in the crust quickly healed as batches of fresh magma bled through giant fis-
sures and poured onto the surface, creating a magma ocean. The continued
destruction of the crust by heavy volcanic and meteoritic activity explains
why the first 700 million years of Earth history, referred to as the Hadean or
Azoic eon, are missing from the geologic record.
The original crust was quite distinct from modern continental crust,
which first appeared about 4 billion years ago and now represents less than
0.5 percent of the planet’s total volume. During this time, Earth spun wildly
on its axis, completing a single rotation every 14 hours, thus maintaining
high temperatures throughout the planet. Present-day plate tectonics (the
interaction of crustal plates) could not have operated under such hot condi-
tions, which produced more vertical bubbling than horizontal sliding.
Therefore, modern-style plate tectonic processes were probably not fullyFigure 5Saturn’s
heavily cratered moon
Mimas from Voyager Iin
November 1980.
(Photo courtesy NASA)
Historical Geology