Mount Qualibou (saint lucia)

Introduction

On Monday the 15th of august 2022 I visited the calderas of Mount Qualibou, the Pitons and the sulphur springs.

The geological history of Saint Lucia

Saint Lucia is on a plate boundary where the American plate is submerged underneath the Caribbean plate. It is this action that has created the island of saint Lucia, as well as most other Caribbean islands, by causing weak cracks in the crust for magma to rise through.

19-9 million years ago small volcanos occur forming the area from Castries to the far north of the island. Then 6-5 million years ago more volcanic activity caused magma to rise and crystalise forming the separate activity southern landmass. 2 million years ago intense volcanic activity from mount Gimie built up the island and connected the two land masses. Finally, under 1 million years ago, volcanoes in Soufriere bay became active.

The geological history of Mount Qualibou and its 2 overlapping calderas

The volcanic complex began forming 400,000 years ago when magma reached the surface. the lava that erupted out crystalised to creat a series of small volcanic cones made of basalt (the most abundant rock on the planet's crust)., these cones also deposited basalt. Over many years the colour of the rock lightened as lava became more silicic and began depositing andesite as opposed to basalt. Over time, the more silicic andesite created more volcanic peaks such as mount Gimie. However, as the lava became more silica rich, eruptions ceased, this is because the magma became to viscous to flow out of the magma chamber. beneath the surface, at high depth, a very large magma chamber continued to expand causing pressure to increase. 300,000 years ago a massive eruption occurred. A plume of ash flew 40km into the air and pyroclastic flows raced down through the surrounding jungles and into the ocean. As a result of the enormous volume of lava erupted, a 6km by 8km area of land collapsed creating a large caldera. This was the largest eruption in the Caribbean in the last 1 million years. 40km cubed of tephra were ejected. 260,000 years ago two large viscous lava domes formed known as the pitons. Over the next 224,000 years, other smaller lava domes formed in the Qualibou caldera.

36,000 years ago yet another massive eruption occurred. As pyroclastic flows raced down the island, a volcanic rock called ignimbrite was deposited across much of Saint Lucia. After the eruption ejected 16 cubic kilometres of tephra, a large area of land within the first caldera collapsed creating another caldera. Said caldera is smaller than the 1st (measuring only 3.5 by 5Km). Over the next 20,000 years, large volume eruptions built several lava domes in the recent caldera, such as Terre Blanche and mount Bonin

The Pitons

Formed 260,000 years ago as a result of the large eruption that occured 40,000 years prior

Highly viscous domes of lava erupted onto the surface and cooled forming prominent cliffs made out of the grey-white rock dacite.

Gabriel's hole

This is a hole in the crust in the sulfur springs. This was created when a tour guide named Gabrial attempted to show how strong the crust was on the sulphur springs by jumping on it. However, the crust was too weak and collapsed under his feet creating a waist deep hole. Luckily, his tourist group pulled him out of the hole. Although he now has 2nd-degree burns from the waist down.