Geoscience Reference
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mantle (its oxygen isotopic ratios are identical to
the Earth's), but are strongly depleted in volatiles.
It was then realized that accreting matter would
form embryonic planets with a large range of
sizes.
The final stages of planetary formation would
involve giant impacts in which bodies of com-
parable size collided at high speed. A giant
impact produces rock-vapor that preferentially
retains the refractory elements as it condensed.
Shortly after the formation of the Earth, a large
object is inferred to have hit the Earth at an
oblique angle, destroying the impactor and eject-
ing most of that body along with a significant
amount of the Earth's silicate portions. Some of
this material then coalesced into the Moon. This
event also melted a large fraction of the Earth.
From the angular momentum of the present
Earth--Moon system the projectile is inferred to
have had a mass comparable to Mars and the
Earth was smaller than it is today. After the col-
lision the Earth is a very hot body indeed. The
idea of a cold primordial undegassed Earth can
no longer be entertained. The present lower man-
tle is more likely to be refractory and gas-poor
than to be primordial and gas-rich. The
giant
impact theory
is the now dominant
theory
for the formation of the Moon
. The the-
ory was proposed in 1975 by Hartman and Davis
(see Hartman
et al
., 1986).
Table 2.2
Composition of the continental
crust
Species
A
B
C
SiO, (percent)
58.0
63.7
57.3
TiO
2
0.8
0.5
0.9
Al
2
O
3
18.0
15.8
15.9
FeO
7.5
4.7
9.1
MnO
0.14
0.07
—
MgO
3.5
2.7
5.3
CaO
7.5
4.5
7.4
Na
2
O
3.5
4.3
3.1
K
2
O
1.5
2.0
1.1
P
2
O
5
—
0.17
—
Rb (ppm)
42
55
32
Sr
400
498
260
Th
4.8
5.1
3.5
U
1.25
1.3
0.91
Pb
10
15
8
A: Andesite model (Taylor and McLennan,
1985).
B: Amphibolite--granulite lower-crustal
model (Weaver and Tarncy, 1984).
C: Theoretical model (Taylor and McLennan,
1985).
variance with the observations that the moon
has no substantial metallic iron core, and that
its rocks are similar in composition to the Earth's