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What Causes Geohazards?

We would not have geohazards if the Earth's crust was solid and did not move. The theory of continental drift explains how the land has moved over time due to plate tectonics.

Continental drift

In 1912 the German scientist Alfred Wegener came up with a theory of continental drift. The theory explains how the Earth's land masses moved to where they are now. The theory was based on the Earth’s continents once fitting together like a giant jigsaw puzzle.

Continental drift explains how similar animals and plants lived in countries now widely separated by ocean, and how mountain ranges in different continents were once joined.

250 million years ago the Earth's continents were joined together in one giant super-continent called Pangaea. Slowly, over millions of years, Pangaea broke in two and drifted apart forming the smaller continents of today.

Plate tectonics

Our understanding of how the continents move has improved. The theory of continental drift has now been replaced with the term ‘plate tectonics’.

On the Earth today there are seven large plates and many smaller ones. The plates move in slow motion and are always changing shape. It is thought that convection currents in the mantle of the Earth form the energy to move the tectonic plates from a few millimetres to a maximum of about 15cm per year.

Plate boundaries

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

Types of Plate Boundary. Image: GNS Science.

Spreading boundary (divergent)

A spreading boundary, where two plates move apart so magma, or molten rock, can rise from inside the Earth to fill in the gap. The two plates move away from each other. This can form rift valleys on land or ocean ridges on the seafloor (for example the Atlantic Ocean).

Colliding boundary (convergent)

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

If the plates are continental plates they are the same weight so the plates cannot over-ride each other and the land buckles and folds, creating 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)

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. This makes New Zealand geologically active with many earthquakes, geothermal areas and volcanoes. This plate boundary has shaped New Zealand:

The movement of these two tectonic plates forms the New Zealand landscape that we know and love.

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

Convection currents caused by heating in the Earth's mantle explain how the continents move. Who came up with this theory and what is this theory called? Image: Public Domain.

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

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The two plates push past each other along the Alpine Fault, moving about 38mm per year. What land feature can be seen alongside the Alpine Fault? Image: GNS Science. 

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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.

Complete the 'Pangaea to the Present' activity on continental drift.