Geology of Christchurch, New Zealand

Abstract

Christchurch is situated on the east coast of the South Island of New Zealand in the south Pacific Ocean. The city is located at the coast of the Canterbury Plains adjacent to an extinct volcanic complex forming Banks Peninsula. The site of Christchurch was mainly swamp, behind beach dune sand, and estuaries and lagoons, which have now been drained. Two rivers—the Avon and Heathcote—which originate from springs in western Christchurch, meander through the city and form the main drainage system. Christchurch has developed as a service center, principally in response to the requirements of agriculture, horticulture, orcharding and market gardening on the fertile soil of the Canterbury Plains. Ground water in alluvial aquifers underlying the northern Canterbury Plains provides a plentiful supply of excellent quality water for domestic, industrial, live stock and irrigation requirements. An abundant supply of greywacke gravel and sand derived from erosion of the Southern Alps—the axial mountain range of the South Island—is available from pits excavated on the plains inland from Christchurch. Clay and volcanic rock are available near Christchurch from the Port Hills slopes of Banks Peninsula. A 2.6 km long rail tunnel and an adjacent road tunnel pass through the volcanic rock of the Port Hills, linking Christchurch with the port of Lyttelton in Lyttelton Harbor. Geological constraints of concern to Christchurch include flooding, variable foundation conditions, slope instability on the Port Hills, and coastal erosion. The Waimakariri River with its catchment in the Southern Alps, regularly flooded Christchurch prior to stopbank construction and river realignment, which began shortly after the city was established in 1850. Variable foundation conditions as a consequence of a high water table and lateral changes from river floodplain, swamp, and estuarine-lagoonal environments, impose constraints on building design and construction. Susceptibility to slope failure instability is a problem for urban development on the loessial soils and colluvium which mantles the volcanic rock of the Port Hills. Stable foundation conditions are determined by the identification of potentially active erosion processes in the field and in the laboratory, and the use of appropriate design and construction practices. Coastal erosion processes, and sea and river response to a predicted global warming, must also be considered. The geology, tectonic setting, and active seismi-city of the Christchurch area indicate that future large earthquakes will occur which will have major impact on the city. Earthquakes are expected to produce liquefaction, landsliding, ground cracking, and tsunami. Planning and design to mitigate the consequences of these phenomena are an essential prerequisite for preparedness. The effects of environmental, hydrogeological, and geological factors are being incorporated in urban planning. The identification and quantifying of geological hazards, and the implementation of regulation and planning designed to discourage irresponsible land use, should continue in the future as the geological knowledge and database is expanded.