Norfolk Island and its neighbouring Phillip Island are remnants of a weathered and eroded volcanic complex that began forming after volcanic activity approximately 2 to 3 million years ago.The Island has an area of 35.7 km2 and is largely comprised of basaltic rocks and tuffs that are weathered deeply, with highly permeable soil, suitable (where sufficiently level) for cropping and grazing.
Six types of soil occupy the major part of the Island. They have been formed from late Tertiary basalt flows with interbedded ash and tuff. These soils all have firm strongly structured surfaces with rapid surface infiltration, and no impeding subsurface layers. These volcanic clay soils have a high water holding capacity (~120 mm/m).
Erosion is evident on Norfolk Island from three processes: soil creep, slumping (land slips) and sheet erosion, contributing to infill in lower parts of the landscape and drainage lines.
Soils have clay textures (>35% clay) throughout the profile. Rock and sand suitable for construction are in short supply on Norfolk Island and concrete, which has to be imported to the island, is expensive.
Acid sulphate soils
Norfolk Island has numerous small areas of some of the most acidic soils in the world. Acid sulphate soils are naturally occurring soils, unconsolidated sediments or organic accumulations (peat) in which sulphuric acid may be produced or has been produced. They are formed under waterlogged (anaerobic) conditions in peaty wetlands across Norfolk Island. Although the mapped areas of acid peat sulphate soils are small, they occur in many drainage lines and consequently may affect water quality in wetlands, streams and dams during critical dry periods.
Acid sulphate soils are considered benign, if left undisturbed. Disturbed and aerated they are the ‘nastiest’ soils in the world. Under the 2020 dry climate conditions and in areas of disturbance (draining, excavation, dam building) where acid sulphate materials with sulphide-containing minerals (pH >4) have become exposed to air, they have oxidised and are producing sulphuric acid (pH <4).
From CSIRO (2020). Norfolk Island Water Resource Assessment. A summary report. CSIRO, Australia.
Update November 2022: “Looking after the peat swamps”
Acid sulphate soils have been found in many of the island’s peaty wetlands and drainage lines. The waterlogged peat soils are high in iron pyrite, which forms sulphuric acid when exposed to air (or if dried out), releasing large amounts of acid, aluminium and iron into the water and environment. If detected, managed appropriately, and left undisturbed, acid sulphate soils do not pose any risk to the environment. However, if disturbed, drained, and unmanaged, these soils can result in negative impacts on water quality, human health, agriculture, infrastructure, animals and plants.
This work forms part of the continuation of the Norfolk Island Water Resource Assessment project, funded by the Department of Infrastructure, Transport, Regional Development, Communications and the Arts. It will complement further research by the CSIRO to understand potential impacts on fresh and marine water quality and aid preservation work in the Kingston and Arthur’s Vale Historic Area.
For more information on the Norfolk Island Water Resource Assessment, please visit: https://www.csiro.au/en/research/natural-environment/water/Water-resource-assessment/Norfolk/Assessment-details
Hydrogeology
The foundation work is:
Abell, R.S. & Falkland, A.C. 1991. The hydrogeology of Norfolk Island South Pacific Ocean. Bulletin 234. Bureau of Mineral Resources, Geology and Geophysics. Australian Government Publishing Service. (67pp, 24MB).