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Volcanic activity
What is a volcano?
A volcano is a landform made from magma erupting at the surface. Magma is molten rock within the earth. When magma erupts out of a volcano it is called lava.
The size and shape of a volcano depends on:
- how often it erupts
- the size and type of eruptions
- the type of magma
What causes volcanoes?
Volcanoes would not exist if the Earth’s surface was solid and did not move.
The Earth has five layers: the inner and outer core, the lower and upper mantle, and the crust. The deeper you go below the Earth’s crust, the hotter it gets.
The high temperatures (900°C) and high pressures in the mantle layer of the Earth are enough to melt rock. The high pressure changes the rock into a thick semisolid called magma. The hot molten rock in the mantle does not normally make it through the many kilometres of crust that forms the ground that we walk on. Only in certain areas where there are cracks or breaks in the crust, such as at plate boundaries, can the molten mantle start to creep through.
Plate tectonics
On the Earth today there are six 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 there are three types of movement:
- Spreading boundary (divergent) - A spreading boundary is 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 (eg the Atlantic Ocean).
- 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 felt as an earthquake.
- 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 have the same weight so cannot over-ride each other and the land buckles and folds, forming mountain ranges.
- If the plates are both oceanic plates then island arcs or basins can form.
- If an oceanic plate hits a continental plate the heavier oceanic plate will sink under the continental plate.
Plate Tectonics in Aotearoa
Aotearoa New Zealand sits on the edge of two tectonic plates, the Indo-Australian and the Pacific plates. This makes Aotearoa geologically active with many earthquakes, geothermal areas and volcanoes.
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