Differences from Earth's Volcanism

Mt St. Helens' eruption in 1980

Age
• Volcanism on the Earth is an ongoing process.
• All volcanism on the Moon ended over a billion years ago.
• Many of Earth's volcanoes are less than a few 100,000 years old.
• Most volcanism on the Moon occurred between 3 and 4 billion years ago.
• Because the Moon does not show any evidence for recent volcanic or geologic activity, it is sometimes called a "dead" planet.

  
  Location

• Earth's volcanoes mostly occur within long linear mountain chains.
• Mountain chains like the Andes mark the edge of a plate.
• Mountain chains like the Hawaiian Islands mark past plate movements over a mantle hotspot.
• In contrast, mare typically occur in the bottoms of very large, very old impact craters.
• Therefore, 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.

  
  Shape

• Mare surfaces are very flat and smooth. This implies that mare lavas were very fluid.
• They could both flow very easily and spread out over large areas.
• The low gravity means that any rare explosive eruptions could throw debris further on the Moon than on the Earth.
• 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 explains why large volcanoes are not seen on the Moon.