The Fiordland is one of the most magical places in New Zealand. It is also known for its grandiose geology between fiords and mountains. In this article, we will explain the mountains formation. Its geology is very different from the rest of the South Island. It is also known for its many lakes as Lake Marian. Indeed, as mentioned in a previous article, the formation of the Southern Alps is the result of the collision between the Australian-Indian plate and the Pacific plate.
Here it is a subduction which has led to Fiordland mountains formation.
For all definitions: go to this page.
For the time scale is here!
Fiordland mountains formation
Before 20 million years
400 million years ago (Devonian), New Zealand was in the bottom of the sea. The sediments has accumulated for millions of years, deposited one on the other.
Subsequently, about 300-200 million years ago (Permian-Triassic), the collision between the Australian-Indian and the Pacific plate led to first mountains formation in the South Island.
Around 30 million years (Paleogene), the mountains are back in the ocean following a descent of the earth’s crust.
After 20 million years
Geology diverges for Fiordland from the rest of New Zealand 20 million years ago. The Australian-Indian plate will subdue under the pacific plate.
That means we have a plate that goes under another plate when they join together. This is the same mechanism that can be found in the North Island, which I refer to for more information on subduction. However, if in the north it is the pacific plate that subducts (passes below) the Australian-Indian plate. In the Fiordland we have the opposite! The subdued Australian-Indian plate under the pacific plate in the area called Puysegu Trench.
During this subduction, many rocks rise to the surface from the depth. These characteristic rocks of the earth’s crust will appear in the Fiordland mountains.
The rocks of Fiordland
Plutonic rocks
It is possible to see many volcanic rocks (basalt, diorite, granite, etc.). These volcanic rocks originate from an ancient continent called Zealandia.
These rocks are plutonic rocks directly from the depths of the Earth. Plutonic rocks form by slow cooling of a deep magma. The slower the cooling (typically several tens of thousands of years), the larger large crystals formation(at least visible to the naked eye).
Metamorphic rocks
It is also possible to see other rocks, metamorphic rocks. A metamorphic rock is a rock whose formation originates in transformation to a sedimentary, magmatic or other metamorphic rock. It is the changes in the pressure and temperature parameters that craft rocks. In the Fiordland mountains, most of the metamorphic rocks present are gneiss.
Like plutonic magmatic rocks, these rocks are found in the Fiordlands mountains following their extraction from the depths of the Earth.
Greenstone
The greenstone, translated in French by “jade” and in Māori by “pounamu”, is a gem. It is a crystal formed by a pressure and temperature given to the Earth. In Māori culture there are many traditions and is much used as jewelry, ceremonial object, etc …
For all definitions: go to this page.
For the time scale is here!
Source
Joceyn Thrnton, « The field guide to New Zealand Geology, An introduction to rocks, minerals and fossils»; Pinguin Books
Hamish Campbell & Gerard Hutching; « In search of ancient New-Zealand »; Pinguins books
Geoffrey E. Batt and Jean Braun, «The tectonic evolution of the Southern Alps, New Zealand: insights from fully thermally coupled dynamical modelling», Geophysical Journal International, 1998
J.Davey and Euan G. C. Smith, «The tectonic setting of the Fiordland region, south-west New Zealand», Geophysical Journal International, 1982
Peter Blattner (1978) Geology of the crystalline basement between Milford Sound and the Hollyford Valley, New Zealand, New Zealand Journal of Geology and Geophysics, 21:1, 33-47, DOI: 10.1080/00288306.1978.10420720
