The oldest rock on the Chatham Islands is the schist exposed on the north-west and north-east of the main island, and also on the Forty Fours (Motuhara) in the ocean to the east. It also forms the basement to the younger rocks of the island group.
The schist is continental rock, and 100- to 90-million-year-old fossils on Pitt Island (Rangiāuria) confirm that the Chathams formed part of the Gondwana supercontinent. Around 83 million years ago the Zealandia continent broke away from the rest of Gondwana. At about this time a large volcano formed in the area of the Chatham Islands, and rocks from it constitute the bulk of the southern part of the main island.
The volcano and much land to the west submerged between 70 and 65 million years ago but the Chatham Rise, though underwater, is a reminder of the former land connection to the New Zealand mainland, as are dinosaur fossils from about that time.
Volcanic activity continued under water. Sedimentary rocks also formed on the ocean floor. Less than four million years ago tectonic movement led the Chatham Islands to become the one part of the Chatham Rise above water. Formerly underwater volcanic and sedimentary rock was now exposed. However, there is very little seismic activity in the vicinity of the islands.
Soil and vegetation
For the last 350,000 years peat has built up on the main island, a product of both climatic conditions and poor soil drainage, and peat or peat-derived soils cover 60% of the land area. In some places the peat is 6 metres deep and is still accumulating. Within the peat is found volcanic ash from eruptions of the Taupō volcano on the New Zealand mainland, in particular from the Ōruanui eruption 26,500 years ago.
Before human settlement about 80% of the main island was covered with sedges, rushes, ferns and swamp grasses, a thick canopy of long thin-leaved tarahīnau (a Dracophyllum species) and pouteretere (Leptecophylla robusta) shrub. Tall trees such as podocarps were absent.
The coasts, where sand has mixed with peat, carry Corynocarpus laevigatus (karaka), known in the Chatham Islands as kopi, and other broadleaf forests dominated by the tree daisy Olearia traversiorum, known as akeake (pronounced locally as ‘ak-ee-ak’). This is not related to the New Zealand akeake Dodonaea viscosa, despite the identical Māori name.
Forests in more sheltered or wetter settings were home to Chatham Islands variants of karamū and matipo.
Most plants were either variants of mainland species or closely related to them. The most notable endemic plant is the blue-flowered Chatham Island forget-me-not Myosotidium hortensia.
Small island, tall shrubs
The Chatham Island akeake is the world’s tallest daisy. Barker’s koromiko (Hebe barkeri) is the world’s tallest hebe, and the Chatham Islands matipo, karamū and tarahīnau are also the largest of their respective genera. This phenomenon results from the absence of canopy trees such as podocarps.
Human impact on vegetation
Moriori probably brought Corynocarpus laevigatus to the islands. During the six centuries of human occupation fires burnt huge holes across the peat bogs, which were further compacted after 1840 by grazing animals. The coastal stands of trees were also grazed, reducing them over time to stark standing trunks, or treeless pasture.
Tarahinau remained common in the southern part of the main island. Elsewhere coarse bracken fern and, since 1970, gorse have become prevalent.
The seas around the Chatham Islands are invigorated as subtropical ocean currents meet and mix with colder, less saline, water from the subantarctic. Sea life is so abundant that the Chatham Islands are one of the world’s richest fishing grounds. They are also home to huge flocks of seabirds.
Weka not kiwi
Some Chatham Islanders call themselves ‘weka’, after a bird which is both hunted and eaten on the islands. Weka were only introduced to the islands in 1905, but in the 2000s there was a population of around 60,000.
Land-bird life on the islands dates from the emergence of the current landmass a few million years ago. It is similar to that found on the New Zealand mainland.
At the onset of human settlement about half the 64 species present were distinct from mainland relatives at the species or subspecies level. Many were larger than their New Zealand counterparts. Three birds – two rails and a flightless duck, all now extinct – were distinct at the genus level. Other extinctions since Pākehā settlement include another rail, the local raven and fernbirds. There were no kiwi, nor were there ever any moa.
In November 2003, scientist Colin Miskelly was operating a spotlight used to bring down tāiko (an endangered species) to fit them with transmitters. When his team caught a record four in a night, he had to return to camp for the duct tape used to attach transmitters. As he came back on his quad bike, a tāiko emerged from the dark and flew up the headlight beam. ‘I had to duck to avoid being hit in the head, and still shudder at the thought of what might have happened if it had hit the quad.’1
Native birds are now relatively rare on the main island, which is dominated by introduced species from New Zealand and further away.
The best known surviving natives are the tāiko (magenta petrel) and the black robin. Other endemic species are the local oystercatcher, tūī, warbler, parea (Chatham Island pigeon), Forbes parakeet, and Chatham Island shore plovers and snipe.