Volcanic Islands’ Rocks – Mauritius

2 grey boulders on top of each other.

As one drives around the beautiful island of Mauritius, gentle slopes and deep valleys in brown and green can be observed. These features are carved on the volcanic rocks that created the island. Magma from deep inside the Earth’s crust has welled up and outpoured the thick mineral rich lava. Solidification over millions of years now results in an array of different types 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, 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. are all made up of basically two types of rocks: basalt and granite. Basalt is composed mostly of calcic plagioclase feldspars and pyroxenes. It is normally dark in color 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 result of volcanic eruption, though it is formed right in the magmatic cauldron. It is light colored 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, on the other hand, evolved from alkalic basalt magma; it didn’t go through much differentiation though.

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 colored 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.

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