A GEOLOGICAL LANDSCAPE

The islands of Motutapu and Rangitoto are 'joined at the hip' (by a causeway constructed by US servicemen stationed on the island during WWII) , yet they couldn't be more different. They are a unique combination. Motutapu is Jurassic, sitting on ancient greywacke more than 160 million years old and sedimentary strata some 20 million years old. It is an ancient island dating back to Gondwanaland whereas Rangitoto is NZ's youngest island.

Rangitoto is the product of an eruption 600 years ago. The eruption covered Motutapu with fertile, friable, volcanic solids making it attractive for cultivation by Maori. The last eruption is believed to have been only about 200 years ago.

Millions of Years in the making

Years ago	Epoch	Rocks/local events	Vegetation/Biota
600	Recent	Rangitoto erupts basaltic ash and lapilli over 75% of Motutapu. Overlies greywacke and Waitemata sandstone Erosion by sea of soft sandstone and cliffs continues	Possum, wallabies, rabbits, stock. Bush cleared in places for farming in 1857, and cultivation by Maori, Dog and rats. Bracken. Vegetation affected by ash from Rangitoto. Tree ferns. Gully forest - taraire, mangeao, puriri, rewarewa, totara, kohekohe, mahoe. Coast forest - pohutukawa
1000 - 10000	Holocene	Sea level back to near present day. Motutapu separated from hills of Waiheke and Motuihe	Rimu, tree ferns. Seeds dispersed by birds added to mixture of trees. Late glacial - rise in temp
20000	Pleistocene	Ice Age - fluctuating sea levels(4 Ice Ages)Motutapu a low hill in the Waitemata River ValleyMotukorea (Browns Island) erupts ash on the hill	Continuous podocarp forest quickly grew. Forest in Auckland only in small stands and tall shrublands on north facing slopes
2million - 5million	Pliocene	Volcanic rocks of Great Barrier Island and Coromandel formed - act as barrier to ocean waves for Motutapu - wave action reduced on greywacke. Greater effect on softer rocks
17million	Miocene	Submergence of the land Stacks and coast submerged. Waitemata sediments overlaid greywacke (the Waitemata basin extends over Auckland to the Waitakeres) Sandstones/mudstones rest directly on greywacke	Barnacles from greywacke rocks fell off into Waitemata strata
22million	Oligocene Eocene Paleocene	Motutapu an island. Stacks eroded by ocean waves	Warm ocean currents brought nikau
	Cretaceous	Tasman Sea slowly opening isolating NZ in the Pacific Land in Auckland above sea level Erosion of greywacke Motutapu a greywacke hill mass
135million	Jurassic	Motutapu formed largely of greywacke Ancestral continent of NZ formed of Waipapa basement greywacke (hard, dark grey sandstone) from hardening sediment deposited at eastern edge of Gondwanaland Argillite rocks - scraping by oceanic plate Raised above sea level	Land links with Gondwanaland Ferns, mosses, podocarps, conifers, extinct ancestors of kauri, totara, matai, kahikatea, rimu. Ancestors of weta, ratites, tuataras, frogs, snails Large marine reptiles Fossils - giant barnacles, lamp shells, bryozoans
230million	Triassic	NZ a geosyncline basin under the sea collecting sediment