The arc of the Windward Islands sits on a ring of fire of convergent plate margins between the subducting Caribbean plate and the Atlantic floor. Grenada emerged from a shallow sea as a submarine volcano about 38 million years ago.
The subsea sediments deposited were composed of sand, silt and calcareous mud. In between these sediments, are layers of volcanic minerals and deposits of volcanic origin – turbidite, typically highly stratified volcanic deposits found in ocean troughs.
Turbidite are deposited when underwater avalanches, sliding down the slopes of the continental shelf edge, come to rest with the sand and coarser material settling first, followed by mud and finally particulate matter. Around 25 million years ago, thanks to the subduction of Atlantic Ocean floor beneath the Caribbean Plate, these sedimentary turbidite rocks were faulted, folded and uplifted, as seen north of Levera Beach.
Grenada has two distinct geological regions: the interior and the coastal belt. The rugged, mountainous interior is dominated by peaks of 2700 feet, with steep ridges and deep, narrow valleys with fast-flowing streams in valley bottoms. While the relatively low-lying coastal periphery is much more subdued due to the volcanic centre’s eroded material being re-deposited on the coastal area, including fluvial and lehar mudslide volcanic deposits from the centre ‘smoothing over’ like Polyfilla the existing relief of the lowland areas.
The volcanic vents, like mole hills, have shifted many times over millions of years, resulting in the juxtaposition of rocks of different age, type, and volcanic origin. It has created a jumbled topography with several major peaks, each having numerous ridges radiating from them. The oldest volcanic rocks are the northern domes of andesite, a basalt-like fine-grained, extrusive, igneous rock (light to dark grey) of Mt. Alexander, Mt. Rodney, Mt. William which are about 21 million years old.
The ridge tops, composed mostly of andesite and basalt lavas, have surprisingly retained their narrow tops and steep sides possibly due to the impermeable layer of clay soils, as water is the principal erosive agent of the rock.
The andesite domes of Levera Hill and Levera Island represent more recent volcanic activity, around 5 million years ago. The eruptions of the Southeast Mountain and Mt. Lebanon vents are about this time too, 5-2 million years ago. This was the most intense era of volcanism, with the first major shift of eruptive centres from Mt. Granby towards the south of the island culminating in Fedon’s Camp and Mt. Qua Qua and the extrusion of Basalt lava on its western ridges.
The basalt lava flows are thought to be 3.5 million years old, inter-layered with eroded and redeposited younger volcanic sediments of lahar, massive mudslides of unconsolidated volcanic deposits from the volcano sides. The in-filling Lahar gives the relatively subdued topography of southern Grenada.
In the north of the island, major eruptions re-occurred at the same time, with the Mt. St. Catherine massif being the youngest volcanic structure on the island. Activity began 4 million years ago and continued up to 10,000 years ago. Initially a vent near Mt. Plaisance erupted with basalt lavas, which were overlain by a series of andesite and dacite (containing quartz) lava flows. As the vent migrated southward, andesite and dacite lavas were deposited to the northwest of the present summit. The one-mile-wide crater to the south-east of the mountain was partially filled-in by an andesite lava.
The final stage of volcanic activity involved the formation of explosion craters throughout the island, most notably at Lake Antoine, Grand Etang and St. George’s harbour – giving the island its perfect harbour, which was fought over by the French and British. The three closely-spaced explosion craters at Grand Etang are generally thought to be the youngest volcanic structures on the island, having formed approximately 10,000 years ago.
As well as gases expelled during volcanic eruptions, there is lava – the magma once it starts flowing – and tephra, chunks of solid material of all shapes and sizes exploded through the air; volcanic ash are the smallest particles. The ash becomes compacted into a solid rock, tuff, which is both sedimentary and igneous. This is widespread and is valued by the construction industry for its compactability in foundations. The distillery sits on top of it.
- LAVA FLOW
- LAVA DOME
- MAGMA CHAMBER
Southern Grenada’s surface geology is almost entirely comprised of volcanic mudslide deposits (lahar) and fluvially deposited volcanic material. The rocks exposed in Quarantine Point are of this type. The eastern coast is composed of eroded and redeposited volcanic deposits from the mountainous centre which accounts for the gently rolling topography. The western coast, on the other hand, is a much steeper, abrupt landscape, because of the dominance of eruption of volcanic activity to westward.
Since water is the principle erosive agent on the island, and annual rainfall increases with elevation, the high elevations are eroding at a greater rate than the lowlands; the lowlands filling up with washed down highland material.
In recent times, volcanic activity on the island of Grenada has been virtually non-existent, with the minor exception of hot springs in the Mt. St. Catherine area that emit sulphurous water and vapour. However, 5 miles to the north of Grenada is one of the most active volcanoes in the Windward Islands, 160 meters below sea level, known as “Kick ‘Em Jenny”. Situated away from her recently-discovered dormant brother “Kick ‘Em Jack”, Jenny has erupted at least half a dozen times this century, sinking a passenger vessel in 1944, and in March 2018 Jenny’s alert status was raised to Orange – the second-most severe level.
Its next eruption may see the volcano emerge to form a new island, just as Grenada.