This is a slightly expanded version of my seventh contribution to the current Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “G is for Green Hydrogrossular Garnet”.
G is for Granite
This is a small granite stone I found on the Te Waewae Bay coast at Papatotara on 16 August this year. A very similar granite formed the bench-top in the Dunedin motel unit I stayed in a week later.
Granite is often used as an example to demonstrate how a rock can be made up of more than one mineral – white and/or pink feldspar, pale quartz, and small specks of black or silvery mica (see Geology Science). Small amounts of various other minerals may also be present, helping to create a variety of types of granite. Granite is tough and chunky – it resists erosion better than other rocks. It can be found in batholiths, huge intrusions of magma that form underground. It cools there slowly, so that the crystals within it are usually visible to the naked eye. Batholiths can be tens or even thousands of kilometres long. New Zealand’s Median Batholith, the source of my Papatotara stone, is thought to be over 10,000 square kilometres in size, and is an unusually long and thin batholith (see the section “Zealandia’s ancient granite backbone” in this GNS media release on the mapping of Zealandia). About eight years ago, I visited England and went to Dartmoor and came across their famous tors, including Hound Tor:
The tors of Dartmoor are exposed granite segments of the massive Cornubian Batholith which lies beneath much of Cornwall and Devon – see “Geology and Landforms: Dartmoor Factsheet”. In his book “Quartz and Feldspar – Dartmoor: A British Landscape in Modern Times” (2015), Matthew Kelly writes: “It is hard to think of something more enduringly real or certain than granite’s tough composite of quartz, feldspar and mica” (page 2). Dartmoor granite is quite grey, characterised by a relatively high proportion of dark tourmaline. While climbing on and around Hound Tor, I noticed very large white crystals (“phenocrysts”) in the rock – of what I now know to be feldspar, and a result of the very slow cooling of the rock deep underground. These crystals were up to about eight centimetres long and two centimetres wide. I think I took a photo of one at the time but I can’t find it. I have located a photo my wife took which, when zoomed, in gives an idea of these crystals.
For a little more on Hound Tor, see The Globe Trotter‘s account of a visit, with photos of climbers, and there is a brief introduction to Hound Tor produced by Essentially England. And here is an excellent YouTube clip of drone footage of Hound Tor:
The next entry, “H is for Hematite Poppy Jasper”, is here. An Index for this Alphabetical Series is here.
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