Plate Tectonics

Plate Tectonics

​The Earth's crust and upper mantle is broken into many plates called tectonic plates that are like pieces of a jigsaw puzzle. There are seven major plates that make up 94% of the Earth's surface and many smaller plates making up the other 6%.

Map showing the distribution of plate tectonics, source USGSSource: USGS, http://pubs.usgs.gov/publications/text/slabs.html, Public domain, via Wikimedia Commons

The tectonic plates are in motion and it is thought that they have been in motion since early in earth's history.  The word tectonic refers to the structure of the earth and the processes happening on it. Ireland has a long and interesting tectonic history and therefore, we have a great diversity of rock-types in a relatively small area.  

The areas where these plates meet are known as plate boundaries. There are three types of plate boundary:

 

Divergent or constructive plate boundaries

The plates divert and this causes the construction of new rock.

It happens when two tectonic plates pull apart and rock from the mantle rises up through the opening to form new surface rock when it cools.  It happens at the start of a new ocean and continues at the mid-ocean ridge while the ocean is opening. It is associated with rifting (large-scale faulting) and volcanoes.

Source: domdomegg (Own work) [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

 

An example of this is Iceland, which is formed on the mid-Atlantic ridge.  

The Paleogene rocks in Ireland, including at Cooley, Co. Louth and at the Giant's Causeway in Co. Antrim, formed at the opening of the North Atlantic approximately 60 million years ago.

See volcanoes for more on the effects of divergent plate boundaries (redirect to Topics section).
 

Convergent or destructive plate boundaries

The plates converge and rocks are destroyed. 

This is when two tectonic plates move toward each other and collide. What happens is dependant on the type of plates involved.  It is possible to have the collision of two oceanic plates, one oceanic plate (thin and dense) and one continental plate (less dense and thick) or two continental plates.  

Subduction occurs when there is a difference in the plates and the heavier of the two plates is forced downwards into the hot mantle where it melts and the lighter plate is forced upward and buckles due to the friction and mountains are formed. This occurred​ in Ireland with the closure of the Iapetus Ocean over 450 million years ago.  The Caledonian mountains formed at this time and it is thought they were as high as the modern Himalayas when they formed. 

Diagram of convergent plate boundariesSource: domdomegg (Own work) [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

 

The Himalayas are an example of the collision of two continental plates where the Indian plate is crashing into the Eurasian plate and is being forced upwards. They are continually growing at an average rate of 1cm per year, this will be 10km in 1 million years!

See earthquakes, tsunami, and volcanoes for more on the effects of convergent plate boundaries (redirect to Topics section).

 

Passive plate boundaries

Also known as strike-slip boundaries.

This is when two plates slide past each other. When the plates move, the jagged edges of the plate boundaries snag and catch each other and can get jammed. This causes a build-up of pressure. When the plates eventually pass each other, the pressure is released in the form of an earthquake.  The closest passive plate boundary to Ireland is the boundary between the African and Eurasian plate south of Portugal.

Diagram of passive plate boundariesSource: domdomegg (Own work) [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The movement of the two plates can be in opposite directions or in the same direction but at different speeds for example the San Andreas Fault in California.

 

​​​​​ResourcesLink
Irish examples
Teachers resources

Earth Science Ireland - Violent Earth​

More information
Related topic/s

Natural Hazards: earthquakes, volcanoes, tsunami

Related programme/s

Geohazards: Tsunami warning system

Related publicationsUnderstanding Earth Processes, Rocks and the Geological History of Ireland
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