Geography

Sea of the Hebrides

May 13, 2013, 10:37 pm
Content Cover Image

Rugged basaltic north tip of Staffa, Sea of the Hebrides. @ C.Michael Hogan

The Sea of the Hebrides is an element of the North Atlantic Ocean, located off the western coast of Scotland, separating the Scottish mainland and the northern Inner Hebrides islands (to the east) from the southern Outer Hebrides islands (to the west).

To the north, the Sea of the Hebrides joins The Minch, an expansive saline strait.

caption Regional setting of the Sea of the Hebrides.

Basin Geology

caption Sea of the Hebrides coastline on Mull. @ C.Michael Hogan

The basement of the Sea of the Hebrides has a magnetic basement of the Lewisian gneiss formation, which is exposed to a shallow level beneath the Sea of the Hebrides, Skerryvore Bank and Inner Hebrides Trough, according to N.H.Trewin. Overlying the Lewisian is the Proterozoic Torridonian formation, which is about six kilometres in thickness under the Sea of the Hebrides. The Camasunary Fault separates the present Sea of the Hebrides sub-basin from the Inner Hebrides sub-basin.

In the troughs and basins to the west of Scotland, Permian and Triassic deposition patterns are characterised by sandstones and coglomerates being laid down on alluvial fans which fed braided streams; this deposition involved northward in or marginal movement in both the Sea of the Hebrides as well as the Minch.  

Presence of the Portree shale formation beneath the seafloor of the Sea of the Hebrides is encouraging the exploration of petroleum in an ongoing manner.

Bathymetry and Hydrology

The Sea of the Hebrides exhibits a bathymetry having a broad asymmetric channel aligning roughly north northeast to south-southwest; moreover, this channel descends to a maximum depth of about 240 metres at its western part. Furthermore, the channel can be viewed as partitioned into several banks that have significantly deep local depressions, which banks seem to be due to resistant tertiary bedrock. The channel's western edge is a well-defined scarp following the MInch Fault.

Fisheries

Prominent pelagic fish species in the Sea of the Hebrides include Herring,  Atlantic cod (Gadus morhua),  and Sprat (Sprattus sprattus). Fisheries are the chief industry to support the economy of the outer Hebridean Isles and an important factor for the Inner Hebrides population. Long line fishing is common with local fishermen. Large scale commercial fishing is frequently seen by Norwegian and other vessels.

Trawlers are not typically found in fleets anywhere off the west coast of Scotland, but may be found singly or in a group of up to three vessels. The western sides of Islay or Skye are the favourite locales for trawlers in this region.

Ecology

caption Seal and birdwatching, Islay, Inner Hebrides. @ C.Michael Hogan

The Sea of the Hebrides is home to a variety of pelagic birds, cetaceans and pinnipeds, as well as the fish and invertebrates that dwell in the water column. The Grey seal is one of pinnipeds seen hauling out on land, including small skerries; some of the insights on marine biodiversity derive from stomach contents analysis from captured Grey seals. These analyses reveal gadid populations in the Sea of the Hebrides to be rich in Common ling (molva molva) , Atlantic cod (Gadus morhua) and European Whiting (Merlangius merlangus); correspondingly the chief flatfish in the Grey seal diet are European plaice (Pleuronectes platessa) and Megrim (Lepidorhombus whiffiagonis). The Megrim is typically found at depths exceeding 100 metres. Atlantic herring (Clupea harengus) and Atlantic horse mackerel (Trachurus trachurus) are two pelagic schooling fish found in the natural food chain of the Sea of the Hebrides. Sandeels are found in considerable numbers in the Sea of the Hebrides due to the preferred seabed configuration of considerable smooth areas of gravelly sand. Corrspondingly the Atlantic puffin (Fratercula arctica) feeds on the sandeels and breeds on coastlines of the Sea of the Hebrides, at such places at the cliffs of the north side of Chaffa.

The chief terrestrial ecoregion here is the Celtic broadleaf forests. Notable trees in the basin include Sessile oak (Quercus petraea), Pedunculate oak (Q.robur), Downy Birch (Betula pubescens), Downy Willow (Salix lanata),  Alder (Alnus glutinosa) and Rowan (Sorbus aucuparia). Although there is extensive historic deforestation in much of the terrestrial rim, there are locations of well preserved ancient forest, notably the pygmy forest at the northern side of the Ross of Mull, where a mixed pygmy oak forest thrives on the steep rocky slopes with highly mineralized soils.

Prehistory through Early Christian Era

caption Ruins of early Christian abbey, Iona. @ C. Michael Hogan Human settlement on the Inner Hebridean Isle of Mull and other Hebridean Islands dates to at least as early as 6000 BC. These original settlers arrived from mainland Scotland and Ireland and built Bronze Age menhirs, burial cairns, cists, standing stones, stone circles; archaeological finds of pottery and knife blades give additional evidence of these Neolithic and Bronze Age peoples. Between 600 BC to 400 AD Iron Age inhabitants constructed forts, duns and crannogs.

As far as the Hebridean Isles, the Early Christian period began on Mull and Iona in the 6th Century AD, with 563 AD being a key date, since that date signified the influx of Christianity to Britain by Saint Columba, who arrived from Ireland to set up a monastery on the Island of Iona, off the southwest point of Mull.

Jurisdiction

Due to the marine geometry involved, the Sea of the Hebrides is generally considered saline sea waters under the jurisdiction of the United Kingdom under terms of the United Nations Convention on the Law of the Sea (Anderson. 2008), which was drafted in 1982 and went into effect in 1994; however, the United Kingdom has acknowledged that foreign ships may have the Right of Innocent Passage, except for submerged submarines. This route is frequently used for small vessels in route from Glasgow and Dublin ports to the Baltic Sea.

References

  • David Anderson. 2008. Modern law of the sea: selected essays. Martinus Nijhoff Publishers. 623 pages
  • John Morton Boyd, Ian L. Boyd. 1990. The Hebrides: a natural history. Collins. 416 pages
  • Griffes. North Atlantic, Baltic Sea, North Sea and Mediterranean Sea. ProStar Publications
  • J.S.Hammond, A.J.Hall and J.H.Prime. 1994. The diet of Grey seals in the Inner and Outer Hebrides. Journal of Applied Ecology. 31:4, pp. 737-746
  • Jack Hardisty. 2009. The Analysis of Tidal Stream Power (Google eBook) John Wiley and Sons. 342 pages
  • P.D.W.Haughton. 2001. The petroleum exploration of Ireland's offshore basins. Geological Society of London. 473 pages
  • A.D.Stewart. 2002. The later Proterozoic Torridonian rocks of Scotland: their sedimentology, geochemistry and origin. Geological Society. 130 pages
  • N.H.Trewin. 2002. The geology of Scotland. Geological Society. 576 pages 

See Also

Glossary

Citation

Hogan, C. (2013). Sea of the Hebrides. Retrieved from http://www.eoearth.org/view/article/51cbeedb7896bb431f69ab6c

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