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A very common grey or greenish rock made of sand-sized grains and rock fragments. This rock could be considered the foundation of New Zealand (and the continent of Zealandia). It formed from material deposited in the sea off the eastern shores of 'Australia' when it was part of Gondwanaland.
Sedimentary: Sand-sized fragments cemented together in a muddy matrix (therefore technically a 'clastic sedimentary' rock).
Source location: Canterbury.
Sizes: Choose from dropdown menu either Shelf specimen (300–800g), or Hand specimen (less than 300g)
Note: Images are examples of specimens only. Sizes and shapes will vary.
Greywacke is a sedimentary rock composed of a mixture of quartz, feldspar and sand-sized grains set in a muddy matrix.
Physical properties of greywacke
- Colour: Typically grey, dark grey or dark greenish in colour.
- Lustre: Usually has a dull or earthy lustre.
- Hardness: Has a variable hardness depending on the composition of its grains and rock fragments.
- Density: Ranges from 2.2 to 2.9 grams per cubic centimetre.
General properties of greywacke
- Composition: Primarily composed of sand-sized grains and rock fragments, which are often cemented together by a finer-grained matrix.
- Texture: Has a coarse-grained texture due to the presence of larger grains and fragments.
- Induration: Is typically indurated, meaning it is hardened and compacted through processes like pressure and cementation.
- Depositional Environment: Is commonly associated with turbidite deposits, indicating its formation in submarine or deep-marine environments. New Zealand's greywacke is largely derived from deposits carried in ancient times from Australia's landmass into its eastern ocean.
Uses of greywacke in New Zealand and worldwide
- Early Māori: Greywacke (whatuaho) was used to make pounding tools for preparing muka, the fibre that comes from harakeke (NZ flax).
- Construction: Used as a construction material, particularly in areas where it is locally available. It can be used for building facades, walls, and aggregate for concrete.
- Landscaping: Employed in landscaping projects, including pathways, retaining walls, and decorative features, due to its natural appearance and durability.
- Geological Studies: Studied by geologists to understand sedimentary processes, ancient environments, and the history of Earth's crust.
- Building Stones: Is quarried and used as building stones for architectural purposes, such as in walls and other decorative elements.
- Riprap: Its durability makes it suitable for use as riprap, providing erosion control and stabilisation along riverbanks, shorelines, and slopes.