Trou aux cerfs or Murr's Volcano
To log this earthcache, you need to answer some questions by to the owner:
- Look around you, near the kiosk you will see that the ground is brown but a little further is black, explain in your own words where the black color come from.
- Most volcanoes have a cone and they are rising, others do not have cone like Hawaii, why?
- According to you the crater is dormant forever or it may wake up at any moment?
- Optional but appreciated: Please, provide a picture.
About Murr's volcano
Trou aux Cerfs volcano is a monogenetic volcano, because it has erupted only once. For th last 700 000 years it has been a dormant volcano.
The little island of Mauritius was formed when a big shield volcano erupted some 10 million years ago during the Pliocene period. It is believed that Mauritius was created by three main volcanic eruptions (The Ancient Series, The Early Series and the Recent Series) interspaced by calm periods. These series of volcanic activities gave rise to the number of mountains, gorges and valleys that can be seen all over the island. The ranges of Port Louis – Moka and some scattered isolated mountains are believed to be the remains of the large crater (caldera) of the huge volcano that initially created Mauritius. The last volcanic eruptions are thought to have been 600, 000 to 700,000 years ago and involved about 20 volcanoes found along the North-North-East and South-South-West axis of the island; the last volcanic eruption. L’escalier crater at Nouvelle Decouverte, was about 20,000 years ago.
There are four famous volcanoes in Mauritius that are thought to have been active during the past 700,000 years: Trou aux cerfs, Trou Kanaka, Bassin Blanc and Grand Bassin.
What is a volcano?
A volcano occurs when an opening of the earth's crust allows magma to surface. At great depth the rocks partially melt. The liquid formed, less dense than the surrounding rock, rises to the surface. During the trip, magma may stop in a tank called magma chamber, and stay several centuries. The ascent of magma is then facilitated by the cracks in the crust. The ductwork is called "volcanic vent." Surfacing, volcanic magma releases different products: liquids (lava); solid products (blocks, bombs, slag, ash, crystals), and gas.
A volcanic eruption marks the arrival on the Earth's surface of the deep magmas. The gradual accumulation of these products constructs a form that can take different shapes. The energy released during an eruption is enormous: during major eruptions this can reach millions of times the power of the atomic bomb dropped on Hiroshima in 1945.
Some volcanoes live hundreds of thousands of years or even a few million years. Others are born and after only a few days of activity, fall asleep forever. By convention, the term "active volcano" refers to a volcano that has had at least one eruption in the last 10,000 years.
It is the nature of the magma that determines the speed of a volcanic eruption and the form of the visible volcano. Very fluid lava will spread in the form of very long lava flows. When it is sufficiently rich in gas, they are expelled in the form of relatively moderate explosions, resulting in the formation of a slag cone. In contrast, viscous magmas that reach the surface are often preceded by loud explosions due to their rich gas. Not having the ability to flow, they accumulate near the fireplace and give rise to domes or needles.
About this volcano
Trou aux Cerfs is one of the most well defined and impressive volcanic structures of the island, formed during the late stages of volcanism on the island, 700,000 to 600,000 years ago. It is a dormant volcano with a central vent, a well-defined cone and crater which has a perfect circular despression. On top of the volcano, you can see the crater about 180 m in diameter.
Where are the volcanos?
Today, about 1670 active volcanos are listed worldwide. Sixty erupt each year (not including the numerous submarine volcanoes). These volcanos are mainly concentrated along the boundaries of tectonic plates. A hundred of these volcanos are considered particularly dangerous because large populations live nearby. Only some of them are subject to specific monitoring with the presence of a dedicated observatory.
Active volcanoes are not distributed randomly on the globe. They mainly related to three geodynamic environments:
- Volcanism accretion, where plates move apart from each other. It is estimated that at least 20,000 current or very recent eruptive centers punctuate these limits, which more than 99% are not observable because they are hidden in the deep ocean. Volcanism of accretion zones is best known to the few areas where the ridges are emerging. This is the case of Iceland.
- Subduction volcanism where plates converge: Current volcanism manifests itself in the most obvious way along subduction zones, generally very explosively. The best known area is the Pacific Ring of Fire, which borders the ocean on all sides except the south. It concentrates more than 60% of active emerged volcanos on the planet.
- Volcanic "hot spot" independent of plate boundaries, is related to thermal plumes lifts deeply rooted in the planet. This type of volcanism is relatively rare (a few dozen volcanos maximum).
About this volcano
Trou aux Cerfs volcano is in Mauritius which is a volcanic island formed there are 8 million years ago. This island is part of an archipelago containing the Reunion Island and Rodrigues.
The volcano is on the African plate, it's not an accretion or subduction volcanism, it's a "hot spot" volcano.
Two types of volcanos
Polygenetic volcano
A polygenetic volcano erupts repeatedly, in contrast with monogenetic volcanoes, wich erupt only once. Polygenetic volcanic fields generally occur where there is a high-level magma chamber. These volcanic fields may show lithological discontinuities due to major changes in magma chemistry, volcanotectonic events, or long erosional intervals, and may last more than 10 million years.
Unlike monogenetic volcanoes, polygenetic volcanoes can reach massive sizes.
Monogenetic volcano
A monogenetic volcano erupts only once, as opposed to polygenetic volcanoes, which erupt repeatedly over a period of time. Many monogenetic volcanoes are cinder cones, often with lava flows. Some monogenetic volcanoes are small lava shields. Other monogenetic volcanoes are tuff rings or maars.