Plate Tectonics - Geological Survey Ireland

​​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%.

Source: 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:

DivergentorconstructiveplateboundariesThe plates diverge 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: Wikimedia Commons

An example of this is 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). 

ConvergentordestructiveplateboundariesThis is when two tectonic plates move toward each other and collide. The result depends on the type of plates involved.  It is possible to have the collision of two oceanic plates, an oceanic plate and a continental plate or two continental plates.  

Subduction occurs when there is a difference in the density of the plates. Oceanic crust is typically denser than continental crust and is forced downwards into the hot mantle when it collides with continental crust.  The less dense conitinental crust is forced upward. 

This occurred​ in Ireland with the closure of the Iapetus Ocean over 450 million years ago. At the start the Iapetus oceanic crust was subducted beneath continental crust, but eventually the oceanic crust was gone and continents collided.  The Caledonian mountains formed at this time and it is thought they were as high as the modern Himalayas when they formed. 

Source: 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).

Passiveplateboundaries

Also known as strike-slip or transform ​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.

Source: Wikimedia CommonsThe 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.