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Published byReynard Ross Modified over 8 years ago
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Land of lava and strata
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Gondwana land – ancient southern super- continent (break up in the early Jurassic about 184 Mya) Great thickness of sand and mudstone deposited off the eastern edge Sediments slowly metamorphosed into hard grey sandstone and mudstone (greywacke) An upheaval event raised this new land mass above sea level, forming mountain ranges - ancestral New Zealand
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These mountains were eroded down and the sea advanced By about 30 mya, NZ consisted of a few scattered islands About 25 mya, two large tectonic plates of the earth’s outer crust started impacting and sliding past each other The country split along the alpine fault The Pacific plate began subducting under the Australian plate.
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Volcanic activity in the Northland area under the sea. The Auckland area subsided to become a deep marine basin. For 5-6 mill yrs material from a chain of volcanoes along the western side of Northland eroded and began filling the basin – the Waitemata basin. Volcanic eruptions ceased about 15 mill yr ago in Northland
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The Waitemata basin was filled with sand and mud layers (Waitemata strata) Volcanic vents then began to form in South Auckland Sand began to build along Auckland’s West Coast 0.15 mill yrs ago, basaltic lava began erupting in Central Auckland within a 20km radius of the City centre There were 50+ small volcanic cones and craters.
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Waitemata strata of volcanic andesitic ash and sedimentary rocks Overlaid with more recent volcanic sandstone, rhyolitic ash and scoria REFERENCE Homer,Moore,Ker mode.(2000) Lava & Strata.p4, Inst.GNS
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Rangitoto island is the youngest 600 years ago (just “yesterday”) Field trip At Takapuna beach North end the main geological feature is lava flows At South end are outcrops showing sandstone and mudstone strata of the Waitemata series
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Homer,L, Moore,P and Kermode,L (2000) Monograph Lava and Strata. Institute of Geological & Nuclear Sciences
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