Historical Geology Understanding Our Planet\'s Past

mountain slopes and the sides of large meteorite craters, gouging out deep

canyons in the rocky plain.When the rains ceased and the skies finally cleared,

Earth emerged as a giant blue orb,covered by a global ocean nearly 2 miles

deep and dotted with numerous volcanic islands.

Ancient marine sediments found in the metamorphosed rocks of the

Isua Formation in southwestern Greenland support this scenario for the cre-

ation of the ocean. The rocks originated in volcanic island arcs and therefore

lend credence to the idea that some form of plate tectonics operated early in

the history of Earth.They are among the oldest rocks, dating to about 3.8 bil-

lion years ago, and indicate that the planet had surface water by this time.

During the intervening years between the end of the great meteorite

bombardment and the formation of the first sedimentary rocks, vast quantities

of water flooded Earth’s surface. Seawater probably began salty due to the

abundance of chlorine and sodium provided by volcanoes. However, the

Figure 11The
landscape of Mars from
Viking I, showing
boulders surrounded by
windblown sediment.
(Photo courtesy NASA)

Historical Geology