EOS Volcanology Logo EOS Volcanology Slide Set #2
VOLCANO TOPOGRAPHY

SLIDE #1 (111K)
Summary of the NASA/JPL TOPSAR system, which is an airborne radar interferometer that is flown on the DC-8 aircraft. See Slide #2 for a view of the actual instrument.


SLIDE #2 (125K)
The TOPSAR radar system, developed by JPL, is flown on the NASA DC-8 aircraft. Here we see Mike Kobrick from JPL standing at the rear of the aircraft, next to the radar system. TOPSAR comprises a pair of C-Band (5.6 cm wavelength) radar antennas that are mounted one above the other (one antenna is in the blue area of the plane's paint work, the other is just in sight at the bottom of the aircraft). L-band (24 cm) and P-Band (70 cm) radars also fly on the DC-8 at the same time, and these are the antennas with small squares (the L-Band system) and large squares (P-Band) that are just above the lower C-Band system. Photo by Scott Rowland.


SLIDE #3 (108K)
Space Shuttle photograph of the Western Galapagos Islands, which comprise one of the Geology Super Sites for the SIR-C / X-SAR missions in 1994. These islands are built from a set of seven volcanoes, which in total have erupted over 60 times over the last 100 years. The main focus of the topography studies is Fernandina Island, which is the smaller of the two islands at lower center in this image.


SLIDE #4 (120K)
Geologists from the University of Hawaii ran a field expedition to the summit of Fernandina volcano in September, 1989, in part to study the landforms produced by the eruption the previous year. Standing on the rim of the caldera, this is what we saw! This view shows the 1,100 meter high caldera wall that collapsed in 1988. The haze in the scene is due to the almost continuous landslides that were still occurring from this unstable slope more than a year after the main collapse. The distance to the other side of the crater is about 4 km in this image. Photo by Pete Mouginis-Mark.


SLIDE #5 (118K)
Investigators at the Jet Propulsion Laboratory and the University of Hawaii are collaborating on the analysis of the TOPSAR data of Fernandina Island. Here we see a digital elevation model for the summit caldera which has been shaded such that high areas are light tones, and low areas are dark tones. As described in the caption for Slide #4, the total depth of the summit caldera is over 1,100 meters. Compare this slide with Slide #3 for the visual data, and Slide #6 for a better impression of the relief. Data processed by Howard Zebker, Harold Garbeil, and Pete Mouginis-Mark.


SLIDE #6 (91K)
The digital elevation data shown in Slide #5 can also be processed on a computer to simulate the shading that would be expected if the sun were at a specific angle above the horizon. This gives a much better impression of the depth of the caldera compared to Slide #5, and can be used to assess the variation in slopes on the flanks (see also Slide #8). Compare this single lighting geometry with those shown in Slide #7. Data processed by Howard Zebker and Harold Garbeil.


SLIDE #7 (165K)
Although a single lighting geometry can aid in the interpretation of topography, it is often more informative to try several different viewing perspectives, such as those shown here. The summit of Fernandina is shown under three lighting directions at 120 degrees to each other. Data processed by Howard Zebker and Harold Garbeil.


SLIDE #8 (92K)
The derivative of the topography can also be computed from a digital elevation model in order to calculate the slopes of a volcano. Here we see that the slopes on the flanks of Fernandina range from less than 5 degrees (blue) to greater than 30 degrees (white). What is particularly interesting is that some of the slopes on the outer flanks exceed 30 degrees. (having seen the summit caldera in Slide #4, it comes as no surprise that the inner walls exceed 30 degrees). These are very steep slopes for a basaltic shield volcano, and must have been produced by unusual structural or magmatic processes. Also interesting is the fact that these outer slopes are not symmetrical around the summit of the volcano. Obviously, something interesting has been going on within the volcano to generate these steep, asymmetric slopes. Trying to understand this process is one of our main research areas at the present time. Image prepared by Harold Garbeil.


SLIDE #9 (128K)
The steep side of the eastern flank of Fernandina volcano is shown clearly in this ground photograph taken in 1989. The area shown here is also seen in the nadir view presented in Slide #10. Notice also that almost all of the many individual lava flows that have erupted on the volcano are vegetation-free, indicating their relatively young age. Photo by Pete Mouginis-Mark.


SLIDE #10 (166K)
One of the most instructive uses of the TOPSAR data comes from the comparison of the radar backscatter (left), panchromatic images taken from the SPOT satellite (center), and radar-derived shaded relief maps (right). These three subscenes show exactly the same area (roughly the field of view in Slide #9). Arrows labeled "A", "B", and "C" point to the same features in the three images, so that one can see that in some cases it is easy to identify the cinder cones (i.e., in the radar data), and in other cases it is the visible wavelength data (i.e., SPOT data) that help in the identification of the lava flows. Prepared by Pete Mouginis-Mark.


SLIDE #11 (147K)
A second comparison between the TOPSAR data (top) and SPOT image for the southern flank of Fernandina volcano. In this instance, the radar-measured slopes are compared to the visible data, and show that little information on the distribution of slopes can be inferred from the location of the lava flows. We do not yet understand what is going on here, but perhaps the general topography of the volcano is a relic of an earlier stage of activity, and the surface flows reflect current surface processes. Image prepared by Pete Mouginis-Mark and Harold Garbeil.



Back to Educational Outreach

EOS Volcanology LogoReturn to Main EOS Volcanology Page