Wednesday, 11 May 2016

An island apart

by Lindsey Atkinson
Having recently had the good fortune to visit Barbados and some of the Windward Islands in the Lesser Antilles I was struck by the difference between Barbados and the other islands and curious to find out why.  Now you’ll have to excuse me as I am not a geologist so I am wandering into new territory with this blog...

The Windward Islands are the more southerly islands of the Lesser Antilles, including  Martinique, St Lucia, St Vincent and the Grenadines, and Grenada.  They lie near the eastern edge of the Caribbean tectonic plate and are part of the Lesser Antilles volcanic arc.  Being largely volcanic in origin the larger islands are mountainous with a rich volcanic soil and they still have active volcanoes.  Seismic activity in the area is monitored by the University of the West Indies Seismic Research Centre.  
Sulphur springs, La Soufrière, St Lucia

Sulphur springs in La Soufrière, give away St Lucia’s volcanic origins while its namesake, La Soufrière (1234m) on St Vincent, last erupted in 1979 replacing the lake that used to lie in the crater with a lava dome.

Inside the crater, La Soufrière, St Vincent
Barbados stands apart from the other islands being the most easterly of the Caribbean islands, 160km east of St Lucia.  It also differs in being a relatively low lying island, with the highest point at Mount Hillaby (340m), and it differs in origin from its nearest neighbours.

Former coral colonies, Little Bay

Sedimentary layers, Little Bay
Unlike the Windward Islands, Barbados was not formed by volcanic action and it lies at the very edge of the Caribbean tectonic plate.  As the South American plate was subducted under the Caribbean plate sediment was scraped off the South American plate, including deposits of pelagic organisms, forming an accretionary prism.  These layers were subsequently covered by a coral cap.  Both former coral colonies and sedimentary layers  can be seen exposed on the east coast, as here at Little Bay (left).
The movement of the plates resulted in uplifting of these deposits until eventually the island was exposed above sea level.  This happened in stages resulting in ridges which are visible across the island.

Harrison Caves
Little Bay

The island is therefore predominantly limestone, with little surface water as the water filters through the rock.  Beneath the surface are caves such as Harrison Caves with stalactites and stalagmites while on the surface there are dry gullies.  Some of these gullies may have formed when limestone cracked during uplifting or, as in the case of Welchman Hall Gully, where a cave roof has collapsed.  

Erosion has also done its work as the pounding Atlantic waves on the east coast have resulted in the dramatic cliffs of Little Bay and the limestone ‘mushroom’ rocks  of Cattlewash Beach. 
'Mushroom' rock on Cattlewash Beach

And of course erosion of the coral rocks has created the beautiful sandy beaches so beloved of tourists!
Crane Beach, South Coast

Barbados National Trust 

Donovan SK and Harper DAT (2005) The geology of Barbados: a field guide.
Caribbean Journal of Earth Science 38: 21-33.
Radtke U and Schellmann G (2006) Uplift History along the Clermont Nose Traverse on the West Coast of Barbados during the Last 500,000 Years - Implications for Paleo-Sea Level Reconstructions. Journal of Coastal Research 22: 350-356

Saunders et al. (1984) Stratigraphy of the Late Middle Eocene to Early Oligocene in the Bath Cliff Section, Barbados, West Indies.  Micropaleontology 30: 390-425

The Soufrière Foundation
University of the West Indies Seismic Research Centre