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Isles of Scilly Geodiversity

Updated: Jul 8, 2023

The Isles of Scilly has a geological history dating back 290 million years and forms part of an interconnected chain of six deep magma rock intrusions known as plutons that stretch from the Isles of Scilly to Dartmoor as part of the Cornubian Batholith.

The Tresco channel between Tresco and Bryher on the Isles of Scilly
The Tresco channel between Tresco and Bryher on the Isles of Scilly

The Isles of Scilly are an idyllic protected landscape known for their stunning natural beauty and tranquillity provided by its remoteness, dominance of the sea, pristine beaches and its unique wildlife. Underlying this charming and idyllic landscape is its geodiversity that along with the marine environment, contributes to the unique landscape and fragile biodiversity of the islands.

The Isles of Scilly is an archipelago or island group located in the offshore waters of the Atlantic Ocean known as South West Approaches about 45 kilometres southwest of Land's End in Cornwall.

Land’ End looking out to the Atlantic and Longship’s Lighthouse
Land’ End looking out to the Atlantic and Longship’s Lighthouse

The Isles of Scilly archipelago covers an area of about 41 square kilometres and is composed of approximately 200 low-lying granite islands. With a resident population of 2,300 people only five of the islands are inhabited including St Mary's, Tresco, St Martin's, Bryher, and St Agnes.

The Archipelago of the Isles of Scilly, UK - Image by NASA ISS014-E-16597
The Archipelago of the Isles of Scilly, UK - Image by NASA ISS014-E-16597

The Isles of Scilly are a popular holiday destination and are connected to the mainland by the 19-seater Skybus a 20 minute flight between from Land’s End Airport to St. Mary’s Airport and Penzance harbour to Hugh Town from where the passenger ship RMV Scillonian III sails.

The RMV Scillonian RMV was built in 1977 at the Appledore boatyard in North Devon and is managed by the Isles of Scilly Steamship Company. The passenger ferry service operates daily sailings daily (some Sundays) from March to October to the Isles of Scilly and takes around 2 hours 45 minutes. Be advised, having taken this journey you will need to dose up on ginger to help prevent and treat motion sickness.

The RMV Scillonian III is one of only three ships including the oldest operating steam driven vessel in North America and Cunard’s flagship the RMS Queen Mary to still carry the status of Royal Mail Vessel (RMV).

The Scillonian lll docking at Hugh Town on St. Mary's in the Isles of Scilly
The Scillonian lll docking at Hugh Town on St. Mary's in the Isles of Scilly

The Isles of Scilly have a rich history dating back thousands of years. Archaeological remains suggest human habitation on the islands since the Stone Age, and various ancient structures, burial sites, and artifacts have also been discovered. The islands have a rich maritime history of shipwrecks and tales of smuggling and piracy.

The archipelago of the Isles of Scilly is also home to a unique and fragile ecosystem and ecologically important colonies of sea birds, rare plants, and marine life. The Isles of Scilly have been designated a Conservation Area, Area of Outstanding Natural Beauty, Heritage Coast, European Marine Site and has areas of significant importance including 27 Sites of Special Scientific Interest, 238 Scheduled Ancient Monuments, 130 Listed Buildings and RAMSAR sites for the protection of birds.

Overall, the Isles of Scilly are a captivating destination known for their scenic beauty, tranquillity, and rich natural heritage. Whether it's exploring the picturesque beaches, discovering the archaeological sites, or immersing oneself in the unique island life, the Isles of Scilly offer a charming and idyllic escape.

This back drop as a charming and idyllic is built upon one of the most complex geological regions in the South West of England.

The granite outcrops of the Isles of a Scilly have been shaped by a combination of the Cornubian batholith, Variscan Orogeny and the erosive powers of wind, sea, temperature change and a touch of the last ice age.

During the last Ice Age when the outer edge of the Irish Sea Ice Stream reached the the islands of Bryher, Tresco and St Martin’s. Glacial meltwater channels, moraines of flint from the sea bed and organic material locally known as ram formed the Pernagie bar and White Island bar of St Martins. Ice scoured rock surfaces can still be found on the islands today.

The Cornubian batholith is a large granite intrusion that covers much of the South West peninsular of England. Estimated to have a thickness of several kilometres the Cornubian batholith was a significant magmatic event associated with the tectonic processes and crustal evolution of the region that happened during the ancient mountain-building event known as the Variscan Orogeny.

Plutons are large intrusive igneous bodies of magma that form deep within the Earth's crust and move to the surface and cools and solidifies slowly underground. They are typically composed of coarse-grained rocks such as granite, granodiorite and diorite.

During the Variscan orogeny, the ancient supercontinent of Gondwana collided with the southern margin of the Laurussian continent, resulting in the formation of the supercontinent Pangaea. The collision produced intense compression and deformation of the Earth's crust, leading to the uplift and formation of the Variscan mountains that extends across western Europe for more than 3,000 Km.

The Variscan mountains stretch from the Isles of Scilly to Iberian Mountains in Spain and Portugal to Frances areas of Brittany, Massif Central, Vosges, and Corsica, and the Black Forest of Germany into the Czech Republic and Bohemian Massif.

The Cornubian batholith is composed mainly of granitic rocks, particularly biotite granite and muscovite granite. This granite is typically coarse-grained, consisting of quartz, feldspar and mica minerals. The Cornubian granite is known for its pink or grey colouration.

The Variscan orogeny also led to the development of mineralisation and mineral deposits in Cornwall, including tin, copper, and tungsten. These metals have historically been mined in the region and have contributed to Cornwall's mining heritage. It’s here, and in the geothermal waters washing through the subterranean rocks that lithium is also found.

The predominantly exposed coarse-grained granite on the Isles of Scilly is known as Scillonian granite and is responsible for the distinctive landscapes and rocky outcrops found on the islands.

The Isles of Scilly intrusion is comprised of two common types of granite. Firstly, a coarse-grained granite with porphyritic crystals of feldspar which is dominant across the islands known as Outer Granite. The second type is a finer grained and non-porphyritic intrusion more restricted outcrop in the north and west of St. Mary's and the south part of Tresco and Bryher known as the Inner granite.

White sandy beaches of the Isles of Scilly
White sandy beaches of the Isles of Scilly

The erosion of this granite over time years has been the source of the quartz and feldspar grains that have created the white sandy beaches of the Isles of Scilly.


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