Orbicular granites are quite rare, especially those with large and clearly defined orbicules like the one found by Levis Johnson in the early 1940s.
A rare find
Johnson came across the 380 million year-old granite boulder in Elfords Creek, Karamea (South Island) while out deer stalking.
A couple of years after his discovery, Johnson organised for a bulldozer to shift the boulder onto a truck, which then transported it to his house. The estimated 4-5 ton boulder with a diameter of 1.2-1.5 metres sat on Johnson's front lawn for nine months until Pat Marshall, a famous New Zealand geologist then working for the Public Works Development Department, recognised the significance of the boulder. Johnson declined Marshall's initial offer of £5, believing his discovery must have been worth more. In the end, Marshall paid £50 for the boulder - which today would be around $4500. The Geological Society of New Zealand newsletter reported that Johnson 'felt like a millionaire'.
Marshall then had his new purchase carefully transported to the Public Works Laboratory in Wellington. After being cut and polished, slices of the boulder were sent to both national and international museums and Geology Departments, including in Australia, Britain, and here at the Auckland Museum.
What is orbicular granite?
Granites are coarsely crystalline plutonic igneous rocks which form a major part of the Earth’s continental crust. In New Zealand outcrop of true granite is restricted to the South Island and other orbicular granites have been described from the Separation Point Granite of the Motueka area in northwest Nelson.
The word orbicular / orbicule is derived from the Latin orbis meaning circular, as in orbit or orbis terrae (the Earth). In geology it refers to onion-like, concentric layers in a rock or mineral. For example many concretions are orbicular in form.
In geology terms
Exactly how orbicular granites form is not clearly understood but they may consist of a spheroidal or squarish core either alone or with one to several concentric shells. The core is essentially plagioclase feldspar (often radiating mosaics) and the shells either of coarse alkali feldspar (with quartz) or concentrations of magnetite (an iron oxide) and biotite mica are arranged concentrically or tangentially through plagioclase. The concentration of the black magnetite and mica causes the dark layers. The orbicules may be deformed or even fragmented indicating a dynamic environment of formation and accumulation in the magma during its consolidation.
The orbicular granite can be found in the Origins Gallery on level one of the Museum.
Grapes, R. 1996. Orbicular granite from Karamea. Geological Society of New Zealand Newsletter 110: 19-22.
Cite this article
Grenfell, Hugh and Fleming, Paige. .
An expensive piece of granite . Auckland War Memorial Museum - Tāmaki Paenga Hira. First published: 7 October 2016. Updated: 2 November 2016.