Lunar and Planetary Institute






 

Compiled by
Peter Mouginis-Mark

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Introduction

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.

Eruption Types and Resultant Landforms (Slides 1-20)

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.

Rocks and Boulder Fields (Slides 21-25)

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.

Radar Studies of Lava Flows (Slides 26-31)

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.

Volcanic Structures and Landforms (Slides 32-40)

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.

Acknoledgements

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.

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