Spatio-temporal evolution of temperature and fluid flow through a new “thermo-lithological” boundary; the case of a pit crater of Karthala volcano (Comoros archipelago) refilled on January 13th 2007 by a lava flow

Abstract

On January 13th 2007 an effusive eruption on the top of Karthala volcano, in the Comoros islands, emitted a lava flow which has been perfectly constrained inside the cylindrical shape of the Choungou Chagnoumeni pit crater; 225 m in diameter and with a thickness of about 7 m.This eruptive event, with a known geometry before the eruptive event, give a unique opportunity to study a “thermo-lithological” structural boundary recently born, and follow its thermal and fluid flow evolution in time.The motivation of this survey is to better understand some scientific questions such as: How fluid flows evolve in time along a new thermo-lithological boundary? How lava temperature cooling induce changes in hydrothermal circulation? Different data set have been acquired on the field in order to discuss these problematics. On March 2008, a temperature monitoring was installed with a data logger (CR 1000 Campbell Scientific) and 11 sensors placed at 30 cm depth, 1 m apart, inside the crater, along a straight profile, perpendicular to the western rim of the Choungou Chagnoumeni crater. Data were recorded during 275 days out of 677 days, through 4 different periods between 3rd March 2008 and 8th January 2010.On June 2009, high resolution electrical resistivity tomography (ERT) coupled with soil CO2 diffuse degassing was performed above the monitoring line of temperature sensors along a profile of 128 m long, with ERT electrodes and CO2 measurements located each 2 m.Numerical modeling of the temperatures was also tested on this case study, thanks to a finite element method (FEM) code using the Rheolef C++ library. The aim of using a numerical code, integrating a maximum of parameters, have been to better interpret the other data acquired on the field.The lateral permeability transition induced by the crater refilled with lava flow material was attested by the soil diffuse degassing profile.Coupling ERT and temperature modeling allow evidencing the heat transfer with conductive processes.The most striking result has been the inversion of the lateral temperature gradient which evolves during the first three years of cooling, from the highest temperature in direction to the center of the crater to the highest temperature in direction to the boundary of the crater.Such a result highlights the importance of structural boundary in dragging hydrothermal fluids and has been interpreted by a change in the main parameter governing the heat transfer close to the boundary of the crater, which evolved from (1) a lava heat source toward (2) a higher permeability area constituted by the boundary of the crater. This study clearly displays the importance of the lateral permeability gradient in changing the hydrothermal circulation in time.