The Unique Virunga Volcanoes: Part 1
By: Joel Atwood, ED2
Everyone who lives in Rwanda knows about the Virunga Mountains. In the North West, their volcanic cones dominate the horizon. During a clear morning they can be seen all the way from the Southern Province. They symbolize Rwanda and are home to its greatest tourist attraction, the famous mountain gorillas. These volcanoes are the source of much of Rwanda's rain and provide porous mineral rich soil for growing crops. They, however, pose a potential danger to 100,000 people that live in the flat volcanic lave plateau that fill the valley at the base. What make these mountains unique are their size, shape, and the lava they produce.
Rwanda is a geologically active country. Most of the country is uplifted into a series of mountain ridges that form its nickname, the Land of a Thousand Hills. These ranges are a result of regional extension, in other words, the Earth is slowly being ripped apart. These breaks are not a one big uniform crack but a series of cracks that form long flat valleys surrounded by the mountains. Rwanda contains two major rift valleys: one in the west where the Lake Kivu sits in the resulting valley and one in the east where many lakes reside and the Akagera River weaves its way towards Lake Victoria in Uganda. The land in between two rift valleys tends to be squashed up into high mountains and plateaus because of tech tonic forces creating the rift valleys. A dramatic example of this is the Rwenzori Mountains of Uganda and the DRC where a non-volcanic mountain range has been pushed up between two rift valleys so high that the third highest peak in Africa is contained in them.
The series of rift valleys in Rwanda is all part of what is known as the West African Rift, a series of rift valleys that pass through the middle of the continent of Africa. The other great series of rift valleys is commonly known as the East African Rift and it is responsible for creating the volcanoes in Tanzania and Kenya. Why are there rifts? Because Africa is splitting apart. The rift valleys indicate the formation of a new ocean basin. In a few million years, a small sea or ocean will replace the great lakes of Africa and the continent will effectively become two. Rwanda sits on the fractured edge on the newly forming Somalian plate and the DRC resides on the African plate. Give or take a few million years and Rwanda will never have to worry about rebels in the DRC because there will be an ocean in between them!
The Virunga volcanoes play a critical role in the formation of this new ocean. They erupt lava, which fills the cracks and makes the valley become wider and wider. This new rock will become the ocean floor. The Virunga volcanoes sit on a fracture in the Earth that spans the rift valley. Lava reaches the surface because as the continents move farther and farther apart, pressure is released on very hot rocks in the mantle. As the presser decreases, those rocks begin to melt. Liquid rock, being less dense than solid rock, moves up through cracks in the surface and creates volcanoes.
So, what is unique about these volcanoes? The Virunga and almost all volcanoes associated with rifting produce a type of lave that hardens into a rock called basalt. This rock is dark brown to black in color, very dense, and flows relatively quickly. The Big Island of Hawaii contains two classic example of a basalt producing volcanoes. The two active volcanoes are called Kilauea and Mauna Loa. Both are known as a type of volcano called a shield volcano. Shield volcanoes are volcanoes shaped like a shield resting on the ground. They have very large bases and a gradual slope. This is because basalt lava flows tend to flow 5 to 15 kph and can spread lava over a large area creating a broader shaped mountain. However, the Virungas are different. They are basalt producing volcanoes, but are huge with very steep slopes. Muhabura and Mikeno (in the DRC) are great examples of just how steep these volcanoes are. Why are they so steep? That is a good question that no one knows for sure.
Rwanda is a geologically active country. Most of the country is uplifted into a series of mountain ridges that form its nickname, the Land of a Thousand Hills. These ranges are a result of regional extension, in other words, the Earth is slowly being ripped apart. These breaks are not a one big uniform crack but a series of cracks that form long flat valleys surrounded by the mountains. Rwanda contains two major rift valleys: one in the west where the Lake Kivu sits in the resulting valley and one in the east where many lakes reside and the Akagera River weaves its way towards Lake Victoria in Uganda. The land in between two rift valleys tends to be squashed up into high mountains and plateaus because of tech tonic forces creating the rift valleys. A dramatic example of this is the Rwenzori Mountains of Uganda and the DRC where a non-volcanic mountain range has been pushed up between two rift valleys so high that the third highest peak in Africa is contained in them.
The series of rift valleys in Rwanda is all part of what is known as the West African Rift, a series of rift valleys that pass through the middle of the continent of Africa. The other great series of rift valleys is commonly known as the East African Rift and it is responsible for creating the volcanoes in Tanzania and Kenya. Why are there rifts? Because Africa is splitting apart. The rift valleys indicate the formation of a new ocean basin. In a few million years, a small sea or ocean will replace the great lakes of Africa and the continent will effectively become two. Rwanda sits on the fractured edge on the newly forming Somalian plate and the DRC resides on the African plate. Give or take a few million years and Rwanda will never have to worry about rebels in the DRC because there will be an ocean in between them!
The Virunga volcanoes play a critical role in the formation of this new ocean. They erupt lava, which fills the cracks and makes the valley become wider and wider. This new rock will become the ocean floor. The Virunga volcanoes sit on a fracture in the Earth that spans the rift valley. Lava reaches the surface because as the continents move farther and farther apart, pressure is released on very hot rocks in the mantle. As the presser decreases, those rocks begin to melt. Liquid rock, being less dense than solid rock, moves up through cracks in the surface and creates volcanoes.
So, what is unique about these volcanoes? The Virunga and almost all volcanoes associated with rifting produce a type of lave that hardens into a rock called basalt. This rock is dark brown to black in color, very dense, and flows relatively quickly. The Big Island of Hawaii contains two classic example of a basalt producing volcanoes. The two active volcanoes are called Kilauea and Mauna Loa. Both are known as a type of volcano called a shield volcano. Shield volcanoes are volcanoes shaped like a shield resting on the ground. They have very large bases and a gradual slope. This is because basalt lava flows tend to flow 5 to 15 kph and can spread lava over a large area creating a broader shaped mountain. However, the Virungas are different. They are basalt producing volcanoes, but are huge with very steep slopes. Muhabura and Mikeno (in the DRC) are great examples of just how steep these volcanoes are. Why are they so steep? That is a good question that no one knows for sure.