Haig Fras
Encyclopedia
Haig Fras is a 45 km long submarine granitic
rocky outcrop in the southern part of the Celtic Sea
, lying about 95 km northwest of the Isles of Scilly
. It covers an area of 356.5 square kilometres (137.6 sq mi). At one point it reaches within 38 m of the sea surface. It is protected as a Special Area of Conservation
(SAC) because of the diverse fauna developed in its bedrock
reef
habitat. It was discovered in 1962 and named by Smith and others in 1965.
. The granite outcrop is surrounded by an area of Devono
-Carboniferous
sedimentary rock
s. Evidence from gravity data suggests that the extent of the intrusion is greater than the observed outcrop, based on its association with a linear negative gravity anomaly
. It is dated at 277 Ma, Early Permian in age, within the range of intrusion ages for the granites that make up the Cornubian batholith
. Although the two intrusions have the same trend and are of similar age, they are not thought to be related. In detail three separate outcrops have been identified using a combination of radiometric data
and seabed sampling. The granite is fine to medium-grained, unlike the megacrystic granite typical of the Cornubian batholith intrusions, although this may be due to the low level of sampling available. A foliated
granite is locally developed as a marginal facies.
The granite is thought to be cut by a swarm of basic dykes
inferred from magnetic and seismic reflection data
. This swarm is interpreted to be 56 km long and 2.5 km across and at least six separate intrusive bodies have been identified. The proposed intrusions are though most likely to be of Mesozoic
age.
) and Devonshire cup corals (Caryophyllia smithii
) are found at the top of the outcrop. Encrusting sponges, crinoids and Ross coral Pentapora fascialis
are found near base of the outcrop. Mobile fauna observed include urchins
such as Echinus esculentus, brittle star
s and the squat lobster
Munida
.
in August 2008. It was selected on the basis of being an Annex I habitat, 1170 (reefs), in the terms of the EU
Habitats Directive.
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
rocky outcrop in the southern part of the Celtic Sea
Celtic Sea
The Celtic Sea is the area of the Atlantic Ocean off the south coast of Ireland bounded to the east by Saint George's Channel; other limits include the Bristol Channel, the English Channel, and the Bay of Biscay, as well as adjacent portions of Wales, Cornwall, Devon, and Brittany...
, lying about 95 km northwest of the Isles of Scilly
Isles of Scilly
The Isles of Scilly form an archipelago off the southwestern tip of the Cornish peninsula of Great Britain. The islands have had a unitary authority council since 1890, and are separate from the Cornwall unitary authority, but some services are combined with Cornwall and the islands are still part...
. It covers an area of 356.5 square kilometres (137.6 sq mi). At one point it reaches within 38 m of the sea surface. It is protected as a Special Area of Conservation
Special Area of Conservation
A Special Area of Conservation is defined in the European Union's Habitats Directive , also known as the Directive on the Conservation of Natural Habitats and of Wild Fauna and Flora...
(SAC) because of the diverse fauna developed in its bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
habitat. It was discovered in 1962 and named by Smith and others in 1965.
Geology
The Haig Fras outcrop consists mainly of granite, forming a WSW-ENE trending elongate intrusionIntrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
. The granite outcrop is surrounded by an area of Devono
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
-Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s. Evidence from gravity data suggests that the extent of the intrusion is greater than the observed outcrop, based on its association with a linear negative gravity anomaly
Gravity anomaly
A gravity anomaly is the difference between the observed acceleration of Earth's gravity and a value predicted from a model.-Geodesy and geophysics:...
. It is dated at 277 Ma, Early Permian in age, within the range of intrusion ages for the granites that make up the Cornubian batholith
Cornubian batholith
The Cornubian batholith refers to the group of associated granite intrusions which underlie the south-western peninsula of Great Britain. The main exposed masses of the batholith are seen at Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land's End and the Isles of Scilly. It formed during the...
. Although the two intrusions have the same trend and are of similar age, they are not thought to be related. In detail three separate outcrops have been identified using a combination of radiometric data
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
and seabed sampling. The granite is fine to medium-grained, unlike the megacrystic granite typical of the Cornubian batholith intrusions, although this may be due to the low level of sampling available. A foliated
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
granite is locally developed as a marginal facies.
The granite is thought to be cut by a swarm of basic dykes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
inferred from magnetic and seismic reflection data
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
. This swarm is interpreted to be 56 km long and 2.5 km across and at least six separate intrusive bodies have been identified. The proposed intrusions are though most likely to be of Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
age.
Fauna
The bedrock reef at Haig Fras supports a range of fauna. Jewel anemones (Corynactis viridisCorynactis
Corynactis is a genus of colonial anthozoans similar in appearance to sea anemones and in body format to scleractinian stony corals. These animals are cnidarians in the family Corallimorphidae.-Species:Species so far described in this genus include:...
) and Devonshire cup corals (Caryophyllia smithii
Caryophylliidae
Caryophylliidae is a family of stony corals. In addition to the tropics they are also found in temperate seas and in very deep water.The coral colonies are often composed of only a few polyps, especially in Catalaphyllia jardinei, living on sandy land in partly cloudy water, associated with partner...
) are found at the top of the outcrop. Encrusting sponges, crinoids and Ross coral Pentapora fascialis
Bitectiporidae
The Bitectiporidae is a family within the bryozoan order Cheilostomata. Colonies are encrusting on shells and rocks or upright bilaminar branches or sheets. The zooids generally have at least one adventitious avicularia on their frontal wall near the orifice. The frontal wall is usually covered...
are found near base of the outcrop. Mobile fauna observed include urchins
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
such as Echinus esculentus, brittle star
Brittle star
Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to in length on the largest specimens...
s and the squat lobster
Squat lobster
Squat lobsters are decapod crustaceans of the families Galatheidae, Chirostylidae and Kiwaidae, including the common genera Galathea and Munida. They are not lobsters at all, but are more closely related to porcelain crabs, hermit crabs and then, more distantly, true crabs...
Munida
Munida
Munida is the largest genus of squat lobsters, with over 240 species.-Species:*Munida abelloi Macpherson, 1994 *Munida acacia Ahyong, 2007 *Munida acantha Macpherson, 1994 *Munida aequalis Ahyong & Poore, 2004 *Munida affinis A...
.
Special area of conservation
The area of the proposed SAC is 481 square kilometres (185.7 sq mi), which includes a margin of 330 metres (1,082.7 ft) around the mapped bedrock area, derived from the maximum water depth of 110 metres (360.9 ft) and an assumed length to depth ratio of a fishing warp of 3:1. The area was first proposed in 2004 and was included in the first tranche of candidate SACs submitted to the European CommissionEuropean Commission
The European Commission is the executive body of the European Union. The body is responsible for proposing legislation, implementing decisions, upholding the Union's treaties and the general day-to-day running of the Union....
in August 2008. It was selected on the basis of being an Annex I habitat, 1170 (reefs), in the terms of the EU
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
Habitats Directive.