Soft-rock hill country
Two types of landslide are common in New Zealand’s hill country, particularly in the North Island – rotational landslides and creeping earthflows.
Rotational landslides are dominant in the soft, Tertiary (less than 65 million years old) marine sedimentary rock that makes up much of New Zealand’s hill country. They generally consist of a series of rigid blocks that have slumped downward, often rotating on a curved surface. Small ponds often form on the surface of the landslide, and extensive flow lobes (tongues of material) spread down the slope.
Such slides can deform whole hillsides, and many block, at least temporarily, the local drainage systems. If the rock layers are not strongly deformed (such as those in the Taranaki–Whanganui region), hillsides tend to collapse with little sideways movement of material. However, if the layers are dipping out of the hill slope, landslides can travel for considerable distances.
Herbert Guthrie-Smith wrote of his problems with slips in the soft mudstone at his Hawke’s Bay farm:
‘Sometimes a whole hillside will wrinkle and slide like snow melting off a roof, its huge corrugations smothering and smashing the wretched sheep, half or wholly burying them in every posture.’ 1
Creeping earth flows
Also common in soft-rock hill country are deep-seated creeping earth flows. They move a few metres each year, normally during the winter months when the ground is saturated. Like glaciers, they flow and slide over their base, producing crevasse-like tension cracks and hummocky terrain. Earth flows tend to occur on gentle slopes and in soft, clay-rich rocks containing minerals that swell and shrink as they absorb and release water.
Regolith landslides are New Zealand’s most widespread and noticeable type of landslide. Regolith (from the Greek meaning blanket of stone) is the covering of soil and loose rock fragments that overlies bedrock. Regolith landslides occur mainly in this surface material and involve very little of the underlying bedrock. Although they can occur individually, it is more usual for large numbers to occur simultaneously over wide areas during intense rainstorms, such as Cyclone Bola on the East Coast in 1988, and the Manawatū–Whanganui rainstorm of 2004.
The typical landslide in these events is shallow and rapid, immediately converting into fast earth flow (with the consistency of wet cement). On average these run out to distances equivalent to three times the length of the initial slide, and occasionally up to 10 times. Regional events, numbering hundreds of individual landslides, spread over thousands of square kilometres of land, are often yearly occurrences. They also take place whenever large earthquakes or rainstorms occur.
The amount of soil erosion they cause was first documented in 1921 by naturalist Herbert Guthrie-Smith in his book Tutira. Later, geographer Kenneth Cumberland systematically examined regolith landslides in Soil erosion in New Zealand (1944). While landslides of this type are generally not life-threatening, they exact a huge toll each year on roads, railways, farm infrastructure and pasture production.
Deforestation has greatly increased susceptibility to these events. Within a short time they have extensively depleted productive soils formed over thousands of years. They represent New Zealand’s most costly landslide hazard.
Submarine landslides occur most commonly offshore on the continental slope. Bathymetric surveys, carried out by boats bouncing sound waves off the sea floor, indicate that some are much bigger than any seen on land. As they are an important trigger of tsunamis, recent attention is being given to understanding them.