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AFRICAN GREAT RIFT




Main African volcanic activities are situated along the Great Rift Valley, a hundreds of kilometres wide depression, which spans from Mozambique in the south to Valley of Jordan in the north, 6500 km long. About 80 % of the volcanoes are found alongside or within the Great Rift Valley. These are one of the most spectacular but least-known volcanoes on the planet. In the past 150 years, more than 110 eruptions have been reported from 18 locations, while another 112 volcanoes have been diagnosed as potentially active.
Most southerly volcanic fields are dominated by three massifs, of which the greatest is Kilimanjaro (Shining Mountain), 6 000 cu km of lavas and pyroclastics that tower to 5 967 m above sea level. Neighbouring Mt. Meru (the Black Mountain, 4 568m) appears but a modest companion.

SOUTHERN VOLCANOES
Most southerly volcanic fields are dominated by three massifs in Tanzania, of which the greatest is Kilimanjaro (Shining Mountain), 6 000 cu km of lavas and pyroclastics that tower to 5 967 m above sea level. Neighbouring Mt. Meru (the Black Mountain, 4 568m) appears but a modest companion. Mt. Meru, 75 km from Kilimanjaro, is not complete. Its summit has been worn down by ancient glaciers, while massive collapses have removed 3 km of eastern flank. The third great volcano of Tanzania is Ol Doinyo Lengai (The Mountain of God).
North of Lengai, the rift valley dissects Kenya with another 16 active centres, from Longonot, Suswa and Menegai to Teleki-Likaiyu and volcanic islands in the middle of Lake Rudolph. Since 1870, less than dozen of eruptions have been reported, mostly small effusions of lava from Teleki-Likaiyu and the impressively-named Emuruangogolak.
CENTRAL (VIRUNGA) VOLCANOES

Further 700 km west, beyond Lake Victoria and within a second branch of rift valley, Virunga volcaoes (border of DR of Congo, Rwanda and Uganda) cause of 80 % of African volcanic activity. Activity is focused upon the neighbours Nyiragongo and Nyamuragira, enormous volcanoes built by lavas that are surprisingly liquid, topped by calderas over a kilometre across and perpetually filled by lakes of glowing lava.

From the 1930s until 1977, the most famous of all lava lakes was that within Nyiragongo (3470 m). For more than 40 decades, the midnight glow from the lake lured climbers and geologists. Then on 10h15 a.m. on 10 January 1977, a fissure from beneath the summit allowed sudden drainage of the lake onto southern slopes 10 km away, burning villages and leaving 70 people dead within 25 minutes. The unexpected outpouring produced the greatest known number of victims from a lava flow. Since 1982, the caldera has again filled with lava. The road to the volcano wades in the mud of a damp track, in a soaking forest where the tree trunks hide murkily in the mist. Then one hour of steep climb and then a few sombre rocks standing out to cross in an icy wind, and the summit of the volcano is reached. Then 200 m of vertical rockface descend that guards entrance to the crater, and one can view (if no mist) a lake of molten lava. Descending the spur of a broken platform, one overhangs a lake of molten lava, a few metres away from its surface. The lake is covered in an elastic skin which shines in a magnificent metallic glare in the sunlight. This skin undulates and inflates slowly above the pulsating lava. Suddenly, the whole lake bursts into life. Activity starts with fountains bursting into life all around circumference of the lake at the foot of the rockface. Some of these gush up right up to the platform, and bigger and bigger bubbles of gas burst with huge detonations. The lava lake suddenly inflates and erupt into the whole depression., sweeping the whole crater.

NORTHERN VOLCANOES
Persistent lava lakes are one of the hallmarks of Africa's historic volcanism. Another renowned example is Erta Ale, 2000 km north-east from Nyiragongo. Erta Ale (the Fuming or Devil's Mountain) lies in the north-east end of The Rift Valley segment that runs across Ethiopia and Djibouti to the Red Sea. The summit lava lake sits at an elevation of about 500 m, while the base of the volcano rests 75 m below sea level. Erta Ale and its neighbours have grown from the most low lying part of the Great Rift Valley on land. One hundred kilometres away the rift has been flooded to create the Red Sea. After rifting has started 20 million years ago, the crust of the continent eventually split between modern Ethiopia and Arabia, allowing a new crust to from upwelling lava. What is happening now at the Red Sea is possibly what happened between the American and Afro-European continents when the Atlantic first developed 2000 million years ago.
Erta Ale is a shield volcano, part of the East African rift system. In its vast summit caldera there are two pit craters. The smaller crater now has an active lava lake. Even now, after the end of the war, the Erta Ale range remains one of the most inaccessible places on earth. There are no roads and temperatures are extremely high. The tribe living there, the Afar people, are proud and strong warriors, often hostile to foreigners.
The lava lake is permanently stirred by fountains of lava and the skin that covers it in some cooler spots makes it seem alive. The lava of Erta Ale is basaltic, which remains molten at 1100C. The lava lake has a surface area of 4 000 sq m, so the thermal radiation is intense. The surface of the lake is covered in a layer of cooler lava, floating on the molten core like the skin on milk. The lake is moving - a current ripples the lava and makes the skin undulate. This current originates in a fountain spraying molten lava brewed by the high temperature gas that it emits. The skin is black in the shade, and as brilliant as silver in the sun. The fountain is scarlet, becoming orange as its temperature rises. The flow of the lava tears the skin into a huge panels separated by fissures through which lava shows red. Bubbles of gas regularly break the surface of the lake, popping the elastic skin with spurts and incandescent jets.


WESTERN VOLCANOES

Rifting has also been essential to Africa's second great chain of volcanoes, which runs for 1600 km along the Cameroon-Nigeria border to the lonely island of Pagalu, in the Gulf of Guinea. The Y-shaped chain is dominated by Mt Cameroon (the Cavern of Gods) which, rising from the coast to 4 070m, is the highest peak in West Africa. Due to an eruption in the fifth century BC, Mt Cameroon became the first volcano outside the Mediterranean known to the west's ancient world.
Many of the volcanoes have water lakes filling their summit craters. Twice in the 1980s (at Lake Monoun in 1984 and Lake Nyos in 1986), clouds of carbon dioxide unexpectedly escaped from the lake water and, being heavier than air, they rolled downslope close to the ground, before dissipating into the atmosphere. Together, the silent, deadly clouds suffocated more than 1700 people and hundreds of cattle and wild animals. The volcanoes were not obviously active, it appears that carbon dioxide may have been seeping gently and underdetected into the base of the lakes. Once enough gas has been dissolved, the waters in the lakes became critically stable, so that only a small disturbance (heavy rainfall or weak earthquake) could force the lake water to overturn, bringing the gas rich layers to the surface and allowing the carbon dioxide to bubble out and escape. Several ways of reducing the hazard are currently being studied, and include siphoning away the gases before they achieve dangerous concentrations. The lakes of Cameroon show that not even a sleeping volcano can be ignored.