St. Lucia Geology - Anse La Raye Falls
Regional Setting
Saint Lucia lies in the Lesser Antilles arc between the islands of Martinique in the north and St. Vincent in the south. The islands of the Lesser Antilles form an arcuate line along the eastern margin of the Caribbean Sea that stretches ~700 km from Sombrero in the north to Grenada in the south and that marks the boundary between the North American and Caribbean plates. The islands of the Lesser Antilles have formed over millions of years by volcanic processes related to the westward subduction (under thrusting) of the North American plate beneath the Caribbean plate. These processes are still on-going today. When the North American plate reaches depths of about 100 km, the crust is melted to form magma. This magma is less dense than the surrounding rock, and rises up to the surface where it erupts to form volcanoes. This process happens all the way along the plate boundary, and the line of volcanoes that is produced forms the arc.
Geology – Island Pfofile
Saint Lucia is made up almost entirely of volcanic rocks. Like all of the islands of the Lesser Antilles, Saint Lucia began its life as a series of submarine volcanoes. After many eruptions over millions of years these volcanoes built large topographic features that slowly rose above the surface of the water, joined with neighbouring volcanic islands, and grew to the island we see today.
Newman (1965) divided the volcanic centres in Saint Lucia into 3 broad groups based on age and geographic distribution. From oldest to youngest these groups are the Northern, Central and Southern series. This subdivision is somewhat confusing, as several of the centres within the Northern Series are actually located in the south of the island. Furthermore, subsequent age dates obtained for the volcanic rocks of Saint Lucia show that several centres that were originally classed as part of the youngest Southern Series more likely correlate with the older centres of the Northern Series. We prefer to use a slightly revised version of the original subdivision, grouping the volcanic rocks of Saint Lucia as follows:
Group 1: Eroded basalt and andesite centres (a revision of the ‘Northern Series’)
Group 2: Dissected andesite centres (called the ‘Central Series’)
Group 3: The SoufrièreVolcanic Centre (a revision of the ‘Southern Series’)
The eroded basalt and andesite centres are the oldest rocks on Saint Lucia and are located in the northern and southernmost parts of the island (see map above). Age dates for the centres in the north range from 18 - 5 Millions of years. The centres in the south, including Mt. Gomier, Morne Caillandre/Victorin, Moule a Chique/Maria islands, Savannes, Beauséjour, St. Urbain and Mt. Tourney, have published ages ranging from 10.1 Millions of years (lava near De Mailly) to 5.2 Millions of years (lava from Savannes). The age of these eroded centres indicates that they are unlikely to erupt again. However, there is some shallow seismicity and cold fumarolic (a fumarole is a crack in the surface of the Earth - usually associated with volcanoes - from which gases and steam are emitted) activity associated with some of the southern centres and these centres should be monitored closely for any signs of reactivation.
The dissected andesite centres comprise the central and eastern part of Saint Lucia and are somewhat younger than the eroded dominantly basaltic centres to the north and south. Age dates for these centres range from 10.4 Millions of years (lavas west of Dennery) to 2.8 Millions of years (lavas from Derriere Dos). These old ages indicate that these centres are unlikely to be the site of future volcanic activity. The youngest volcanic activity in Saint Lucia produced the rocks of the Soufrière Volcanic Centre.
The last 500-600 meters to the waterfall can only be reached on foot. The path is a nature trail that is partly unpaved and leads over stones and small rocks. The stream must also be crossed twice. Depending on the weather and water level, this can only be done by wading through the water. So the shoes might get wet.
Bathing at the waterfall and in the brook is forbidden. The water is also used as drinking water for the villages.
At the beginning of the path there is a small hut. With a little luck you can find "Rasterman Junior" here. If necessary, he will be happy to help you with walking sticks and can also show you the way to the waterfall and explain something about nature.
Please pay attention to nature and don't leave the path!
Log conditions:
1.
a) In which group (divided by Newman 1965) are the “Anse La Raye Falls” located?
b) What kind of rock do you find here directly at the waterfall? Andesite or basalt? Give a short explanation. What is the difference between these two types of rock?
2.
How high is the waterfall? Estimate the height.
Optional, no log condition:You can also optionally measure the GPS height from below and then determine the difference from the top of the waterfall.
3.
Take a picture of yourself or your GPS with the waterfall in the background and upload it to your log.
Send the answers of the questions to Fam.Weiss with your GC-name in the subject-line. We contact you if something is wrong.
Attention – Please note!
Please don´`'t write the answers into your internet-log at GC!
Log entries without answering the questions will be deleted by us directly without any further enquiry!
Attention: To fulfill the log condition you have to send a reply mail with the answers for the questions per Geocaching account! A "multiple answer mail" for several Geocaching accounts therefore only fulfills the log condition for the respective sender! The logs of the other accounts are deleted without comment!