Intra Plate Volcanics, Hot Spots

Intra Plate Volcanics, Hot Spots

Intra Plate Volcanics, Hot Spots

A sunny ferry trip to Rangitoto Island

 

 

 

 

A broad gently sloping cone. Rangitoto is a shield volcano formed from fluid basalt lava flows

Where does all the basalt come from?

…..mid ocean ridges ridges to begin with then it is subducted under the east coast of the North Island

  • Most basalt is produced at mid ocean ridges.
  • Basalt produced at mid ocean ridges is rafted away as oceanic crust is formed.
  • The process of subduction produces mainly basaltic magma
  • This basalt is not rafted away from the site of eruption.
  • The subducting slab comprises of basalt and gabbro with a thin veneeer of seafloor sediment.
  • Water and other fluids move into the overlying “mantle wedge” where they promote partial melting.
  • The resulting basalt magma makes its way into the continental crust
  • The basalt can end up feeding large volanos such as Mount Ruapehu.
  • Broad gently sloping cones such as Auckland’s Rangitoto Island, Lyttleton and Akaroa on the Banks Peninsula.
  • In the Auckland region steep sided scoria cones are also formed by the process of fire fountaining.

 

 

 

 

  • Most basalt is produced at mid ocean ridges.
  • Basalt produced at mid ocean ridges is rafted away as oceanic crust is formed.
  • The process of subduction produces mainly basaltic magma
  • This basalt is not rafted away from the site of eruption.
  • The subducting slab comprises of basalt and gabbro with a thin veneeer of seafloor sediment.
  • Water and other fluids move into the overlying “mantle wedge” where they promote partial melting.
  • The resulting basalt magma makes its way into the continental crust
  • The basalt can end up feeding large volanos such as Mount Ruapehu.
  • Broad gently sloping cones such as Auckland’s Rangitoto Island, Lyttleton and Akaroa on the Banks Peninsula.
  • In the Auckland region steep sided scoria cones are also formed by the process of fire fountaining.

Plate tectonics and the rock cycle

Watch the video of work we have done in class and check your knowlege by doing the quiz.

 

Stop Press.

Patrick wins a Mars bar for the first homework completion. Another mars bar goes to the first person with all correct answers.

…another couple of Mars bars for Cici and Alex because they got the work done quickly.

Still waiting for a completely correct set of answers.

Rangitito, Auckland’s youngest volcano

Rangitoto Island.

  • Rangitoto is Auckland’s youngest volcano. Early eruptions broke out under the sea about 600 million years ago.
  • Maori inhabitants on nearbye Motutapu witnessed these eruptions leaving trace footprints in ash. Scoria.
  • As with all volcanos in the Auckland the driving force for activity on the surface is the subduction of the pacific plate beneath the australian plate.
  •  fountained from a central cone and basalt lava flowed freely from its base, overlapping to produce a symetrical shield shape. Mangere mountain is another example of a scoria cone.
  • The lava flows cooled to from a hard crust on the surface. In places this was broken into slabs by the drag effect of lava flowing beneath.
  • Lava sometimes drained to leave behind caves which are now open to view after roof collapse.
  • On the foreshore at the wharf thick syrupy flows can be  seen.
  • Older volcanos North head and mountain Victoria lie accross the Waitemata.
  • After 600 years there is very little soil on the surface of rocks for plants to grow.
  • Mosses and lichens would have been the first colonisers. As they died a thin layer of humus formed
  • Pohutukawa were the first trees to take hold in a very dry environment. The base rock is porous.
  • After wind borne invaders arrived they attracted birds who brought with them more seeds to grow and develop.

Geology.

 The photos.

How to get there.

By Fullers Ferries from Auckland Wharf. The cruise out to Rangitoto also offers the option of taking the Volcanic Explorer a tractor drawn train up the volcano rather than walking. You can cover more ground this :

Bibliography

A Field Guide to Auckland by Ewen Cameron, Bruce Hayward and Graeme Murdoch. Published by Random House New Zealand 2001.

Lava And Strata by Lloyd Homer, Phil Moore and Les Kermode. Published by Landscape publications limited in association with The Institute of Geological and Nuclear Sciences

 

Nga Tapuwai o Mataoho, footprints of Mataoho. Mangere Mountain field trip

Thanks to Ian and Farrell at Mangere Mountain Education Centre for sharing their knowledge with 3 parties of Green Bay High School students as they researched surface processes for science 1.13. Walk the present, discover the past.

Students focussed their research  on:

  • the shape of the scoria cone (steep sided compared with Rangitoto a shield volcano where the molten basalt flowed freely without a high gas content.)
  • major landsape features and processes (explosion crater, large southern cone with a smaller cone to the north)
    Other landscape features including a breach on the eastern side of the southern cone that allowed basalt to flow freely where the football fields are now. Another minor breach that local Maori had to fortify to prevent incursions by unwelcome guests
  • Assorted lava bombs and evidence of fire fountaining as volcanic activity waned
  • Human impact through stone gardens, kumara pits. There was substantial quarying as durable basalt and scoria were removed for building purposes.

Watch out for some of the features in the video of the field trip.

Untitled from john west on Vimeo.

The basaltic magma is produced 70Km below the surface as the pacific plate is subducted beneath the Australian plate. It gradually makes it way to the surface and it is only a matter of time before another volcano joins 50 others dotted accross the Auckland landscape

Some more on Auckland volcanos

Mangere Mountain

Maori Bay a 17 million year old Auckland volcano….. yep those plates were colliding then!

Lake Pupuke, one of the oldest volcanos in the Auckland field.

Te Henga Bethell’s Beach, evidence for a 20 million year old volcano

A look at the subduction zone responsible for all the volcanic activity in the North Island and the devastating earthquake in Christchurch. Why Tonga has a precarious existence

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