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Tectonics in the British Isles Find out more

British Isles past volcanoes

 There are no active volcanoes left in the British Isles however there are extinct volcanoes, such as Arthur’s seat in Edinburgh. Just like the castle rock on which Edinburgh Castle is built, it was formed by an extinct volcano system of Carboniferous age (approximately 350 million years old), which was then eroded by a glacier moving from west to east during the Quaternary (approximately the last two million years), exposing rocky crags to the west and leaving a tail of material swept to the east. This is how the Salisbury Crags formed and became basalt cliffs between Arthur's Seat and the city centre. 

There are many examples of INTRUSIVE igneous features in the British Isles too.  Batholiths are huge scale features where magma is intruded into the ground and cool very slowly, giving coarse grained rocks with large crystals.  The outer edge of the magma is in contact with existing rocks, and the heat and pressure changes the rock through metamorphosis creating a metamorphic aureole.  If magma is squeezed HORIZONTALLY along bedding planes through layers of existing rock a layer of rock can cool which is known as a Sill.  If magma is squeezed VERTICALLY upwards cutting across bedding planes of sedimentary rocks a Dyke is created.  The Isle or Arran in Western Scotland is essentially an exposed Batholith, which has dykes and sills, including the Drumadoon Sill.

 

British Isles Earthquakes

The British Isles also gets numerous small magnitude earthquakes per year, despite being quite far from the nearest plate boundary, the Mid Atlantic Ridge (over 1600km away at its nearest point to Cornwall, measured on Scribblemaps).  Although infrequent and of low magnitude, they can still cause damage.  On average, the UK may expect: 

  • an earthquake of 3.7 ML or larger every 1 year 
  • an earthquake of 4.7 ML or larger every 10 years 
  • an earthquake of 5.6 ML or larger every 100 years

 

According to the British Geological Survey;

Where do British Earthquakes occur?

Most earthquakes occur on the western side of the British mainland. Earthquakes are almost completely absent from eastern Scotland and north east England. Similarly, Ireland is almost completely free of earthquakes. 

What has been the biggest?

The largest known British earthquake occurred near the Dogger Bank in 1931, with a magnitude of 6.1. The most damaging UK earthquake was in the Colchester area in 1884. Some 1200 buildings needed repairs, chimneys collapsed and walls were cracked. 

Why?

Although Britain is far from any plate boundaries we are still being squeezed by motion of the Earth's tectonic plates. Northern Britain is also still being uplifted due to the melting of the ice sheets that covered many parts of Britain thousands of years ago. This deformation results in occasional earthquakes. 


The Market Rasen Earthquake
The most recent large earthquake was the
2008 Lincolnshire earthquake which struck Lincolnshire.  It happened on the 27th Feb 2008 at 1am. It was a 5.2 magnitude earthquake on the Richter scale with the epicentre 2.5 miles (4 km) north of Market Rasen and 15 miles (24 km) south-west of Grimsby.

The earthquake was caused by the sudden rupture and motion along a strike-slip fault, 18.6 kilometres (12 mi) beneath Lincolnshire. The actual motion of the earthquake happened over a period of 2 minutes but it was most intense and was felt at the surface for just 10 to 30 seconds. The maximum vertical ground motion at the epicentre of the earthquake was only ~1 mm.   

 

The earthquakes of Great Britain are said to be intraplate (within the plate not at the edge) earthquakes, meaning they are not close to tectonic plate boundaries that cause most of the world’s earthquakes.  These intraplate British Isles earthquakes are produced by distant tectonic stresses – a combination of E–W North Atlantic Ridge and N–S African Plate regional stress fields, and local mantle conditions.  This means that the British Isles are in effect being squeezed by movement of the Mid-Atlantic Ridge to the West and North West, and Alpine mountain building where the African Plate is being driven under the Eurasian plate to the South.

 

The earthquake had some minor impacts, and little damage was sustained.  Many people contacted the police to report the earthquake.  Building damage was minor, but large buildings such as apartment blocks were reported to have shaken for up to 30 seconds afterwards. There were no deaths, but a 19 year old man in Barnsley suffered a broken pelvis when a piece of chimney fell through the roof onto his attic bed. The St Mary Magdalene church at Waltham on the Wolds had its spire damaged and was to be rebuilt at a cost of £100,000. (source for facts)