Emplacement of the mid-Miocene Yatta lava flow, Kenya: Implications for modelling long channelled lava flows

Abstract

The emplacement of the 13.51 m.y. old Yatta lava flow in Kenya has been investigated using evidence from field observations combined with a novel method of modelling length-dominated lava flows along channels. The Yatta lava erupted as an individual flow from a single vent on the eastern rim of the present-day East African Plateau during the extensive volcanism that occurred in mid-Miocene times. It then followed an old river valley for nearly 300 km, thus forming one of the longest phonolitic lava flows on Earth. For our modelling we combined a composition and temperature dependent viscosity equation with empirical cooling and morphological relationships. By using an average channel width and the known length of the Yatta lava flow but varying the mean thickness and underlying topography, we have improved flow rate calculations for the internal part of the lava, close to the front of the lava flow. Within this zone the lava's motion was treated as steady, uniform, and laminar, following a stepwise cooling from the eruption temperature to the temperature at the cessation of flow. Comparison of eight different compositions ranging from basaltic to rhyolitic has revealed that the length-dominated Yatta lava flow emplacement was rapid (~ 7 days), approximately isothermal (cooling at 0.71 °C/km), and the result of high effusion rates (~ 7900 m 3/s). This study shows that morphology of a lava flow, and in particular its length, is not a simple function of rheological properties and effusion rate, but is also affected by many other parameters. Small changes in H 2O compounds in the lava chemistry can affect melt viscosity significantly and thus lava flow morphology. Therefore H 2O content, together with slope angle and the mean lava flow thickness, ultimately control the length of a lava flow within a channel.