1. Olympus Mons, Mars-Hawaii Comparison
Compiled by Peter Mouginis-Mark, University of Hawaii
(Underlined terms are defined in the glossary found at the end of the slide descriptions.)
Because of their frequent occurrence and relatively nonexplosive character, Hawaiian eruptions are some of the most extensively studied in the world. Observing the emplacement of lava flows or the dispersal of ash from a fire fountain provides detailed information on the way that volcanos work. Because of their similar composition and shape, the two most active Hawaiian volcanos (Mauna Loa and Kilauea) are also studied by geologists who look at the other planets. Indeed, if you can't actually be working on the Moon, Mars, Venus, or the jovian moon Io, Hawaii is the next best place to be!
This slide set is intended to show that although Hawaiian volcanos are small when compared to those found on the other planets, we can learn a considerable amount about how the extraterrestrial features were formed. Through the comparison of the size of lava flow fields and the dimensions of channels on volcanos, the mode of emplacement of deposits seen in Viking Orbiter, Apollo, Voyager, and Magellan images can be inferred. Furthermore, by using Hawaiian volcanos as test sites for the analysis of remote sensing instruments such as imaging radars and thermal infrared spectrometers, the planetary volcanologists can better infer the physical characteristics of volcanos on the different planets.
The views of Hawaiian volcanos and volcanic features provided here are intended to show the diversity of volcanic landscapes in Hawaii as they may relate to volcanos on the planets. This is not meant to be a comprehensive view of the geology of Hawaii, nor are all the volcanos found in Hawaii presented in this set. Rather, type-examples of lava flow structures, cones, types of eruptions, and their deposits are compared to features that are believed to have a similar origin on the planets. Whenever possible, the scale of the planetary example is shown by means of an insert of one or more of the Hawaiian Islands--demonstrating the amazing size of features on Mars, Venus, and Io.
The content of the slide set is arranged so that five general topics are reviewed. These topics may form the focus of individual high-school classes, or they may be useful as segments in an undergraduate class. A brief discussion of the relevance of each topic to planetary volcanology is included prior to each group of slide captions.
Lava Flows and Flow Fields on the Planets
Lava flows and flow fields are often among the first geomorphic features that help identify a volcanic center on a planetary surface. For example, radar images from the Magellan spacecraft clearly show the radial distribution of flows from a volcano without a summit crater being identifiable. There are two different types of lava flows in Hawaii. Pahoehoe lava has a relatively smooth surface and is typically less than 1 meter thick. A'a flows are much more rugged and can be up to 20 meters thick. This difference in flow morphology is caused by variations in the rate of eruption of the lava and the different viscosities; pahoehoe is produced with low-effusion-rate eruptions (less than a few cubic meters per second) that have lower viscosity, while a'a is produced by higher-volume discharge of higher-viscosity lava.
It is currently unknown what types of lava occur on Mars and Io, but the strong radar backscatter from many flows on Venus suggests that a'a may be common. The large volumes of flows on Mars would suggest that discharge rates were probably at least as high as big a'a eruptions on Earth, otherwise the eruption duration would have had to have been hundreds of years in order to produce the observed volume of individual flows. Therefore, the research objectives for the analysis of Mars Observer data will include the investigation of the meter-scale morphology of the flows to attempt to identify features found on pahoehoe or a'a flows, and an analysis of the topography of the flows to better understand their slopes and volumes.
1. Olympus Mons, Mars-Hawaii Comparison
The martian volcano Olympus Mons (18°N, 133°W) is one of the largest volcanos in the solar system, measuring over 600 kilometers across and rising more than 27 kilometers above the surrounding plain. In this view, Olympus Mons is compared to the Hawaiian Islands, and demonstrates the value of comparative planetary volcanology. Notice that the Island of Oahu would easily fit inside the summit caldera of the martian volcano. Volcanologists on Earth have a good understanding of Hawaiian volcanism, but the eruptive style and duration of planetary examples such as Olympus Mons remain poorly understood. (Viking Orbiter 646A28.)
2. The 1859 Mauna Loa Lava Flow, Hawaii
This view of the west coast of the Big Island of Hawaii, taken from the space shuttle, shows the entire 50-kilometer length of the 1859 lava flow from Mauna Loa volcano. This is one of the largest lava flows in the world that has been erupted in historic times. The eruption occurred at the 11,000-foot elevation level (close to the summit at right), lasted about 200 days, and produced a pahoehoe and a'a lava flow that traveled all the way to the coast (left). Although one of the largest young lava flows on Earth, the Mauna Loa flow is small compared to most of the flows seen on Mars (slide #6), Io (slide #7), and Venus (slide #8). (NASA S09-46-1841.)
3. A'a Lava Flow, Hawaii
A'a lava flow is typically blocky, usually approximately 3-20 meters thick, and rolls over itself across the ground like a tank track. The jagged flow front normally creeps forward and steepens until a section becomes unstable and breaks off, revealing the incandescent central core. This flow is about 4 meters thick. (Photograph courtesy of S. Rowland.)
4. Pahoehoe Lava Flow, Hawaii
As a pahoehoe flow spreads out across the ground, the flow surface cools and the majority of lava transport takes place through a series of tubes. In this view the leading edge of the flow field is advancing via a series of break-outs of lava from such a tube system. As can be seen from the scale bar in the image (which is marked in 5-centimeter increments), pahoehoe flows are much thinner than a'a flows, sometimes being only 30-50 centimeters thick. (Photograph courtesy of P. Mouginis-Mark.)
5. Pahoehoe Lava Flow, Hawaii
The classic "ropy" texture of a pahoehoe lava flow is shown here. Scale bar is marked in 5-centimeter increments. (Photograph courtesy of P. Mouginis-Mark.)
6. Lava Flow Field, Elysium Planitia, Mars
There are several fine examples of compound martian lava flow fields. This one is just west of the volcano Hecates Tholus, within Elysium Planitia (33°N, 214°W). Several lava flows more than 100 kilometers in length can be found (the direction of flow in this image is from the bottom to the top of the slide). The insert compares these martian flows to the recent activity at Pu'u O'o on the East Rift Zone of Kilauea, Hawaii. Shown in red are the Pu'u O'o lavas erupted between 1983 and 1986, and in blue are the flows erupted from the Kupaianaha lava pond between 1986 and 1991 (slide #19). The distance between the Kupaianaha vent and the coast is approximately 10 kilometers. (Mosaic of Viking Orbiter 651A07-14.)
7. Ra Patera, Io
Long lava flows exist on other planetary bodies including Io, a moon of Jupiter. At top right we see a sequence of flows over 150 kilometers in length that have been erupted from Ra Patera (8.4°S, 325.3°W). The state of Hawaii, which is approximately 130 × 150 kilometers in size, has been superimposed in red on this image for a scale comparison. (Voyager 16390.06; image enhancement by A. McEwen.)
8. Sapas Mons, Venus
Numerous large overlapping lava flows are shown in this Magellan radar image of Sapas Mons on Venus (8°N, 188°E). Sapas Mons is approximately 400 kilometers across and 1.5 kilometers high. Smooth (pahoehoe) lava flows typically appear dark in radar images, while rougher (a'a) flows will have a higher radar backscatter and so will appear bright in Magellan data. From this scene, we can infer that long lava flows on Venus are comparatively rough at the radar wavelength of 12 centimeters. The Big Island of Hawaii, which is approximately 130 × 150 kilometers in size, has been superimposed in blue for scale comparison. (JPL Magellan MGN P-38360.)
Eruption Types and Resultant Landforms
With the exception of Io (where eruptions still occur today), we have to infer what volcanic eruptions were like on the planets through analysis of deposits preserved on the surface and by analogies with eruptions seen on Earth. In Hawaii, the frequent eruptions and fine field exposures allow us to find many planetary analogs. Good examples of lava channels forming in the middle of lava flows can be observed, as can the rate of cooling of materials thrown from explosive eruptions and the temperature distribution of active lava lakes.
Field studies in Hawaii provide important new insights into the way that eruptions built volcanos on the planets. Only by observing the dynamics and temperature distribution of an active lava lake in Hawaii was it possible to show that the crust of a lava lake can be as cool as the surfaces seen in lava lakes on Io. Similarly, observations of explosive eruptions that were produced as lava flows entered the ocean gave insights into the formation of large ash cones found on the island of Oahu. These observations in turn aid the interpretation of ash deposits found in Hawaii and the larger, old, highland volcanos of Mars, such as Tyrrhena Patera.
9. Pu'u O'o Lava Channel, Hawaii
During many of the eruptions of Pu'u O'o Volcano, Hawaii, pahoehoe lava flows became channelized so that the margins (called "levees") became more solid and the lava was moving most rapidly in a central channel. These central channels can also be found within certain lunar and martian lava flows and are thought to be an indicator of high discharge rates (more than 10 million kilograms per second). The central channel shown here is approximately 4 meters wide. (Photograph courtesy of S. Rowland.)
10. Thurston Lava Tube, Hawaii
In Hawaii, many lava flows form tubes that may extend for several kilometers. This is an efficient way for the lava to travel comparatively large distances without significant cooling. Thurston Lava Tube, located close to the summit of Kilauea Caldera, is a fine example of this type of landform. Here the tube is approximately 3 meters in diameter. Note two lava benches on the wall on the left. (Photograph courtesy of P. Mouginis-Mark.)
11. Marius Hills, Moon
Although most lunar volcanism produced the broad lava flows that infill the lunar maria, in a few places, such as the Marius Hills (14°N, 56°W), it is possible to find volcanic domes. In this scene we can see several lunar domes. Some of these domes are quite smooth and low, while others are more rugged and heavily cratered. Two large sinuous rilles similar to Hadley Rille (slide #12) can also be seen cross-cutting a mare ridge. (LOV-214M.)
12. Hadley Rille, Moon
The Apollo 15 mission visited Hadley Rille near the Apennine Mountains (26.4°N, 3.7°E). Hadley Rille is a sinuous rille that was probably formed as a lava channel not unlike the Hawaiian example shown in slide #9, except that the lunar example is much larger than any lava channel found on Earth. In the foreground we can see astronaut James Irwin attending the lunar rover. At this locality, Hadley Rille is approximately 1.5 kilometers wide and approximately 300 meters deep. (Apollo 15 AS15-85-11451.)
13. Fire Fountain Eruption, Pu'u O'o, Hawaii
Scientists studied fire fountains from the phase-34 eruption (August 1984) of Pu'u O'o in Hawaii to investigate the dispersal and cooling of ejected materials. In this view, the incandescent part of the plume is approximately 200 meters high. Large clasts from this fire fountain retained their heat to such an extent that they coalesced on the ground to form a lava flow that moved away from the vent (at right in this view). Pu'u O'o is located about 15 kilometers downrift from the summit of Kilauea Caldera, on the East Rift Zone. (Photograph courtesy of P. Mouginis-Mark.)
14. Alphonsus Crater, Lunar Nearside
This is an oblique Apollo 16 view of Alphonsus Crater on the lunar nearside (13.4°S, 2.8°W). This crater is particularly interesting because, although it was formed by an impact, it has fractures on its floor and there are symmetric dark patches around some of these fractures. The dark patches are believed to be volcanic in origin, and were most likely produced by explosive eruptions similar, perhaps, to the Pu'u O'o example shown in slide #13. Alphonsus Crater is approximately 120 kilometers in diameter. The island of Oahu, which is approximately 45 × 65 kilometers in size, has been superimposed in red for a scale comparison. (Apollo 16 AS16-M-2468.)
15. Lava-Sea Interactions, Kilauea, Hawaii
Several littoral cones were produced at the coast by the lava flows from Kupaianaha Lava Lake on the East Rift Zone of Kilauea Volcano. At the shoreline, small strombolian eruptions associated with seawater flooding an open lava tube produced cones up to approximately 10 meters high in only a few hours. These littoral eruptions may be similar not only to the ones that produced Koko Crater on Oahu (slide #17), but also Tyrrhena Patera, Mars (slide #18). (Photograph courtesy of P. Mouginis-Mark.)
16. Ash Deposits, Koko Crater, Oahu, Hawaii
Close inspection of the deposits from Koko Crater shows the multiple episodes of explosive activity that were parts of this shallow marine eruption. Here a scientist stands next to a large bomb sag produced by a fragment of coral reef that was ejected from the terrain that existed prior to the eruption. Evidently the layers of ash (which are typically less than 1 centimeter thick) were very wet on their emplacement, since the limestone block penetrated almost a meter into the preexisting surface of the cone, only to be buried by sediments that were derived by surface wash from the eruption. (Photograph courtesy of P. Mouginis-Mark.)
17. Channels on Koko Crater, Oahu, Hawaii
The flanks of Koko Crater on the island of Oahu, Hawaii, show signs of extensive gully erosion. Koko Crater is approximately 400 meters high. Here we see valleys 3-5 meters deep that have been caused primarily by surface water flow and, close to the summit, by sapping. Although spaced further apart on martian volcanos such as Tyrrhena Patera (slide #18), similar valleys may have formed on the older volcanos on Mars as water from the original explosive eruptions was released at the surface. Note, however, that the valleys on Tyrrhena Patera may be 3-5 kilometers wide, which is wider than the entire Koko Crater cone. (Photograph courtesy of P. Mouginis-Mark.)
18. Summit of Tyrrhena Patera, Mars
This radial valley system on a martian volcano is found at Tyrrhena Patera (22°S, 253°W), which also appears to be one of the oldest highland volcanos on the basis of the number of large superposed impact craters. The formation of this volcano may have been characterized by phreatomagmatic eruptions similar to those seen at the coast on the Big Island (slide #15), except at a much larger scale. At Tyrrhena Patera, multiple layers of material (probably ash?) can be seen exposed in the walls of the valleys, which in some places may be 200-300 meters deep. The grid on the image is 50 kilometers. The island of Oahu, Hawaii, has been superimposed in red for scale comparison. (Viking Orbiter 87A14; rectified image produced by M. Robinson.)
19. Temperature Measurements, Kupaianaha Lava Lake, Hawaii
Using a spectroradiometer, scientists were able to measure the radiative temperature and thermal output from the Kupaianaha Lava Lake. This instrument collects spectra from 0.4-3.0 micrometers in over 800 channels, thereby permitting an accurate determination of the blackbody temperature of the surface. Such studies are of value because they show that the surface of a lava lake (or a lava flow) is remarkably cool--perhaps only a few hundred degrees centigrade--compared to the eruptive temperature of approximately 1150°C. Such temperatures are quite similar to those the Voyager 1 spacecraft measured for the volcanic activity on Io (slide #20), suggesting that silicate lavas, as opposed to molten sulfur, could exist within lava lakes on Io. (Photograph courtesy of P. Mouginis-Mark.)
20. Volcanos on Io
Voyager 1 revealed many fresh volcanic landforms on Io, the active moon of Jupiter. In addition to the famous explosive eruption plumes that first attracted the attention of the planetary geology community, Io also has several active lava lakes. The largest of these lakes is at Loki Patera, seen at the lower right of this image. Loki Patera is over 200 kilometers in diameter. (Voyager 16389.42; image enhancement by A. McEwen.)
Rocks and Boulder Fields
When we look at a volcano from orbit we obtain a valuable regional view. However, this is a radically different view from the one that is obtained by a field volcanologist working on Kilauea or Mauna Loa Volcanos; it is at this scale that many of the key observations of volcanic processes are made. For planetary volcanic features, we have views on the surface of the Moon thanks to the Apollo astronauts, and the two Viking landers imaged the surface of Mars. Soviet Venera spacecraft also imaged the surface of Venus, but the origin of these landscapes is unclear, so no Venus views are included.
Close inspection of the rocks seen on the planets, as well as the spatial distribution of rocks and boulders around impact craters and volcanos, can aid in our analysis of orbital images. The identification of volcanic rocks is particularly difficult without the advantage of geochemical data. Lunar rocks returned by the Apollo 15 and 17 missions have many pits (called "vesicles") that were formed when gas became trapped in volcanic rock. Similar pits are also seen in rocks at the Viking Lander 2 site, suggesting that they too have a volcanic origin. However, we do not know for sure that this is the case, since martian rocks will also be eroded by salts from the soil and scoured by fine particles carried by the thin martian wind. Through a comparison of the shapes and sizes of vesicles in Hawaiian rocks, it may prove possible to identify the mode of origin of the martian rocks.
21. Vesicular Volcanic Rock, Hawaii
Pits or vesicles in volcanic rocks are caused by exsolved volatiles emerging as gas bubbles on eruption. The vesicles seen here are in lavas erupted from Mauna Ulu Volcano in 1969. A camera lens cap provides the scale. (Photograph courtesy of P. Mouginis-Mark.)
22. Apollo 15 Rock, Moon
Highly vesiculated basalts have been found on the Moon as well as Earth (slide #21). In 1971 the Apollo 15 mission to Hadley Rille (26°06'N, 3°39'E) returned sample 15556, which has a mass of 1538 grams. This basalt was returned from station 9A, about 60 meters from the rim of Hadley Rille (slide #12). Basaltic lavas from this chemical group at the Apollo 15 landing site formed about 3.3 billion years ago. Scale (in centimeters) is given on the bar at the top of the image. (NASA JSC S-71-45243.)
23. Boulder Field, Halemaumau Crater, Hawaii
The views provided of the lunar (slide #24) and martian (slide #25) surfaces provide tantalizing glimpses of the meter-scale geology of these worlds. On both the Moon and Mars the landscapes are strewn with rocks ranging from millimeters to approximately 1 meter in diameter. Trying to find terrestrial analogs to these planetary rock fields is often difficult because of the nature of the fragmentation processes involved in the formation of ejecta blocks. The boulder field produced by the May 1924 eruption of Kilauea Volcano, Hawaii, provides one of these rare terrestrial analogs because an extensive boulder field was produced by the rapid fragmentation of basaltic rock during an explosive eruption. In this instance, a phreatomagmatic eruption of Halemaumau Crater fragmented preexisting volcanic country rock and scattered this material across the adjacent landscape in a process that was probably very similar to the formation and emplacement of ejecta blocks during an impact cratering event. This view of the rim of Halemaumau Crater shows many angular fragments that at first impression appear similar to the boulder fields seen at Taurus-Littrow and Utopia Planitia. The largest blocks seen in this view are approximately 1.2 meters in diameter. (Photograph courtesy of P. Mouginis-Mark.)
24. Surface of Moon at Taurus-Littrow
This is a surface view of the ejecta blanket of the impact crater Camelot, close to station 5 at the Apollo 17 landing site at Taurus-Littrow (20°10'N, 30°46'E). Camelot Crater (at left) is about 650 meters in diameter. The blocks within the ejecta blanket are primarily basalts that are now partly buried by soil. These blocks vary from cobble to boulder size, are subrounded to subangular, and cover approximately 30% of the surface. Sample numbers 75015 and 75035 were collected from within the field of view at lower right. Footprints in this area provide the scale. (Apollo 17 AS17-145-22160.)
25. Viking 2 Landing Site, Utopia Planitia, Mars
Although no samples were returned to Earth from either of the Viking Lander sites, several rocks at the Viking Lander 2 site at Utopia Planitia (48°N, 226°W) appear to be volcanic in origin because of their deeply pitted surfaces. How these rocks actually came to be scattered across the surface is still a question for debate, since they may be the products of nearby volcanic activity or by the transport of impact ejecta over larger distances. In this view, the footpad of the lander is seen at bottom right. The rock just at the top of the footpad is approximately 35 centimeters across. (Viking Lander 22A001.)
Radar Studies of Lava Flows
The Magellan spacecraft has returned some spectacular data for the surface of Venus using its imaging radar system. Unlike normal photographs, radar images show brightness variations that are caused by spatial variations in the roughness of the surface at the scale of the radar wavelength (12 centimeters). Thus a pahoehoe lava flow should be smooth to the radar, and will reflect only a small amount of the signal back to the spacecraft. This will result in a radar-dark image of a pahoehoe lava flow. Conversely, a'a flows are topographically rough at the wavelength of the Magellan radar, and so will reflect a significant amount of energy back to the spacecraft. This produces a radar-bright image of the a'a flow.
There have been several radar experiments in Hawaii that have used data collected from the space shuttle (the SIR-B experiment in 1984) and the NASA/JPL aircraft radar. These experiments were in part intended to study how easy it is to identify known lava flow types, fractures, and volcanic craters.
26. SIR-B Radar Data and Air Photography, Kilauea, Hawaii
A direct comparison between the space shuttle imaging radar (SIR-B) radar data and air photography of the December 1974 lava flow (Ka'u Desert, Hawaii) shows how easily the a'a portion of the flow can be identified against a background of poorly reflecting ash deposits. In contrast, the pahoehoe portions of the flow (at right) are barely visible in the SIR-B data because of their specular characteristics (at 48° incidence angle most of the radar energy is reflected away from the radar antenna). Compare this remote-sensing view with the ground photographs of the same area shown in slide #27. North is to the right in this view. (SIR-B 115.2.)
27. December 1974 Lava Flow, Kilauea, Hawaii
A ground photograph of the December 1974 lava flow (slide #26) shows the difference between the radar-bright a'a flows (dark in this image) and the smoother pahoehoe flows. The field of view is about 1 kilometer from side to side and is looking west across the Ka'u Desert. The profile of Mauna Loa is in the background. (Photograph courtesy of P. Mouginis-Mark.)
28. A'a Topography, December 1974 Lava Flow, Kilauea, Hawaii
The radar roughness of the December 1974 Kilauea lava flow was investigated using a template that measured the undulations of the surface at an interval of 2 centimeters horizontal spacing. This part of the a'a flow is about 5 kilometers away from the vent. Compare this view with the roughness of typical pahoehoe flows (slide #29). (Photograph courtesy of L. Gaddis.)
29. Pahoehoe Topography, December 1974 Lava Flow, Kilauea, Hawaii
The same template that was used to measure the a'a portion of the December 1974 flow (slide #28) is seen here on a pahoehoe surface within 1 kilometer of the vent. (Photograph courtesy of L. Gaddis.)
30. Radar-Dark Lava Flows, Lavinia Planitia, Venus
There are few large, smooth lava flows on Venus, and they are rarely found on the rolling plains. Although the range of incidence angles provided by Magellan is not optimal for the mapping of radar-dark flows on Venus that might be the equivalent of pahoehoe flows on Earth, there are a few examples such as the one seen here. In this example from Lavinia Planitia (55°S, 354°E), the dark flow at the top of the image may be a compound pahoehoe flow field similar to the smooth compound lava flows seen on Kilauea Volcano (slide #4). (Magellan F-MIDRP.55S355;1.)
31. Lava Flows at Summit of Sif Mons, Venus
The radar characteristics of lava flows on Venus can be quite instructive regarding the distribution of lava flow types. In this Magellan radar image of the summit area of the Sif Mons volcano (slide #33) on Venus (24°N, 352°E) we can see that radar-smooth flows (pahoehoe?) are found close to the western rim of the summit caldera, while radar-bright (a'a?) flows make up the more distal flows and surface units on the eastern flanks. This relationship is the same as the one observed in the Ka'u Desert with the SIR-B data (slides #26 and #27), perhaps indicating similar eruption characteristics. (Magellan F-MIDRP.20N351;1.)
We can learn a lot about the inside of a volcano from an analysis of the spatial distribution of fissures and small vents on its flanks. In Hawaii, Mauna Loa and Kilauea show clear trends in the location of eruptions along lines of weakness called "rift zones." These rift zones mark the locations of subsurface magma transport within the volcano.
On Oahu, we can actually see inside the 2-million-year-old Koolau Volcano because of the large amount of erosion that has taken place on that island. Sections of the volcano that were once 1 kilometer beneath the summit can now be seen at the surface, so that we can study the distribution and number of dikes. In addition to furthering our understanding of the way that Hawaiian volcanos work, information on rift zones and fissures is also valuable for the interpretation of volcanos on Venus such as Sif Mons, and the caldera complex at the summit of Olympus Mons on Mars.
Field observations of Hawaiian rift zones provide detailed knowledge of the structure of the volcano, but to be of value to planetary volcanology, it is also important to explore additional methods for the identification of these features. Through the use of data collected from the Landsat Thematic Mapper (TM) and the airborne Thermal Infrared Multispectral Scanner (TIMS), Mauna Loa is being used as a test site that may help develop methods for the identification of similar features on Mars using data from the Mars Observer spacecraft.
32. Mauna Loa Volcano and Mauna Kea Cinder Cones
The shallow slopes of Mauna Loa are clearly seen is this photograph taken looking south from the southern flank of Mauna Kea. Notice some of the cinder cones of Mauna Kea in the foreground; these cones are up to 400 meters high and are included in the set shown in slide #38. (Photograph courtesy of P. Mouginis-Mark.)
33. Sif Mons Volcano, Venus
This view of the volcano Sif Mons (24°N, 352°E) was obtained by combining Magellan altimetric measurements and a radar backscatter image. Sif Mons is about 2 kilometers high and about 300 kilometers in diameter. A series of bright and dark lava flows extends for a distance of approximately 120 kilometers from the summit area, suggesting very fluid lavas. If Venus lava flows have the same structures as those on the Earth, the bright lava flows may be a'a lava, while the darker flows may be pahoehoe. Note that the vertical scale of the image has been exaggerated to accentuate the subtle relief of the volcano. (JPL Magellan MGN P-37342.)
34. Vent Area, December 1974 Lava Flow, Kilauea, Hawaii
The December 1974 lava flow erupted from a series of en echelon fissures close to the summit of Kilauea Caldera. Here is an example of a fissure approximately 2 meters wide that is invisible on the SIR-B radar image (slide #26). Fissures of comparable dimensions may also have been the vents for the flows on the flanks of Sif Mons, Venus (slide #33). (Photograph courtesy of L. Gaddis.)
35. Oahu, Hawaii
The Island of Oahu, Hawaii, provides many opportunities to see the inside of a basaltic shield volcano because of the considerable degree of erosion of the Koolau Volcano. Here at a road-cut we see numerous volcanic dikes 0.5-1.0 meters wide that formed along the main rift zone of the Koolau Volcano. Similar dikes are almost certainly present within the rift zones of Kilauea and Mauna Loa Volcanos, and act as pathways for the magma as it moves from the magma chamber to the surface. (Photograph courtesy of P. Mouginis-Mark.)
36. Summit Caldera, Olympus Mons
The summit caldera of Olympus Mons comprises several major collapse features that indicate a protracted period of activity at the summit. Parts of the caldera floor possess a complicated sequence of graben and wrinkle ridges that suggest a complex tectonic history. The lowest parts of the floor are over 4 kilometers below the rim of the caldera (slide #37). (Viking Orbiter 890A68.)
37. Olympus Mons Caldera Topography and Chronology
Olympus Mons was a target for some high-resolution (approximately 15 meters/pixel) imaging during the final phase of the Viking Orbiter Survey Mission, and a high-quality image dataset was developed, from which a topographic map and a chronology of summit collapse could be derived. Here the topography (in kilometers above the 6.1-millibar datum) and the sequence of collapse events as determined by Mouginis-Mark and Mathews (1987) are compared. See slide #36 for the geomorphology of the caldera. (Slide courtesy of M. Zuber.)
38. Mauna Kea Summit
The Landsat Thematic Mapper (TM) obtains data of volcanos on Earth in six spectral bands between 0.4 and 2.4 micrometers at a resolution of 30 meters/pixel, and one band between 8.0 and 12.0 meters at 100 meters/pixel. Here, TM data allow iron oxides (shown in red) in the cinder cones of Mauna Kea (slide #32) to be identified. Also visible in blue is the distribution of debris generated during the last ice age, which shows that Mauna Kea has not been active for several thousand years. North is at the top. (Landsat 5 path 63, row 40; image enhanced by H. Garbeil.)
39. Mauna Loa, Hawaii
The summit area and northeast rift zone of Mauna Loa, Hawaii, are shown in this Landsat Thematic Mapper image, which highlights pahoehoe lava flows in red and a'a flows in black. Compare this view of the summit to the oblique view of the northwest flank (slide #2) and the summit caldera (slide #40). North is at the top. (Landsat 5 path 63, row 40; image enhanced by H. Garbeil.)
40. Mokuaweoweo Caldera, Mauna Loa, Hawaii
Experience with the analysis of Hawaiian volcanos using thermal infrared (8.0-12.0 micrometers) data has shown that it is much easier to map subtle lava flow boundaries using such data rather than conventional photography or near-infrared (1.0-2.4 micrometer) images. Such observations suggest that mapping lava flows on martian volcanos may also be possible by using the Mars Observer Thermal Emission Spectrometer (TES), which operates in the same spectral range. Here we see three different band combinations of data of the summit caldera of Mauna Loa Volcano (Mokuaweoweo Caldera) that were obtained by the NASA/JPL airborne Thermal Infrared Multispectral Scanner (TIMS) instrument. Mokuaweoweo Caldera is approximately 4.0 kilometers long by 2.5 kilometers wide. North is at the top of each image. (Images courtesy of H. Garbeil.)
The following people provided some of the images that appear in this set: L. Gaddis, H. Garbeil, A. McEwen, P. Mouginis-Mark, M. Robinson, S. Rowland, and M. Zuber.
a'a (AH-ah) - A type of basalt lava flow characterized by an extremely rough clinkery top and a dense interior. A'a flows are usually 110 meters thick, have flow-front velocities between 10 and 2000 meters/hour, and are associated with high-fountaining high-effusion-rate eruptions.
blackbody temperature - The temperature of an object if it is reradiating all the thermal energy that has been added to it. If an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.
bomb sag - A depression left in finer-grained pyroclastic layers by the impact of a larger bomb or block.
caldera (call-DAIR-uh) - A depression in the summit of a volcano (usually greater than 1 kilometer across) caused by subsidence into a magma chamber.
clast - A fragment of rock that has been transported, either by volcanic or sedimentary processes.
ejecta blanket - The material deposited around the rim of a crater formed by the impact of a meteorite or comet on the surface of a solid planet. Ejecta may be thrown to several times the radius of the crater, depending on the planet where the crater forms. High atmospheric pressure and high surface gravity tend to concentrate the ejecta close to the crater rim, and vacuum conditions and low gravity promote a widely distributed ejecta blanket.
en echelon fissures (on ESH-uh-lon FISH-ures) - Fissures that are parallel in trend to each other, but offset to either the left or right.
graben (GRAH-bun) - A depression formed by the dropping of a central block between two parallel inward-sloping normal faults. Graben indicate a zone of horizontal tension.
impact crater - A crater formed by the collision of a meteoroid and the surface of a planet. The floor of an impact crater is always lower than the level of the terrain in which the crater is excavated. Impact craters can have any size, from micrometers to hundreds of kilometers in diameter.
incidence angle - For radar imaging, the angle (measured from vertical) between an object and a satellite that is observing the object. An incidence angle of 0° is called a "nadir" (NAY-deer) view.
littoral (LIT-uh-ral) cone - A pyroclastic cone formed of fragments of a lava flow thrown out by steam explosions that take place when the lava flow enters the ocean.
mare (MAH-ray) (pl. maria; MAH-ree-ah) - The dark areas of the Moon, which consist of basalt lava flows erupted into depressions (typically old large impact craters called "basins") a few tens to hundreds of millions of years after the impact event.
pahoehoe (pa-HO-ee-HO-ee) - A type of basalt lava flow characterized by a smooth glassy skin, and constructed of innumerable "flow units" called "toes." Pahoehoe flows advance at rates between 1 and 10 meters/hour, and are associated with low-effusion-rate eruptions with little to no fountaining.
phreatomagmatic (free-AT-oh-mag-MAT-ic) - A style of volcanic eruption that takes place when the rising magma contacts shallow groundwater, or erupts under shallow oceanwater or lakes. The water and magma mix and generate strong steam explosions that blast apart the magma (and any preexisting rock) into fine-grained material. The resulting deposits form distinctive structures called tuff rings or tuff cones.
radar backscatter - The radar energy that is reflected back to a detector. The amount of backscatter is high if the surface is perpendicular to the path of the radar wave, so unless it is perfectly oriented, a smooth surface will bounce the radar energy off in some other direction and appear to be dark. A rough surface, on the other hand, has lots of small surfaces in all orientations. If enough of them happen to be oriented perpendicular to the radar path, the surface will reflect enough energy to appear bright.
rift - In general a rift is a fracture or crack in the Earth's surface caused by extension. On some volcanos subsurface intrusions are concentrated in certain directions and this causes tension at the surface and also means that there will be more eruptions in these "rift zones."
sapping - A process of erosion where water leaks to the surface through the pores of rocks. As the water flows away it slowly removes material to form valleys and channel networks.
shield volcano - The type of volcano that forms when the erupting lava has a low viscosity such as basalt. Such lava cannot construct steep slopes, so shield volcanos are characterized by broad gentle slopes.
sinuous rille (SIN-you-us RILL) - Narrow, winding, ditchlike channels found on the Moon and Mars. Sinuous rilles are believed to be tubes and channels that carried flowing lava.
spectroradiometer (SPEC-tro-RAY-dee-om-it-er) - A device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths.
specular (SPEC-you-ler) - A surface that is smooth at the wavelength of the energy falling on it, permitting a mirrorlike reflection of the incident energy.
strombolian (strom-BOH-lee-un) - A style of eruption characterized by repeated relatively small explosions, and associated with basaltic and andesitic volcanos. Strombolian eruptions form large cinder cones.
subangular - Describes a fragment or clast that shows evidence of having been broken off a larger piece, but the original sharp edges and points have been abraded a little.
subrounded - Describes a fragment or clast that broke off a larger piece, but has undergone enough abrasion since then that the edges and points are almost unrecognizable.
wrinkle ridge - A sinuous ridge common on the Moon and Mars that may be a few kilometers in width and hundreds of kilometers in length. Wrinkle ridges are most likely tectonic features formed by horizontal compression.
Carr M. H. and Greeley R. (1980) Volcanic Features of Hawaii: A Basis for Comparison with Mars. NASA SP-403. 211 pp.
Cattermole P. (1989) Planetary Volcanism. Ellis Horwood, Chichester, England. 443 pp.
Decker R. W. and Decker B. B. (1991) Mountains of Fire. Cambridge Univ., New York. 198 pp.
McDonald G. A., Abbott A. T., and Peterson F. L. (1986) Volcanos in the Sea. Univ. of Hawaii, Honolulu. 517 pp.