SLOPES, LAVA FLOW VOLUMES, AND VENT DISTRIBUTIONS ON VOLCAN-FERNANDINA, GALAPAGOS-ISLANDS

Digital elevation data from TOPSAR, an airborne synthetic aperture rad ar system that uses interferometry to derive topography, are used to d etermine slope distributions and lava flow thicknesses on Fernandina V olcano, Galapagos Islands. Four extracaldera slope regions are defined (from the coast inland): A coastal plain (average slope similar to 2 degrees), an apron (6 degrees-12 degrees), steep slopes 250-600 m high (20 degrees-43 degrees), and a 0.5- to 1-km-wide Summit platform (sim ilar to 10 degrees). Lava Bows and vents are mapped using Shuttle Imag ing Radar-C (SIR-C), SPOT, Landsat Thematic Mapper (TM), and air photo s. Over 500 flows are identified and categorized as young, intermediat e, or old based on albedo, vegetation cover, and margin preservation. By area, young flows constitute 55% of the island, 34% are intermediat e, and 7% are old. The aa:pahoehoe ratio of the young flows is 85:15, whereas for the intermediate flows it is 58:42. Of the 423 vents that were classified, 236 are radial and 80% of these are in the apron, 143 are arcuate and 94% of these are in the summit platform, and 46 have transitional orientations and are about equally divided between the st eep slopes and summit platform; 95% of all vents are within 13 km of t he caldera center. TOPSAR data allow flow volumes to be estimated, and young flows range from < 0.01 to 0.12 km(3), with a total volume of 2 .3 km(3). By volume, 91% of the young lava erupted from radial vents b elow the steep slopes, many of which are concentrated within the SE ap ron about 5-6 km from the caldera. Similar concentrations to the NE, N W, and SW consist of young and intermediate flows. Different proportio ns of lava flows and vents form the different slope regions; the coast al plain averages 0.1 vents/km(2) and the slightly steeper apron avera ges 0.6 to 0.9 vents/km(2), increasing inland. The summit platform ave rages 4.7 vents/km(2), and this concentration supports previously prop osed mechanisms for producing higher elevations and steeper slopes in the central part of the volcano. Temporal changes in the plumbing syst em and/or magma supply. rate are suggested by the change in aa:pahoeho e ratio; it appears that in the past, low effusion rate eruptions were more common (perhaps from a filled caldera), whereas more recently, h igh effusion rate eruptions have dominated.