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What Causes Geohazards?

If the Earth's crust was solid and did not move we would not have geohazards. Plate tectonics explains how the surface of the Earth moves.

Earth’s structure

The Earth is made up of several main layers.

Inner core - Solid, very hot and thought to be made of iron.
Outer core - Surrounds the inner core. Made up of only slightly cooler liquid metal.
Mantle - The thickest and still very hot layer.
Crust - The outermost layer - a thin, cool, and brittle shell.

Plate tectonics

Plate tectonics explains how the Earth's surface moves. Continents are sitting on huge chunks of the Earth’s crust called tectonic plates. These tectonic plates are always moving.

Convection currents

Heat from the core causes super-hot convection currents that move around in the Earth’s mantle. Convection currents are the main reason for tectonic plate movement (sometimes called continental drift).

The crust is like a lid on a jar, keeping the mantle under the Earth’s surface.

Plate boundaries

At the edge of the tectonic plates one of three things can happen:

Spreading boundary (divergent)

This is where two plates move apart from each other. Magma (molten rock) will then rise from inside the Earth to fill in the gap. This can form rift valleys on land or ocean ridges on the seafloor.

Colliding boundary (Convergent)

Where two plates push together. Different things will happen depending on what type of plates are pushing together

If the plates are continental plates they are the same weight so the plates cannot over-ride each other. The land buckles and folds, making mountain ranges.
If the plates are both oceanic plates then island arcs or basins can form.
If an oceanic plate collides with a continental plate the heavier oceanic plate will sink (subduct) under the continental plate.

Sliding boundary (Transform)

This is when two plates slide against each other. But rather than sliding smoothly, the plates build up stress then release this with a burst of movement. This movement is felt as an earthquake.

Plate Tectonics in New Zealand

New Zealand sits on the edge of two tectonic plates, the Indo-Australian and the Pacific plates. The movement of these two tectonic plates creates earthquakes, geothermal areas and volcanoes, and has shaped the New Zealand we know today.

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Earth is made up of several main layers. Image: LEARNZ.

Earth is made up of several main layers. Image: LEARNZ.

Continents are sitting on huge chunks of the Earth’s crust called tectonic plates. Image: GNS Science.

Continents are sitting on huge chunks of the Earth’s crust called tectonic plates. Image: GNS Science.

Convection currents caused by heating in the Earth's mantle explain how the continents move. Image: Public Domain.

Convection currents caused by heating in the Earth's mantle explain how the continents move. Image: Public Domain.

New Zealand sits on the edge of two tectonic plates; the Indo Australian Plate and the Pacific Plate. How do you think this affects New Zealand? Image: LEARNZ.

New Zealand sits on the edge of two tectonic plates; the Indo Australian Plate and the Pacific Plate. How do you think this affects New Zealand? Image: LEARNZ.

The Indo Australian Plate is like a bulldozer pushing at the weaker Pacific Plate causing it to rise up into the mountain range known as the Southern Alps. What else does this plate movement cause? Image: LEARNZ.

The Indo Australian Plate is like a bulldozer pushing at the weaker Pacific Plate causing it to rise up into the mountain range known as the Southern Alps. What else does this plate movement cause? Image: LEARNZ.

Watch the GNS Science animation (519k), showing the future shape of New Zealand if the current plate movement continues.