Geoscience Reference
In-Depth Information
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
Search WWH ::




Custom Search