2. I NTRODUCTION 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 x10 8 km 2, 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 10 22 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.

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).