Motupōhue (Southland) EarthCache
Motupōhue (Southland)
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Geology of Bluff Hill/Motupōhue
Bluff Hill - Older than Aotearoa
Much of the rock from which Southland is made was formed long before New Zealand began its drift away from the super continent Gondwana 80 million years ago. During the Permian period (280 - 235 million years ago), a chain of volcanoes was building land by erupting molten rock from below the surface into the shallow seas around. Motupōhue was formed when molten rock oozed up through the seabed but did not quite reach the surface. Insulated beneath layers of sediment, it cooled slowly, forming a mound of coarse-grained rock called norite. Norite was quarried from the slopes of Motupōhue until the mid 20th century. It is a durable stone and was used to build the retaining walls of Bluff’s Island Harbour. Cut and polished, it was used for headstones and decorative panels. The Bluff War Memorial and the map on the summit of Bluff Hill/ Motupōhue are made from polished norite.
Cooked sediments
Because the sediment layers that formed during this period were kilometres deep in places, the weight of the upper layers compressed and hardened those underneath. This process, combined with changes to sediments caused by contact with molten rock, created fine-grained Tiwai argillite. This stone was used by Tangata Whenua to make high quality adzes in pre-European (pre metal) times.
A whanau of mountains
From the summit of Motupohue you can see the ‘Permian family’; the Takitimu Mountains, the Livingstone Mountains, the Longwood Range and Mount Hananui/ Mount Anglem on Rakiura/ Stewart Island, all of which are built of rock formed during the same period.
Rock Classification
- Sedimentary rock - made of sediments like mud, silt, sand and calcium carbonate (from shells and other sea life)
- Metamorphic rock - rock that has been changed by heat and/or pressure
- Igneous rock - molten rock from deep in the Earth’s crust
- Plutonic rock - a type of igneous rock made from magma that has cooled and solidified below the surface of the Earth
Logging
- At GZ you are surrounded by polished norite, in your own words describe the texture, colour and the composition.
- The surrounding exposed rocks are natural norite, there is a predominant hue (other than grey), please identify the colour?
- Give the local name of norite.
- From the above rock types what kind of rock is norite?
- Optional, take some photos at GZ, you should anyway with the great views.
Sources
Super Site for Education - Bluff Hill/Motupohue Scenic Reserve
DOC Booklet.
Additional Hints
(No hints available.)