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Plate tectonics

We can blame plate tectonics for many catastrophes over time – earthquakes, volcanoes, geothermal activity, tsunamis and landslides – while people living on mountain ranges or small islands can thank the work of plate tectonics for providing them with somewhere to live. So what are tectonic plates and how do they cause all this change?

Before the 1960s, it was thought that continents were set in the same position forever. Realising the plates are dynamic led to new insights into our understanding of the planet and revolutionised and unified the Earth sciences – from the study of fossils (paleontology) to the study of earthquakes (seismology).

How do tectonic plates form and move?

The lithosphere (which includes the crust) is the outermost layer of the Earth and is cooler and more rigid than the layer below. The asthenosphere underneath is hot and flexible, transferring some of its heat through convection. In this case, the molten rock (magma) moves towards the cooler section, welling up through 100 km deep cracks in the lithosphere. When the two sides of the crack move apart, the cooled rock forms new tectonic plate material.

The plates ride on the asthenosphere, sometimes moving apart, sometimes past each other and other times moving together at plate boundaries. By using the global positioning system (GPS) over a period of years, the plates have been measured as moving from a few millimetres to up to 20 cm per year. Over many millions of years, this can mean thousands of kilometres of movement.

Nature of Science

In principle, all science knowledge is subject to change. Scientists have explained geology in many ways over time – from Aristotle's explanation in the 4th century BC that earthquakes derived from wind in subterranean caverns, to the early 20th century hypothesis of continental drift, which led to the development of the tectonic plates theory.

Where are the tectonic plates?

The major plates are the Antarctic, African, Eurasian, Australian, Pacific, North American, South American and Nazca. There are also a number of minor plates, such as the Indian, Arabian, Caribbean and Philippine. Some plates (such as the Pacific plate) have only oceanic crust, which is thinner and generally lies below sea level – others have both oceanic and continental crust that projects above sea level.

These plate boundaries are associated with geological events such as earthquakes and the creation of mountains, volcanoes and oceanic trenches. Most of the world’s active volcanoes occur along plate boundaries – the Pacific plate’s Ring of Fire is the most active.

New Zealand is right on the edges of two tectonic plates – the Australian and Pacific – and we get plenty of the action that goes with it!

Three types of plate boundaries

The plate boundaries are characterised by the way the plates move, and they are associated with different types of surface phenomena.

Transform boundaries – The plates move past each other, but because of friction, they cannot just glide past each other so build up stress, which is released as an earthquake. New Zealand’s Alpine Fault is an example of this type.
Divergent boundaries – The plates slide apart  each other, and the space that this creates is filled with new crust from the magma formed below in the asthenosphere. These boundaries can create great fracture zones that are a major source of submarine earthquakes.
Convergent boundaries – The plate slide towards each other. When one plate moves underneath the other, it is known as a subduction zone, associated with deep marine trenches and volcanoes – some of the most explosive on Earth.

Where two continental plates collide they either buckle and compress, or one plate will go under or override the other, creating extensive mountain ranges. When two plates with oceanic crust collide, they usually create an island as one plate is descending (subducting) beneath the other.

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