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.

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

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.

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

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.

• 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.

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.

The structure of the continental crust is defined

seismically to consist of three layer upper, middle, and

lower crustal.

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 CrustDepth 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 Crustranges of

thickness 9 - 21 km.

7.1 - 7.6 4th lies on the mantleLower Crust

In most cases, itis either very

thin or missing.

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.

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.

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

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

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.

• Christensen and Mooney (1995), proposed a middle

crust of:

50% tonalitic gneiss,

35% amphibolite, and

15% granitic gneiss.

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

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

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

• 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.

Table (4): Composition of the lower continental crust

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.

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).