1. What is the crust of the Earth composed of?
a. a great variety of oxygen and nitrogen
b. a great variety of granite and metamorphicfold belts
c. a great variety of igneous,metamorphic, andsedimentary rocks
2. Which part of the mantle is composed mostly of peridotite?
a. middle b. upper c. lower
3. How thick is the continental crust?
a. from 30 km to 50 km as thick as oceanic
b. from 30 km to 50 km much harder than oceanic
c. from 30 km to 50 km slightly less dense than oceanic
4. Why does continental crust extend both above and below the oceanic crust?
a. because it is harder b. because it is thicker c. because it is slighter
5. Where does the water run and collect?
a. above the rock b. above the ocean crust c. above the upper mantle
6. How is the continental crust called sometimes?
a. sial b. sima c. gabbro
7. Why is the lithosphere broken into tectonic platesthat move?
a. because the temperature of the crust increases with depth
b. because of convection in the underlying plasticuppermantle and asthenosphere
c. because the geothermal gradient is smaller in deeper crust
8. How old is the bulk of the continental crust?
a. It is much older than 200 million years.
b. It is only about 200 million years.
c. It is less than 200 million years.
9. Why isn’t the underlying mantle asthenosphere readily destroyed by subduction?
a. It has been estimated to be about 2.0 billion years.
b. It is linked to periods of intense orogeny.
c. It is less dense than elsewhere in the earth.
Exercise 7. Read the text «Composition of the continental Crust» and
Composition of the Continental Crust
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Oxide |
Percent |
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SiO2 |
60.6 |
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Al2O3 |
15.9 |
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CaO |
6.4 |
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MgO |
4.7 |
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Na2O |
3.1 |
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Fe as FeO |
6.7 |
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K2O |
1.8 |
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TiO2 |
0.7 |
|
P2O5 |
0.1 |
Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number. The rarest elements in the crust (shown in yellow) are not the heaviest, but are rather the siderophile (iron-loving) elements in the Goldschmidt classification of elements. These have been depleted by being relocated deeper into the Earth's core. Their abundance in meteoroid materials is higher. Additionally, tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.
This table shows the abundance of elements in Earth's crust. Numbers show percentage in mass. The continental crust has an average composition similar to that of the igneous rock, andesite. Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle. Although the continental crust comprises only about 0.6 weight percent of the silicate Earth, it contains 20% to 70% of the incompatible elements.
All the other constituents except water occur only in very small quantities, and total less than 1%. Estimates of average density for the upper crust range between 2.69 g/cm3 and 2.74 g/cm3 and for lower crust between 3.0 g/cm3 and 3.25 g/cm3.
A more detailed and comprehensive list of elemental composition for the upper crust is given in the main article abundance of elements in Earth's crust.
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Key-word:
1. composition, 2. mantle;