Tectonic Plates

The theory of plate tectonics says that Earth’s outer layer is made up of large, moving pieces called plates. All of Earth’s land and water sit on these plates. The plates are made of solid rock. Under the plates is a weaker layer of partially melted rock. The plates are constantly moving over this weaker layer.

As the plates move, they interact at their boundaries in different ways. At one type of boundary the plates slide alongside each other. At another type, the plates crash into each other. In this case the edge of one plate may slide under another plate and be destroyed. Or the two edges of the plates may rise up and form mountains. At the third type of boundary the plates move apart from each other. When they do, the melted rock beneath the plates rises up. This melted rock, or magma, cools as it rises and forms new crust.

Earthquakes and volcanoes often happen along plate boundaries. There are so many earthquakes and volcanoes at the edges of the Pacific Plate that this region is called the Ring of Fire.

Continental Drift

The plates have moved across Earth’s surface for hundreds of millions of years. As the plates move, the continents on them move, too. This movement is called continental drift.

Scientists think that it takes about 500 million years for all the continents to join together into one big continent—or supercontinent—and then break apart again. A number of supercontinents have formed and broken apart on Earth over time. The most recent supercontinent, called Pangea (or Pangaea), had formed by about 270 million years ago. Pangea lay across a plate boundary. Slowly, Pangea broke apart at this boundary. Rising magma filled in the space, which eventually became the floor of the Atlantic Ocean.

One piece of Pangea included parts of what are now North America, Europe, and Asia. This piece, called Laurasia, drifted north. The other piece of Pangea included parts of what are now Antarctica, Africa, South America, and Australia. This part, called Gondwanaland, drifted south. Eventually Laurasia and Gondwanaland broke apart, too. The continents then slowly moved to the places they are now.

The continents continue to move today. Scientists believe that in about 250 million years they will join together again.

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:

1. Divergent or constructive plate boundaries

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.

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.

2.Convergent or destructive plate boundaries

This 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 continental 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.

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.

3.Passive plate boundaries (Conservative 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.

The Composition of the Earth