Introduction

Dominica is built on a broad submarine ridge of (probably) Early Tertiary age (that is about 60 million years ago) or perhaps even older deposits. It is one of the Volcanic Caribees, the more western line of a double island-arc, the other line of which is mainly built of limestone. Dominica is almost entirely composed of volcanic rocks.

GEOLOGICAL HISTORY

The earliest rocks visible on Dominica are found along the east coast (Pagua-Rosalie and some remnants further north and south.) These are lavas and breccias of late Miocene and dykes of Pliocene age (up to 7 million years old). The lavas may appear as red, deeply weathered materials and they are often partly covered by much younger ash deposits.

In early Pliocene time (perhaps 2-3 million years ago), coarse sedimentary rocks comprised of volcanic materials and some limestones were laid down (and are exposed in west coast cliffs). They form the basal part of Morne Diablotins. Foundland and the Cochrane area, which form the basal part of Morne Trois Pitons are older, possibly late Miocene.

In middle Pliocene time (about 2 million years ago) basaltic-andesitic shield volcanoes were built in the south of the island and west of the earlier lavas. Some of the deposits are pillow lavas deposited under the sea, and examples can be seen near here, (Springfield), by the public road at Antrim.

But the most important mountain building period was in the Pleistocene, when a line of dacitic-andesitic composite volcanoes was superimposed over the less lofty earlier shield volcanoes: Morne au Diable (Pelean), Morne Diablotins, Morne Watt, Morne Anglais, Morne Plat-Pays, are all about 400,000-500,000 years old.

Still later, only 30,000 years ago, a very large eruption, the Roseau ignimbrite eruption took place from near where Trois Pitons stands. Ash and pumice flows filled several valleys and the deposits extended several hundred km from Dominica. The whole deposit is equal in volume to about 60 cubic kilometres of solid rock.

After this, large dome complexes, of which Morne Trois Pitons is the most spectacular example, were formed , usually in the crater of a large volcano, the latest being that of Morne Patates,the most recent dated deposits of which are about 450 years old (Holocene).

THE HISTORIC PAST AND THE PRESENT

Activity during the historic period has been mainly of the form of phreatic and phreato-magmatic explosions, (phreato meaning that water draining from the surface was involved), the last having been located in the Valley of Desolation in 1880, often leaving maare (lakes like that at Woodford Hill) or craters (Du Mas Estate, top end of Morne Prosper e.g)

Present activity:

• soufrieres (Soufriere, Roseau Valley, Morne aux Diables, Valley of Desolation)
• hot springs (Check Hall, Layou,Titou, etc)
• geysirs (Wotten Waven)
• Boiling Lake
• seismic activity (mainly in the south, especially La Plaine area
  

THE ROCKS

Main rock types:

• basaltic lavas (Antrim, eg)
• andesitic tuffs, (some welded) and ashes, (some dacitic) and agglomerates (widespread, but dacitic materials which include little quartz crystals of diamond shape are confined to the NE, eg Calibishie)
  
• dacitic andesitic lavas (Micotrin, Trois Pitons) [also microdiorite, (Imray's)]
  
• reef limestones and conglomerates (west coast)

USES

Economic geology:

• geothermal power (potential high, twice that of St Lucia) untapped
• aggregate, including lightweight (pumice) some exported
• minor accumulations of minerals (low grade sources)
• exploration has found copper
• minor diatomaceous earth, impure clays, low grade thin layer alumina and limestone
• "sponge" which holds and guides water to our very productive rivers
  

TECTONICS

"Plate tectonics" -- for those interested in modern interpretations of the geology:

The Caribbean 'plate' is overriding the Atlantic 'plate', which is being 'subducted' down and providing materials which become part of the magma which fuels the Lesser Antillean volcanoes.

The locus of the edge of the Caribbean 'plate' is perhaps 50 miles east of Dominica, and the crumbling edges of both plates are responsible for the seismic activity so common off east Dominica and in Southern Dominica .