A volcano is a rupture or opening on the surface of the earth where magma breaks through releasing gas, ash, lava and molten rock.  Magma mostly comes from the asthenosphere below the lithosphere.  The asthenosphere is weaker than the lithosphere and close to its melting point. The depth and pressure from above keeps the rocks in the asthenosphere from melting.  When two plates spread, the lithosphere becomes weaker and does not exert as much pressure on the asthenosphere below, resulting in decompression melting and the forming of magma which rises to the surface.  Most volcanoes are located in divergent or convergent plate boundaries.  In subduction zones, volatiles are released as one plate moves under the other.  Volatiles are chemical compounds like water, carbon dioxide and sulfur dioxide that when released can lower the melting temperature of rocks in the asthenosphere causing the magma to rise.

     Madagascar has five volcanoes.  They are Ambre-Bobaomby, Ankaizina, Ankaratra Field, Itasy Volcanic field and Nosy-Be.  All of these volcanoes are dormant.  The last eruption occurred in 6050 BC in the Itasy Volcanic Field.  Since there is no active volcanic activity now or in the recent pass, Madagascar does not have any real preparedness plans in case of an erupting volcano.  Madagascar does have two active volcanoes off its coast.  Both of them are shield volcanoes.  Shield volcanoes are wider than they are taller and produce the most lava flow due to low viscosity. The eruptions are usually not that explosive and are most common in hot spots. The Volcano Piton De La Fournaise on Reunion Island is 300 miles east of Madagascar and is one of the most active volcanoes in the world.  There have been more than 150 recorded eruptions since the 17th century with the most recent eruption on August 1^st of this year. The volcano Karthala on The Comoros islands is around 200 miles northwest of Madagascar and has erupted more than 20 times since the 19^th century with the last eruption on May 29, 2006. Most of the eruptions have been mild but phreatic explosions have occurred in 1918, 1948, 1991 and 2005.  Phreatic explosions occur when magma heats surface water so quickly that it turns to steam causing an explosion of steam, rock, water and Ash.  In 2005, 30,000 residents were evacuated after an eruption because there was a risk of lava flow and deadly volcanic gas.  They returned to their homes and found ash tainted water.  These volcanoes don’t appear to pose much of a threat to Madagascar.  If one of them had a huge eruption causing part of it to fall into the ocean, it could potentially cause a tsunami.  If Karthala had a huge phreatic explosion, ash could potentially reach Madagascar which might pollute the water, damage crops, and cause respiratory problems for animals and humans. 

 REFERENCES

 

Kellor, Edward A., Devecchio, Duane E., (2015) Natural Hazards, Earth's processes as Hazards, Disasters, and Catastrophes (4TH ED).

 

Piton De La Fournaise, www.Wikipedia.org

Mount Karthala, www.Wikpedia.org

Karthola Volcano, Comoros – John Seach, www.volcanolive.com/Karthola.html

Phreatic eruption, www.Wikpedia .org