and is the highest of the extinct volcanic mountains of the Cooks with a relief of over 4,876 meters.

Niue, once within the political boundaries of the Cook Islands (1901/3), does not belong to any of the Southern Cooks' mountain chains. It is an isolated coral island near the head of the Tongan Trench, entirely composed of limestone, which caps a submerged volcano. About 30 km to the east are two Seamounts: the Endeavour and Lachlan Seamounts. A third, smaller seamount, lies 30 km to the south-east.

The eruptive phase of the Cook Islands volcanoes, except Rarotonga, took place in the early Tertiary age (Palaeocene), some 65 to 60 million years ago. It is thought that the Northern group volcanoes stood high above sea level at that stage. They subsided during the Eocene (60 to 38 million years ago).

In the raised coral reef of Mangaia fossils of the Oligocene (38 to 26 million years ago) and Miocene (26 to 7 million years ago) have been found. page 66 Penrhyn, Manihiki-Rakahanga and Suwarrow are underlain by limestone 14 to 22 million years old (Miocene). The coral reef growth at Atiu began in early Pliocene (7 million years ago), while the Suwarrow reef, the reef rock at Rakahanga, Pukapuka (south of Ngake) and Manihiki (south of Tauhunu) are probably of Pleistocene age (2 million and less years ago).

The Mangaia reef at plus 2m yielded radiometric ages of 90 plus/minus 20 and 110 plus/minus 20 thousand years. The makatea at Ngatangiia, Rarotonga, may be of similar age. The main reef formation at Rarotonga and Aitutaki was constructed before the last ice-age (15,000 years ago). A raised reef at Avarua, Rarotanga, at plus 1 m above low water, was dated as 2,130 years B.P. (before the present), while the nearby reef at plus 2 m is older than 43,000 years B.P.

Rarotonga is the youngest of the Cook volcanoes. Lavas showed radio-metric ages of 2.3 - 2.8 million years (Pliocene). Some of the phonolitic eruptions, which followed the basaltic eruptions, might be as recent as the Pleistocene (less than 2 million years ago). The volcanic core is surrounded by Pleistocene gravels and sands.

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