Geology

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

 
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