Some early astronomers thought that the basaltic lavas plains on the moon were actually water. As such, they were falsely labeled using the Latin word for sea – “mare.”
Most of the moon’s volcanism seems to have occured between 3 and 4 billion years ago. Unlike Earth, which has active volcanoes, the moon’s volcanism seems to have subsided about a billion years ago. Another interesting feature of some of the moon’s lava flow is due to the weaker gravity of the satellite. While Earth’s volcanic activity is often subject to violent eruptions, some of the moon’s volcanic activity is believed to have been more of a flowing, pooling effect that spread the lava evenly. This would account for the water-like appearance that misled some early astronomers.
Of course, as Samuel mentioned, there were obviously violent eruptions on the moon as well. Due to the weaker gravity, such explosions hurled debris greater distances than it would on Earth. This prevented the creation of mountain-like lava formations.
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This entry also posted in the American Military University Introduction to Astronomy newsgroup.