and tell of the earliest history of Earth, when the crust first formed some 4.2
billion years ago.
Among the oldest rocks are those of the 4-billion-year-old Acasta gneiss,
a metamorphosed granite in the Northwest Territories of Canada. Its existence
suggests that the formation of the crust was well underway by this time.The dis-
covery is used as evidence that at least small patches of continental crust existed
on Earth’s surface at an early date. Earth apparently took less than half its history
to form an equivalent volume of continental rock as it has today.THE MOON
A popular contention for the creation of the Moon supposes a collision
between Earth and a large celestial body. According to this theory, soon after
Earth’s formation, an asteroid about the size of Mars was knocked out of the
asteroid belt either by Jupiter’s strong gravitational attraction or by a collision
with a wayward comet. On its way toward the inner solar system, the asteroid
glanced off Earth (Fig. 7).The tangential collision over a period of half an hour
created a powerful explosion equivalent to the detonation of an amount of
dynamite equal to the mass of the asteroid. The collision tore a huge gash in
Earth. A large portion of its molten interior along with much of the rocky
mantle of the impactor spewed into orbit, forming a ring of debris around the
planet called a protolunar disk, similar to the rings of Saturn.
The force of the impact might have knocked Earth over, tilting its rota-
tional axis about 25 degrees. Similar collisions involving the other planets, espe-
cially Uranus, which orbits on its side like a rolling ball, might explain their
various degrees of tilt and elliptical orbits. The glancing blow might also have
increased Earth’s angular momentum (rotational energy) and melted the planet
throughout, forming a red-hot orb in orbit around the Sun. As a result, Earth
would have spun wildly on its axis, completing a day in only two hours. The
present angular momentum suggests that other methods of lunar formation
such as fission, capture, or assembly at the same time as Earth were unlikely.
Clinching evidence for the collision theory of lunar origin was obtained
by analyzing moon rocks (Fig. 8) brought back during the Apollo missions of
the late 1960s and early 1970s.The rocks appear to be similar in composition
to Earth’s upper mantle and range in age from 4.5 to 3.2 billion years old.
Since no rocks were found younger than this date, the Moon probably ceased
volcanic activity, and the interior began to cool and solidify.
The new satellite continued growing as it swept up debris in orbit
around Earth.In addition, huge rock fragments orbited the Moon and crashed
onto its surface.The massive meteorite shower that bombarded Earth equally
pounded the Moon. Many large asteroids struck the lunar surface and brokeHistorical Geology