Geoscience Reference
In-Depth Information
contains a very large proportion of incompatible
elements (20--70%), depending on element. These
include the heat-producing elements and mem-
bers of a number of radiogenic-isotope systems
(Rb--Sr, U--Pb, Sm--Nd, Lu--Hf) that are commonly
used in mantle geochemistry. Thus the conti-
nental crust factors prominently in any mass-
balance calculation for the Earth as a whole and
in estimates of the thermal structure of the Earth
(Figure 8.2). (
Rudnick crust
).
The Moho is a sharp seismological boundary
and in some regions appears to be laminated.
There are three major crustal types -- continental,
transitional and oceanic. Oceanic crust generally
ranges from 5--15 km in thickness and comprises
60% of the total crust by area and more than
20% by volume. In some areas, most notably near
oceanic fracture zones, the oceanic crust is as
thin as 3 km. Sometimes the crust is even absent,
presumably because the underlying mantle is
cold or infertile, or ascending melts freeze before
they erupt. Oceanic plateaus and aseismic ridges
may have crustal thicknesses greater than 20 km.
Some of these appear to represent large volumes
of material generated at oceanic spreading cen-
ters or triple junctions, and a few seem to be
continental fragments. Although these anoma-
lously thick crust regions constitute only about
10% of the area of the oceans, they may repre-
sent more than 25% of the total volume of the
oceanic crust. They are generally attributed to
hot regions of the mantle but they could also
represent fertile regions of the mantle or tran-
sient responses to lithospheric extension. Islands,
island arcs and continental margins are collec-
tively referred to as
transitional crust
and range
from 15--30 km in thickness. Continental crust
generally ranges from 30--50 km thick, but thick-
nesses up to 80 km are reported in some active
convergence regions. Older regions show a sharp
cutoff in crustal thickness at 50 km; this may
be the depth of the basalt--eclogite phase change
and the depth at which over-thickened crust
founders or delaminates. Based on geological and
seismic data, the main rock type in the upper
continental crust is granodiorite or tonalite in
composition. The lower crust is probably diorite,
garnet granulite and amphibolite. The average
Table 8.1
Bullen's regions of the Earth's
interior
Depth
Range
Region
(km)
A
0
33
continental crust
B
33
410
upper mantle
220
Lehmann discontinuity
C
410
1000
transition region
650
discontinuity
D'
1000
2700
lower mantle
1000
Repetti discontinuity
D”
2700
2900
transition region
E
2900
4980
outer core
F
4980
5120
transition region
G
5120
6370
inner core
Table 8.2
Summary of Earth structure
Fraction
Fraction
Depth
of Total
of Mantle
Region
(km)
Earth Mass and Crust
Continental crust
0-50
0.00374
0.00554
Oceanic crust
0-10
0.00099
0.00147
Upper mantle
10-400
0.103
0.153
Transition (TZ)
400-650
0.075
0.111
Deep mantle
650-2890
0.492
0.729
Outer core
2890-5150
0.308
—
Inner core
5150-6370
0.017
—
V
P
r
12
V
P
10
8
V
S
CORE
6
r
V
S
4
2
MANTLE
0
2000
4000
6000
Depth (km)
Fig. 8.1
The Preliminary Reference Earth Model (PREM).
The model is anisotropic in the upper 220 km. Dashed lines
are the horizontal components of the seismic velocity (after
Dziewonski and Anderson, 1981).
