Entrée American Geophysical Union
Partie du discours nom propre (journal)
Sous-titre Sessions and Posters
Madagascar's central highlands are deeply weathered, with 1-2 m of laterite overlying 10s of metres of saprolite. These unstable materials sit at altitudes on the order of 1000m in recently uplifted, steep terrain characterised by convex hills with slopes averaging 25 degrees and local ridge-valley elevation changes of 100- 500 m. In many areas these slopes bear numerous erosional features called lavakas, with mapped densities up to 25/k$^{2}$ in some areas. Lavakas are tadpole-shaped gullies with vertical sides and flat floors. They are wide at the headwall and taper to a narrow, deeply incised outfall, which connects to the valley drainage. They are not fed by overland flow, but develop by groundwater sapping and subterranean erosion in the porous and friable saprolite beneath the baked lateritic duracrust. The controls on lavaka formation are poorly understood. Interplay between climate, topography, and bedrock geology is known to be important, and human activity is also implicated. However, there are many areas with appropriate rainfall, hillslope geometry and geology where lavakas do not form. Likewise, lavakas form readily in some areas with little human activity, and do not form in other areas that are heavily used. Our analysis suggests that seismic activity may be a significant regional factor driving lavaka formation. Using GIS analysis, we compared the locations of recent earthquakes (1347 events with magnitudes 3.0-5.4, recorded between 1979 and 1995) with the distribution of zones where lavakas are abundant (as mapped by Henri Besairie in 1957), and found that they are strongly correlated. We cannot say whether individual seismic events are responsible for specific lavakas; but the observation that lavakas are most common in zones that are seismically active implies that earthquakes play a significant role. Repeated mild earth-shaking events might aid in loosening the saprolite and making it more vulnerable to lavaka-forming processes; or individual earthquakes might initiate collapse of slopes already weakened by groundwater sapping. More detailed work into the relationships between individual lavakas and local seismicity is needed to determine which of these mechanisms might be active. Our ongoing GIS analysis will quantify the relative importance of geology, topography, and seismicity in the location of lavaka-prone areas [F. Rasoanazamparana]<=> The GLOBE Program is currently collaborating with students, educators, scientists, health department officials, and government officials in Madagascar to develop a program that combines existing GLOBE protocols for measuring atmospheric and water quality parameters with a new protocol for collecting and identifying mosquito larvae at the genus (Anopheles and non-Anopheles) level. There are dozens of Anopheles species and sub-species that are adapted to a wide range of micro-environmental conditions encountered in Madagascar's variable climate. Local data collection is essential because mosquitoes typically spend their entire lives within a few kilometers of their breeding sites. The GLOBE Program provides an ideal framework for such a project because it offers a highly structured system for defining experiment protocols that ensure consistent procedures, a widely dispersed network of observing sites, and a centralized data collection and reporting system. Following a series of training activities in 2005, students in Madagascar are now beginning to collect data. Basic environmental parameters and first attempts at larvae collection and identification are presented. Results from this project can be used to increase public awareness of malaria, to provide new scientific data concerning environmental impacts on mosquito breeding, and to provide better information for guiding effective mitigation strategies [D. Brooks]
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Mis à jour le 2020/08/11