Abstract
A swarm of 14 landslides occurred spontaneously within the Azenge Mountain on November 13 th , 2010 between the hours of 1 and 6 am after torrential rains. The landslides were studied using interviews, field observations and laboratory study of the soil samples collected from the major landslide site. Results of the study show that the event that was reported by many local media in Nigeria as volcanic eruption was a spontaneous massive slope movement along the mountainous terrain of Imande Ukusu, Mon village. About 1,123,918 m 3 of rock and debris were moved along the sliding surface for more than 1.5 km to the toe of the mountain within a very few minutes. The size of the materials moved decreases from the top to the toe of the slope which range from 50 o to 5 o respectively. These materials range in size from block of rocks of various sizes, soil to mud. The largest boulders moved measured 71.2 m and 88.5 m respectively. The rock types in the area include gneisses, granites, basaltic rocks and dolerite dykes which have a general strike of 240 o NW with a dip of 038 o NE and are highly fractured. The fractures and dip are in the direction of the slope. Laboratory result of the soil samples reflect two types of grain sizes. Those with very high degree of susceptibility to sliding above the sliding surface which plot in the failure zone and those less susceptible from the sliding surface and plot outside the failure zone. Torrential rainfall towards the end of the year was the main triggering mechanism of the landslides. Other causes are attributed to geological, morphological and human factors. The landslides led to the death of 1 person, destruction of vegetation, soil structure and texture, farmlands and remodeling of the geomorphology of the area. It is recommended that human activities along the toe of the hills be reduced and settlements should be sited far away from the hills to avoid further loss of lives in future.
Highlights
Landslides are geological phenomenon which occurs as a result of ground movement, rock falls, and failure of unstable slopes; sand and debris flow on slope and can cause a lot of damages with direct and indirect effect on human settlements and physical infrastructure [1]
Results of the study show that the event that was reported by many local media in Nigeria as volcanic eruption was a spontaneous massive slope movement along the mountainous terrain of Imande Ukusu, Mon village
Several landslides have occurred in Nigeria but many of them are not recorded except when people are killed and/or a good number of properties are destroyed
Summary
Landslides are geological phenomenon which occurs as a result of ground movement, rock falls, and failure of unstable slopes; sand and debris flow on slope and can cause a lot of damages with direct and indirect effect on human settlements and physical infrastructure [1]. The CHL has been considered as a hot line resulting from lithospheric cracks (N30o E) tapping a sublithosperic mantle [6] These Basement Complex rocks comprise of igneous and metamorphic rocks including porphyritic granites, migmatites, diorites, pegmatite and gneisses. Identification of the geology of the affected area, determination of the triggering mechanism of the landslides, effects on people and the environment and mitigation measure are discussed This will assist to evaluate the disaster and risks associated with landslides and eventually provide guidance for the management of such events in Nigeria and other parts of the world
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