Landslides in New Zealand

Unstable land

New Zealand has a high number of landslides because:

• land is still being pushed up by plate tectonics
• rocks are often weak from this movement
• there are lots of earthquakes
• the land is hilly and cut by rivers
• slopes can be unstable, weak layers of volcanic ash or loess
• rainfall can be high

Some landslides move whole mountain sides, suddenly. Others are small or slow, moving only a few centimetres a year.

Causes

On unstable slopes, three things are important in causing large landslides:

1. type of land, such as weak rock and steep slopes
2. processes such as deforestation or stream erosion
3. large rainstorms and large earthquakes

Human impact

Clearing forest has had the most impact on how stable the land is. Clearing land for farming has increased landslide activity. Building roads and subdivision earthworks can also increase landslide rates.

Types of landslides

There are different types of landslides, these include:

• alpine landslides – in the mountains, rock avalanches, rock slides, lahars, rock falls and debris flows
• low mountain landslides – in lower mountain ranges, can be single events or slow movement over a long period of time
• layered rock landslides – where one hard, heavy layer of rock slips off a soft layer of rock when there is a lot of groundwater between the layers
• rotational landslides – common in New Zealand’s hill country where a series of blocks slump downwards, often rotating on a curved surface
• creeping earthflows – where the ground on gentle slopes moves a few metres each year, often during winter months when the ground is very wet
• regolith landslides – one of New Zealand’s most widespread types of landslide. These occur when the loose rock and soil above bedrock slides. Often lots of these slides will occur at the same time during rainstorms.
• submarine landslides – happen under the sea. These can be bigger than those seen on land and even start tsunamis.

Cost

Each year millions of dollars is spent clearing slips from roads and railway lines. It is a never-ending task – there are always more floods or earthquakes to come, and there is plenty of rock and soil waiting to fall down.

In general, landslides are more common in New Zealand than many countries because of the steep land, weak layers and high rainfall. However, landslides cause few deaths in New Zealand because there are not many people living in steep, mountain areas.