Volcano World

Cloud banks drape the flanks of the massive Karthala shield volcano at the southern end of Grand Comore Island. A pyroclastic cone of La Grille, another massive shield volcano that forms the northern part of the island, lies along the coast in the foreground. Karthala contains a 3 x 4 km summit caldera, and elongated rift zones extend to the NNW and SE from the summit of the Hawaiian-style shield. More than twenty eruptions have been recorded since the 19th century from both summit and flank vents.

Photo Credit: Copyrighted photo by Steve and Donna O'Meara, 2002.

The southernmost and largest of the two shield volcanoes forming Grand Comore Island (also known as Ngazidja), Karthala contains a 3 x 4 km summit caldera generated by repeated collapse. Elongated rift zones extend to the NNW and SE from the summit of the Hawaiian-style basaltic shield, which has an asymmetrical profile that is steeper to the south. The lower SE rift zone forms the Massif du Badjini, a peninsula at the SE tip of the island. Historical eruptions have modified the morphology of the compound, irregular summit caldera. More than twenty eruptions have been recorded since the 19th century from both summit and flank vents. Many lava flows have reached the sea on both sides of the island, including during many 19th century eruptions from the summit caldera and vents on the northern and southern flanks. An 1860 lava flow from the summit caldera reached the western coast north of the capital city of Moroni. (Description from the SI/USGS Global Volcanism Program)

The Karthala Volcano is notoriously active, having erupted more than 20 times since the 19th century. The volcano rises to a height of 2,361 meters on the southern end of Grand Comore, the largest island in the nation of Comoros. Frequent eruptions have shaped the volcano’s 3 by 4 kilometer summit caldera, which is shown in these Ikonos images. The top image shows the summit caldera on April 19, 2005, just as the eruption was ending. The bright white cloud on left of center, near the edge of the caldera may be steam from the eruption. The lower image shows the summit caldera on August 4, 2002. Some of the differences between the two images are caused by differences in season and time of day. The top image was taken early in the morning when the eastern sun cast long shadows over the eastern side of the crater. The sun was more directly overhead in the lower image. Beyond differences in lighting, the crater has clearly been changed by the recent eruption. A grey field of ash surrounds the crater and the caldera itself seems larger and deeper. The crater lake that dominates the caldera in the 2002 image is gone entirely. The lake formed after Karthala’s last eruption in 1991. In its place are rough, dark grey rocks, possibly cooling lava or rubble from the collapsed crater.
(Image and description courtesy of the NASA Earth Observatory and Space Imaging (now GeoEye).

The Comores Archipelago comprises four principal islands that trend west-northwest over 170 miles (270 km) between Madagascar and Africa. The islands become progressively younger to the northwest. The youngest and largest island, Grande Comore, is made of two active coalescing shield volcanoes, Karthala and Massif de la Grille. Both volcanoes have erupted in historic time. The above photograph shows the nested calderas at the summit of Karthala. The most recent activity at this volcano was a phreatic explosion in 1991. The photograph below shows cinder cones on the flank of Karthala.

Cinder Cones in Comore Islands