Morphology, Orientation, and Distribution of Explosive Craters in the Nyambeni Hills Volcanic Field

Abstract

The Nyambeni Hills (Kenya) is a shield volcano found along the eastern branch of the East African Rift (EAR). This volcano is pierced by a volcanic field populated with scoria cones, maar craters, and lava flows of Late Pleistocene to Holocene age, referred to as the Nyambeni Hills Volcanic Field (NHVF). Volcanic fields composed of monogenetic explosive cones can be observed along the EAR. Through close examination of the morphology and alignments of such features, magma paths can be inferred. Such magma paths are expected to develop perpendicular to the regional tensile stress (σ3) and along fracture systems in a rift zone. Here we present our findings on the analysis of these explosive volcanic features found in NHVF. Google Earth Pro is utilized to map outlines of craters and cones. The morphometry of each cone is analyzed for erosional marks and slope angle to determine the area and shape of the original feature before erosion. The degree of confidence for each feature is reported to allow for a more informed data analysis. ArcGIS is used to find minimum bounding geometry data. Trends of the long axes of high confidence features with a short axis/long axis ratio greater than 1.2 provide an estimate of the orientation of that feature and are graphically expressed in a rose diagram. Data has been collected on >300 cones within the northeastern slopes of the NHVF. Here both the long axis of the most reliable basaltic scoria cones and the linear distribution of the various features, represented by the near angle relationships of all cones, indicate a general NE-SW trend. This bearing diverges from the overall N-S trend of the rift but is similar to the observed orientation of other volcanic features in the Kenya Rift (e.g. Mega Volcanic Field, South Natron and Marsabit). A NE-SW trend could suggest a NW-SE orientation of σ3 or could be explained by the presence of pre-existing lithospheric weaknesses in the host rock influencing the direction of dikes.