Volcanic Islands’ Rocks – Mauritius

2 grey boulders on top of each other.

As one drives around the beautiful island of Mauritius, brightly coloured gentle slopes and deep valleys can be observed. These features are carved on the volcanic rocks that created the island. Magma from deep inside the Earth’s crust welled up and outpoured the thick mineral-rich lava which has solidified over millions of years to result in an array of rocks.

Petrology

The study of rocks, called petrology, is fundamental to understanding a place’s history and evolution. For instance, the recent discovery of minerals in magmatic rocks has revealed the presence of a continental chunk under Mauritius. Fossils of plants and animals also get trapped inside of the rocks which again reveal a lot about past conditions.

Rocks are mainly minerals stuck together under certain conditions; hence their appearance, weight and uses vary from type to type.

There are basically 3 types of rocks:

  1. Igneous rocks – rocks that are formed by magma or lava. There are 2 types of igneous rocks.
  2. Extrusive rocks like basalt – they are formed by the cooling down of lava when a volcano erupts. Once outside, the lava cools down rapidly; minerals have a short time to expand. So, the rocks are fine-grained.
  3. Intrusive rocks like granite – here the rocks are formed inside the Earth’s crust. The magmatic mass takes time to lose heat to the surrounding; it builds slowly giving rise to coarse-grained rocks.
  4. Sedimentary rocks – like the name suggests, they are made of the fine particles of mud, minerals and organic detritus; a common example is sandstone. These rocks normally form under the influence of gravity as materials heap upon one another by layers. They are vital in finding fossil records.
  5. Metamorphic rocks – they are rocks that are literally metamorphosed. They can be any igneous or sedimentary or even another metamorphic rock that is subject to changes in temperature and pressure. The outcome is a new rock different from the original one. Marble is an example of metamorphic rock; it is a fusion of limestone, clay, silt and iron oxides deposited in different layers.

Rocks of volcanic islands

Volcanic islands such as Hawaii, Mauritius, Java etc. basically have two types of rocks: basalt and granite. Basalt is composed mostly of calcic plagioclase feldspars and pyroxenes. It is normally dark in colour and fine-grained.

Two types of basalt can be found: tholeiitic basalt and alkalic basalt.

Tholeiitic basalt contains a high amount of silica and few alkalis like sodium and potassium. Alkali basalt, on the other hand, contains high amounts of alkalis and few silicas; the pyroxene is augite, made up of calcium and aluminium. Both of them can contain olivine phenocrysts (crystals), feldspars or pyroxenes.

Granite is also formed as a result of volcanic eruption, though it is formed right in the magmatic cauldron. It is light coloured and usually contains quartz, amphiboles, feldspars, small amounts of mica and other minerals. These minerals give granitic rocks different tints.

Rocks of Mauritius

Mauritius island was formed in two distinct phases: the old lava series and the recent lava series. For this reason, layers or rocks consisting of different minerals can be found. The old lava series regurgitated pristine magma which was a fusion of tholeiitic and alkalic basalt. Differentiation (changes in magmatic compositions due to temperature and pressures) formed picrite basalt at the base of the original volcanic shield, trachyte in the branched domes, ending in hawaiite and mugearite. The latter two rocks can still be seen on Mountain Blanche and Mountain Fayence. The recent lava series evolved from alkalic basalt magma and did not go through much differentiation.

The main rocks of the island

There are both intrusive and extrusive types of rocks in Mauritius; most are fine-grained while a few are coarse-grained. Since the island emerged from the ocean floor through volcanic eruption, it is mostly made up of rocks in the basalt family.

The rocks of the island, from oldest to most recent, include:

  1. Oceanite – basaltic rock containing (>35%) olivine phenocrysts (crystals). It can be seen in the Black River Ranges.
  2. Ankaramite – contains a large amount of black augite and olivine phenocrysts within a dark background. It is common at the top of Signal Mountain, Le Pouce, Pieter Both, Corps de Garde and Le Morne Brabant.
  3. Hawaiite – composed of andesite, pyroxene, and some olivine. Hawaiite can be seen at Mountains Blanche and Fayence.
  4. Mugearite – it is the most evolved form of basalt containing oligoclase which can again be seen at the Mountains Blanche and Fayence system.
  5. Trachyte – this is a light coloured rock composed mainly of potassium feldspar and small amounts of dark minerals. These rocks did not erupt from the volcano; rather, they accumulated in the domes and were later exposed by erosion.

Rock climbing in Mauritius

Rock climbing is a famous activity that is enjoyed by many extreme adventurers and consists of mounting up and down rock walls using safety equipment to prevent injuries and deaths. Rock climbing requires the walls to be hard and resistant to withstand the weight of climbers and to allow safety materials to be inserted and fixed onto the walls. By nature, basalt is fragile (depending on the types of minerals locked inside it) and weathers faster compared to other rocks. Since Mauritius is mainly of basaltic origin, rock climbing is not common as the mountains are not resistant enough to put in climbing gear. But there are some mountain climbing activities that are out of harm’s way like on Pieter Both mountain and Le Pouce mountain.

The only place in Mauritius that has UIAA Standards with climbing routes is the Belle Vue Cliff at Albion on the west coast. The cliff is 30 m high and gives a pretty good view of the surrounding region. Many tour agencies offer this service and it is recommended to be in good shape, wear appropriate climbing shoes, and avoid rainy periods (can be dangerous) before engaging in rock climbing.

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