The Earth is an unusual planet by having bimodal topography that reflects the two distinct types of crust. Crust is outer part of the Earth and compositionally is consist tow types, continental and oceanic crust. The oceanic crust is thin (~ 7 km ), and composed from denser rocks such as basalt , younger. Whereas the continental crust is thick (~ 40 Km), and composed of highly diverse lithologies, and contains the oldest rocks.

2. INTRODUCTION
• The Earth is an unusual planet by having bimodal
topography that reflects the two distinct types of crust.
• Crust is outer part of the Earth and compositionally is
consist tow types, continental and oceanic crust.
• The oceanic crust is thin (~ 7 km ), and composed from
denser rocks such as basalt , younger.
• Whereas the continental crust is thick (~ 40 Km), and
composed of highly diverse lithologies, and contains the
oldest rocks.

3. The Importance of Determining
Composition of the Crust
• It contains a very large proportion of incompatible
elements (20 - 70%), which include radiogenic-isotope
(Rb -Sr, U -Pb, Sm -Nd, Lu -Hf).
• Determining whether the composition has changed
through time or not which is leads to:
 Understanding the processes by which the crust is
generated and modified.
 Determining whether there is any generations in
the processes or not and evolution of the planet as
well

4. Continental Crust
• It is different from oceanic crust in chemical composition
and structure and by studying these characteristics that give
the origin of the crust.
• The total area of continental crust is 2.1 x108 km2, covering
41.2% of the solid Earth’s surface.
• Its thickness ranges from 14 to 80 km, averaging 41.1 km
with 2.09 x 1022 kg mass.
• Extends vertically from the Earth’s surface to the Moho
discontinuity, change in wave speeds from 7 - 8 kms-1
.
• The lateral extent is marked by the break in slope on the
continental shelf.

5. Crust is vertically stratified in terms of its chemical
composition and heterogeneous from place to place for
crustal structure and composition for different tectonic
settings.
The crust of Archean cratons thin and has low seismic
velocities in some regions (Yilgarn craton), Whereas in
other cratons, the crust is thick and has high velocities
(Wyoming craton).
Similar heterogeneities are observed for Proterozoic and
Paleozoic regions

6. Chemical Composition
• Estimation of the bulk chemical composition of continental
crust was made first by Clarke 1889 based on the average
samples.
• Estimation of the composition for the continental crust from exposed
regions may not represent wholly continental crust mass so it is
supported by seismic waves.
• Poldervaart (1955) based on average chemical compositions
and seismic velocities of individual different rock types. His
estimation has been improved by Ronov and Yaroshevsky
1969.

7. • Recently the estimation done by Wedepohl (1995) the
average chemical compositions at the surface of the
continental crust, by analyzing composite samples
produced by mixing of rocks collected from different
regions in shield area (Canada and China) and then
mixed them to form composite samples, which were
analyzed to obtain average compositions for the
individual different tectonic regions.

8. Why shield?
Condie (1993) suggests that:
(i) Shield are significantly eroded and thus may not be
representative of the 5 - 20 km of uppermost crust that has
been removed from them.
(ii)They include only Precambrian upper crust.
He utilized two methods in calculating an average of upper-crust
composition:
 Using a map distributions, irrespective of the level of
erosion.
 Restoring the eroded upper crust for areas that have been
significantly eroded, assuming it has a ratio of
supracrustal rocks to plutonic rocks similar to that seen
in uneroded upper crustal regions.

9. The structure of the continental crust is defined
seismically to consist of three layer upper, middle, and
lower crustal.

10. Table (1): Based on a characteristic P-wave velocity profile (model of
Rudnick and Fountain (1995)),two to four layers are recognized within
the continental crust.
<5.7 1st
Sedimentary and
volcanic
Upper Crust
Depth 20–28 km
5.7 - 6.4 2ed
Granitic plutons and
low-grade
metamorphic
6.4 - 7.1 3th
Gabbroic cumulate
and metamorphic
rocks of the granulite
facies.
Middle Crust
ranges of
thickness 9 - 21
km.
7.1 - 7.6 4th lies on the mantle
Lower Crust
In most cases, it
is either very
thin or missing.

11. Upper crust
• The most accessible part and easy target of geochemical
investigations.
• There are two basic methods employed to determine the
composition:
1. Establishing weighted averages of the compositions
of rocks exposed at the surface was utilized by Clarke
1889, he take large-scale sampling and weighted
averaging of the wide variety of rocks from Earth’s
surface.
All major-element determined by this method.

12. 2. Determining averages of the composition of
insoluble elements in fine-grained; done by
Goldschmidt 1933, based on the concept that the
process of sedimentation and glaciation averages
wide areas of exposed crust.
 Clastic sedimentary rocks.
 Glacial deposits.
Many trace-element and REE estimated by this method.
• From the two method Clark and Goldschmidt, upper
crust has granodioritic bulk composition, rich in
incompatible elements.

13. Table (2): chemical composition of the upper continental crust major
elements

14. Table (3): trace element (Incompatible element) of the upper continental
crust

15. Middle crust
• Compared to other regions of the crust (upper, lower
and bulk), few estimates have been made of the
composition this crust.
• Shaw et al. (1994) (Canadian Shield) and Gao et al.
(1998) (Eastern China) are based on surface sampling
of amphibolite-facies rocks in the Lewisian Complex.
• Rudnick and Fountain (1995) modeled the middle crust
as:
 45% intermediate amphibolite facies
 45% mixed between amphibolite and felsic amphibolite
facies
 10% metapelite.

16. • Christensen and Mooney (1995), proposed a middle
crust of:
 50% tonalitic gneiss,
 35% amphibolite, and
 15% granitic gneiss.

17. Table (3): Composition of the middle continental crust, major elements

18. Table (3): Composition of the middle continental crust, trace elements

19. Lower Crust
• The deeper reaches of the crust is more difficult to
study, for that most of the information about this crust
comes from three probes:
 Studies of high-grade metamorphic rocks
(amphibolite or granulite facies)
 Studies of granulite-facies xenoliths
 Seismic investigations
• There are fewer studies of amphibolite-facies xenoliths
compared to granulite-facies due to difficulty of
distinguishing such xenoliths from the exposed or
near-surface amphibolite-facies

20. • Interpretation the origin of granulite-facies depends on
unraveling their pressure, temperature and time history.
• They showing evidence for a “clockwise” P–T path
• From the three previous probes; it has been estimated
that the crust lower becomes more mafic with depth.
• Concentration of heat-producing elements drops off
rapidly from the surface downwards.

21. Table (4): Composition of the lower continental crust

22. CONCLUSION
• The Earth is a planet of a bimodal crust, homogeneous
in oceanic and heterogeneous in continental.
• Based on seismic investigations the crust can be divided
into three regions: upper, middle, and lower continental
crust.
• The chemical continental of the crust has been estimated
by different scientist; Clarke, Condie, Gao, Shaw, Taylor
and McLennan, …etc.
• The crust is the Earth’s major repository of incompatible
elements.

23. References:
• R. L. Rudnick and S. Gao. Composition of the Continental Crust. Treatise On
Geochemistry 3, 1 – 64 (2003).
• Yanagi, T. arc volcano of japan, generation of continental crust from the mantle.
Springer 9 – 17 (2011).