Chapter 20: Volcanoes in the Solar System

The Moon

Earth's moon is covered with vast plains of basaltic lava -- but no active volcanoes. Volcanism on the Moon ended billions of years ago.

The Earth's Moon has no large volcanoes like Hawaii or Mount St. Helens. However, vast plains of basaltic lavas cover much of the lunar surface. The earliest astronomers thought, wrongly, that these plains were seas of lunar water. Thus, they were called " mare " (pronounced "mahr-ay"). Mare means "sea" in Latin. In addition, other volcanic features also occur within the lunar mare. The most important are sinuous rilles , dark mantling deposits ,and small volcanic domes and cones . Most of these features are fairly small, however. They form only a tiny fraction of the lunar volcanic record.

1. Oceanus Procellarum	2. Mare Imbrium	3. Mare Cognitum	4. Mare Humorum
5. Mare Nubium	6. Mare Frigoris	7. Mare Serenitatis	8. Mare Vaporum
9. Mare Tranquillitatis	10. Mare Nectaris	11. Mare Humboldtianum	12. Mare Crisium
13. Mare Fecunditatis	14. Mare Marginis	15. Mare Smythii	16. Mare Australe
17. Mare Moscoviense	18. Mare Ingenii	19. Mare Orientale

Differences from Earth:

• Volcanism on the Moon differs in several ways from volcanism on the Earth. First, there is the matter of age. 
  • Volcanism on the Earth is an ongoing process. Many of Earth's volcanoes are quite young in geologic terms, often less than a few 100,000 years old. 
  • In contrast, most volcanism on the Moon appears to have occurred between 3 and 4 billion years ago. Typical mare samples are ~3,500,000,000 years old. Even the youngest mare flows have estimated ages of nearly 1 billion years. These "young" rocks have not been sampled or directly dated, however, so this age is very poorly known. For comparison, the oldest dated rock on the Earth is ~3.9 billion years old. The oldest sea floor basalts on Earth are only about 200 million (0.2 billion) years old. Because the Moon does not show any evidence for recent volcanic or geologic activity, it is sometimes called a "dead" planet.

   

  The settings of mare volcanism reveal another major difference from volcanism on the Earth. 

   

  • Specifically, Earth's volcanoes mostly occur within long linear mountain chains. Mountain chains like the Andes mark the edge of a lithospheric plate. Mountain chains like the Hawaiian Islands mark past plate movements over a mantle hotspot. 
  • In contrast, the mare typically occur in the bottoms of very large, very old impact craters. Thus, most of the mare are nearly circular in shape. 
  • Further, lunar mountain chains form the edges of these impact basins and tend to surround the lunar mare. There is no evidence that any system of plate tectonics ever developed on the Moon. 

   

  • Finally, the lunar mare are primarily found on one side of the Moon. They cover nearly one third of the lunar nearside (see figure), but less than 2% of the lunar farside. The surface is much higher on the farside, however, and the crust is typically much thicker there as well. Thus, the primary factors controlling volcanism on the Moon appear to be surface elevation and crustal thickness.

   

Finally, there are some major physical differences between volcanism on the Earth and on the Moon. 

• First, lunar gravity is only one sixth that of the Earth's. This means that the forces driving lava flow are weaker on the Moon. Thus, the very flat and smooth mare surfaces imply that mare lavas were very fluid. They could both flow very easily and spread out over large areas. 

 

• Also, the low gravity means that explosive eruptions can throw debris further on the Moon than on the Earth. Indeed, such eruptions on the Moon should spread lavas out into a broad flat layer and not into the cone-shaped features seen on the Earth. This gives one reason for why large volcanoes are not seen on the Moon. 

 

• Second, the Moon has essentially no dissolved water. The lunar mare are all bone dry. In contrast, water is one of the most common gases in Earth lavas. Water also plays a major role in driving violent eruptions on the Earth. Thus, the lack of lunar water should strongly affect lunar volcanism. In particular, without water, violent explosive eruptions are much less likely on the Moon. Instead, lavas should just flow smoothly and quietly out onto the surface.

MARS

Major Sites of Volcanism

Overview

• Mars has the largest shield volcanoes in the solar system.  
• It also has a wide range of other volcanic features. These include large volcanic cones, unusual patera structures, mare-like volcanic plains, and a number of other smaller features. However, volcanic features are not common. There are less than 20 named volcanoes on Mars, and only 5 of these are giant shields.  
• Also, volcanism occurs mostly within three regions. Even the mare-like plains cluster near these regions. The main cluster of volcanoes and lavas is in Tharsis. A much smaller cluster of three volcanoes lies in Elysium. Lastly, a few paterae are near the Hellas impact basin.

Differences from Moon

Age

Like the Moon, volcanism on Mars is very old. 

The mare-like plains on Mars are the same age as the lunar mare, roughly 3 to 3.5 billion years old. 

However, volcanism lasted much longer on Mars than on the Moon. It also seems to have changed over time. Volcanism in the highland paterae and mare-like plains on Mars stopped 3 billion years ago, but some of the smaller shields and cones erupted only 2 billion years ago. The giant shield volcanoes are even younger. These volcanoes formed between 1 and 2 billion years ago. The youngest lava flows on Olympus Mons are only 20 to 200 million years old. These flows are very small, however, and they probably represent the last gasp of martian volcanism. Thus, the odds of finding an active volcano on Mars today are very small.

Setting

 

Like the Moon, Mars shows no sign of plate tectonics. 

• It has no long mountain chains, and there is no clear global pattern to the volcanism. 
• Over half of Mars is heavily cratered like the lunar farside. Unlike the Moon, however, most martian volcanism lies outside large impact basins. Instead, the mare-like plains are mostly near the largest volcanoes. 
• These plains also are not limited to the lowest elevations. Indeed, some lava plains are much higher than the cratered uplands. Lava plains may lie at lower elevations as well. However, thick layers of dust and sediment cover both the Northern Lowlands and the large basin floors. These layers reflect a long history of winds, glaciers and flood events. They also hide any volcanism that may have occurred in the low areas on Mars.

Processes

The concentration and duration of volcanism into these two regions are attributed to the evolution of a long-lived mantle hotspot.

 

Links More Mars Volcano Information can be found at the "Geology of Mars" website curated by Albert T. Hsui, University of Illinois at Urbana-Champaign.

As well as:

Mars   http://seds.lpl.arizona.edu/nineplanets/nineplanets/mars.html

Mars Today.com   http://marstoday.com/

NASA?s Mars Exploration Program   http://mars.jpl.nasa.gov

Mars Global Surveyor   http://mars.jpl.nasa.gov/mgs/

NASA Human Spaceflight   http://spaceflight.nasa.gov/mars/

Venus

Overview

These volcanoes come in a variety of forms. Most are either Large Shields or Smaller Shield volcanoes, but there are also many Complex Features, several Unusual Constructs, and a few Large Flow Features. None is known to be active at present, but our data is very limited. Thus, while most of these volcanoes are probably long dead, a few may still be active.

Locations after Head et al. (1992) J. Geophys. Res. 97, 13153.
Base map is Magellan topography data, available at NASA's JPL.

Background

• First, since the atmosphere is mostly CO2, Venus has an extreme Greenhouse Effect. In fact, the surface temperature on Venus is about 470!C (about 880!F). Further, the surface air pressure on Venus is about 90 times greater than that at sealevel on Earth. This is roughly equivalent to the WATER pressure on Earth one kilometer beneath the ocean's surface. These surface conditions have two effects. (1) There is no water on the surface of Venus. Indeed, there is almost no water in the air, either. The clouds are mostly made of sulfuric acid and they are much, much higher than most clouds on the Earth. (2) Due to the high atmospheric pressure, the winds on Venus are also relatively slow. Thus, neither wind nor rain can really affect the surface on Venus. As a result, volcanic features will look freshly formed for a long time.

• Second, Venus shows no evidence for plate tectonics. There are no long, linear volcano chains. There are no clear subduction zones. Although rifts are common, none look like the mid-ocean ridges on Earth. Also, continent-like regions are rare, and show none of the jigsaw fits seen on Earth. Thus, where volcanism on Earth mostly marks plate boundaries and plate movements, volcanism on Venus is much more regional and much less organized.

• Third, volcanism on Venus shows fewer eruptive styles than on the Earth. Almost all volcanism on Venus seems to involve fluid lava flows. There is no sign of explosive, ash-forming eruptions on Venus, and little evidence for the eruption of sludgy, viscous lavas. This may reflect a combination of several effects. First, due to the high air pressure, venusian lavas need much higher gas contents than Earth lavas to erupt explosively. Second, the main gas driving lava explosions on Earth is water, which is in very short supply on Venus. Lastly, many viscous lavas and explosive eruptions on Earth occur near plate subduction zones. Thus, the lack of subduction zones should also reduce the likelihood of such eruptions on Venus.

IO

Jupiter's strange moon is literally bursting with volcanoes. Dozens of active vents pepper the landscape which also includes gigantic frosty plains, towering mountains and volcanic rings the size of California. The volcanoes themselves are the hottest spots in the solar system with temperatures exceeding 1800 K (1527 C). The plumes which rise 300 km into space are so large they can be seen from Earth by the Hubble Space Telescope. Confounding common sense, these high-rising ejecta seem to be made up of, not blisteringly hot lava, but frozen sulfur dioxide. And to top it all off, Io bears a striking resemblance to a pepperoni pizza. Simply unbelievable.

Above: This false color infrared composite of Jupiter's moon Io was produced from images acquired in July and September, 1996 by NASA's Galileo spacecraft. The area shown is 11,420 kilometers in width. Deposits of sulfur dioxide frost appear in white and gray hues while yellowish and brownish hues are probably due to other sulfurous materials. Sulfur dioxide is normally a gas at room temperatures, but it exists on Io's surface as a frost after condensing there from the hot gases emanating from the Io volcanoes. Bright red materials (such as the prominent ring surrounding the currently erupting plume Pele) and spots with low brightness or albedo ("black" spots) mark areas of recent volcanic activity and are usually associated with high temperatures and surface changes.

Since the first volcanic plume was discovered by Voyager in 1979, Io has remained under intense scrutiny. Astronomers using ground-based telescopes can monitor large volcanic eruptions from Earth by recording outbursts of infrared emission. Such measurements combined with Voyager and Galileo data show that some volcanoes on Io have been active for at least 20 years.

For a world dominated by fiery volcanoes, it's curious that Io is also very, very cold. The ground just around the volcanic vents is literally sizzling, but most of Io's surface is 150 degrees or more below 0 C. The moon's negligible atmosphere traps little of the meager heat from the distant Sun. As soon as volcanic gases spew into the air they immediately begin to freeze and condense. The plumes of Io's sizzling volcanoes are very likely made up of sulfur dioxide snow.

The intense volcanism on Io results from 100 meter high tides raised in its otherwise solid surface by nearby Jupiter and the other Galilean satellites. Although this process is fairly well understood, many aspects of Io's forbidding environment remains a mystery. What makes the lava around the volcanic vents so incredibly hot? What are the plumes made of? What causes the many colors of Io's mottled surface?

Above: The bright spots in this image indicate the locations of volcanic vents on Io, which are spewing hot lava. This image and other data from NASA's Galileo spacecraft indicate that the lava at Pillan Patera (marked Pillan) exceeded 1,700 degrees kelvin (2,600 degrees Fahrenheit) and may have reached 2,000 degrees kelvin (3,140 degrees Fahrenheit). The hottest eruptions on Earth today reach temperatures of about 1,500 kelvin (2,240 degrees Fahrenheit), but hotter lava erupted billions of years ago. [more information]

MERCURY

The pictures returned from the Mariner 10 spacecraft showed a world that resembles the moon. It is pocked with craters, contains huge multi-ring basins, and many lava flows. The craters range in size from 100 meters (the smallest resolvable feature on Mariner 10 images) to 1,300 kilometers. They are in various stages of preservation. Some are young with sharp rims and bright rays extending from them. Others are highly degraded, with rims that have been smoothed from the bombardment of meteorites.

Mercury is marked with great curved cliffs or lobate scarps that were apparently formed as Mercury cooled and shrank a few kilometers in size. This shrinking produced a wrinkled crust with scarps kilometers high and hundreds of kilometers long.

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