New Zealand’s Geohazards

What does geohazard mean?

The word geohazard comes from two words:

Geo – from the word ‘geological’ meaning from the earth

Hazard – a situation that poses a risk to property, environment or life

So ‘geohazards’ means the risk of damage caused by a geological process.

Common geohazards in New Zealand

New Zealand is geologically active. This is because it sits across the boundary between the Indo-Australian and Pacific tectonic plates. These moving plates create huge forces that have shaped our landscape; uplifting mountains, carving out lakes and rivers and creating our rugged coastline. These forces continue to form the land today and can also cause:

• landslides
• earthquakes
• tsunamis
• volcanoes
• lahars
• hydrothermal eruptions

Read more: What Causes Geohazards

Earthquakes

About 20,000 earthquakes occur every year in New Zealand, but we only feel about 250 of these. Earthquakes that are strong and close to towns or cities can cause great damage and sometimes loss of life. For this reason it is important for New Zealanders to know how to prepare for and respond safely to earthquakes.

Tsunami

A tsunami is a series of powerful ocean surges often caused by an undersea earthquake or landslide. Most tsunamis are similar to very strong, fast tides, rather than waves. The larger ones can travel a long way inland. Much of the damage from tsunamis is caused by the erosion from the strong currents and floating debris.

Volcanoes

When molten material (magma) reaches the earth’s surface a volcano is formed. A volcano will erupt broken rock and magma (ash) and release steam and gases. New Zealand volcanoes such as Ruapehu, Tongariro, and White Island are often active. Ash from eruptions can spread over large distances so can affect people living a long way from the eruption. Hazards also exist close to the active vent(s).

Lahars

Lahar is an Indonesian word that describes a volcanic mudflow. A moving lahar looks like a mass of wet concrete that carries rock debris. Lahars vary in size and speed and can cause widespread damage.

Eruptions may trigger lahars by quickly melting snow and ice on a volcano or ejecting water from a crater lake. Lahars are also formed by heavy rainfall during or after an eruption, when volcanic ash is washed off the volcano. Some of the largest lahars begin as landslides.

Hydrothermal activity

Hydrothermal systems come from volcanic activity. Geothermal activity starts when ground water is heated. Most of New Zealand’s geoothermal activity occurs in the Taupō Volcanic Zone, an area that extends from White Island to Mt. Ruapehu.

Hydrothermal activity is usually confined to hot springs, geysers and warm ground. Eruptions can occur if this activity becomes unstable. Hydrothermal processes are often acidic and can alter the nature of rocks and soils, leading to the collapse of ground which can cause damage to property and harm people. Hydrogen sulphide gas can also be emitted and if there are high levels it can be harmful.

Landslides

A landslide is a movement of soil, rock and debris down a slope. Landslides often happen as a result of other natural disasters, such as earthquakes, volcanic eruptions and floods (rainfall).

Landslides are common throughout New Zealand. Serious landslides in built-up areas can cause thousands of dollars in damage and threaten lives.