St. Lucia is a volcanic island that is covered in mainly miocene rock, volcanic igneous rock and conglomerate rocks. There is a small section of coral formations in the north of the island, but the majority of the geology of St. Lucia is volcanic.
St. Lucia has several interesting geological formations that are a result of volcanic activity on the island.
There have been no recorded volcanic eruption, but there have been steam eruptions in the very popular Sulfur Springs.
Roseau Valley contains the oldest geology on the island while the Southern part of the island is geologically younger.
Types of Rocks on St. Lucia
The information provided in this article by YesDirt.com is for informational purposes and is subject to change. Laws are updated. Accessibility guidelines and restrictions change. Be sure to confirm the land status and collection rules before you travel to an unfamiliar location or collect any material.
Large basalt deposits are found in the north and south of St. Lucia. Basalt is usually light gray to black in color and it may have a fluid shape or appearance.
Basalt is the most common rock on earth and it consists of between 45-53% silica.
Basaltic lava is the result of lava flows and solidifies into basalt flows. There is a very old basalt flow on the western coast on the island.
The western coast of St. Lucia has large basalt flows with adesites which consist of more silica than basalt.
This volcanic rock is fine grained and light gray to light reddish in color and it is a result of magma flows that formed the island.
This rock forms Mt. Gimie in the Canrise District of St. Lucia. The volcanic rock supports rich vegetation on this highest mountain on the island.
This mountain is still volcanic.
Rhyolite is a type of rock that is extruded and cools quickly at the surface of the earth. Rhyolite has 20-30% quartz.
Rhyolite can have very different properties depending on the other components and conditions when it is forming.
One form of Rhyolite can form into obsidian, a very dark colored smooth stone that breaks in an even cleving pattern.
Obsidian has been used throughout history to make sharp points of spears and knives.
Obsidian may have a shiny glass-like appearance. It was used extensively by native people in the East Coast of the US.
Rhyolite can also form Pumice. Pumice is a dark or light colored highly porous stone that is commonly used in beauty products and concrete stabilization.
The composition of pumice is also 20-30% quartz which gives it hardness and the extrusive action causes the porous formation.
Andesite is another form of lava that is rich in silica, but lacks quartz. It typically creates short lava flows that can form lava domes.
Morne Soufriere is a large dome shaped mountain on St. Lucia 450 meters high and 1500 meters in diameter. This dome was formed by andesitic lava.
Andosite is a light gray, fine grained stone that is often used as fill, but cannot be used in concrete.
Andosite often contains copper ore.
Copper is currently mined on St. Lucia.
Conglomerate rocks made up of alluvial material that has collected in waterways are called fanglomerates.
These rocks are generally made up of sand, silt, rocks and any other materials that have been cemented together by calcium.
On St. Lucia the calcium is plentiful in the warm water as it is provided by marine life. The volcanic rock and debris that is washed down the mountains can form alluvial fans that solidify into fanglomerates.
Breccia is another conglomerate rock that can form at the base of the andesitic lava flows of Mt. Gimie and Mt. Tabac.
Breccia is Italian for rubble as these rocks often contain large pieces of other rocks cemented together in some type of matrix.
Breccia contains at least 30% large pieces of distinguishable rock held together in any type of cement.
There are several types of breccia, but St. Lucia holds volcanic breccia.
Volcanic breccia is formed when rocks are incorporated into lava flow and harden. There is evidence of megabreccia on the caldrea floor where the lava has cemented together stone that was present on the island before the volcanic action that formed the caldrea.
Breccia will have many visible stones cemented together in the dark volcanic rock present on the island.
Breccias are beautiful and have often been used for sculpture and they are a record of the geological history of the island.
Sulfur Springs located in Qualibou Caldera is the most popular tourist destination on the island. Sulfur springs was the location of extensive sulfur mining until 1840.
The caldura is a depression formed by the eruption of a volcano.
Sulfur stones will be very bright yellow in color. Sulfur has traditionally been called brimstone which means ‘burning stone’.
Sulfur springs is a natural spring that is heated by the molten rock below the surface of the earth.
The water of sulfur springs turns black because of the mixture of iron and sulfur. The heating and evaporating of the water and these minerals can result in the formation of sulfur crystals.
The minerals in the soil are said to have health benefits for those who participate in regular mud baths.
When visiting the Quailibou Caldera you may notice The Pitons. These two volcanic plugs formed over thermal vents just off the coast of the island.
The Pitons are surrounded by coral reefs that thrive on the warm water that surrounds the island.
The two Pitons are Gros Piton and Petit Piton. Gros Piton is the second highest peak on St. Lucia after Mt. Gimie.
You may find coral or marine fossils while snorkeling around these amazing geological formations.
Collecting Rocks on St. Lucia
Be sure to read and obey all posted signs as you are collecting rocks on St. Lucia.
Any collected rocks should be declared when you enter your home country.
Be careful when interacting with waterways on St. Lucia.
Bathing in the rivers is strictly forbidden as these freshwater sources are used for drinking water.
Rocks on St. Lucia
The volcanic island of St. Lucia has some very beautiful and accessible geological features.
The location of Sulfur Springs makes it easy to visit by car.
There is a lot to see and to find on the island.
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