Fig. 45 Three types of plate boundaries

Date: 2015-10-07; view: 504.

We'll discuss the subject: «Plate tectonics in action». Pay attention to the words and expressions and pronunciation of some of the geographical names. Read the text, which will give you some necessary information. Use the earlier communicative formulas (Unit 7). Pay special attention to the questions, which will help you to underline what facts are more important.

DISCUSSION (R.P – 8.1.5)

You will hear part of a radio report about the oceanic crust. For statements 1-12, complete the notes, which summarize what the speaker says. You will need to write a word or short phrase in each sentence. You will hear the recording twice.

LISTENING COMPREHENSION

1. The oceanic crust is different from_______________________.

2. Besides aluminum and calcium, ocean crust has a high content of_______________.

3. The combination is called ____________________.

4. The ocean crust has _____________________.

5. The first layer consists of __________________________.

6. They may be _______________________, which lie up on ________________.

7. The next layer is mainly _____________________, which is found at ___________________________.

8. The third layer is made up of _____________________.

9. While the last layer is ________________________.

10. It is made up of peridotite, chiefly __________________.

Use the diagrams – «Plate tectonics in action», «Tectonic plates». (R.P. 8.2.10, 8.2.11)

Geographical Names

EARTH'S CHANGING SURFACE

Our planet's solid surface is a restless jigsaw of abutting, diverging and colliding slabs called plates (lithospheric plates). How plates behave forms the subject known as plate tectonics.

Each plate involves a slab of oceanic crust, continental crust, or both, coupled to a slab of rigid upper mantle. Collectively these plates make up the lithosphere. This rides upon the asthenosphere, a dense, plastic layer of the mantle. Heat rising through this layer from the Earth's core and lower mantle seemingly produces convection currents that shift the plates above.

Plate activities produce three main kinds of plate margins.

· Constructive (divergent) margins are oceanic spreading ridges where new lithosphere is formed between two separating oceanic plates.

· Destructive (convergent) margins are oceanic trenches where an oceanic plate dives down below a (less dense) continental plate.

· Conservative (transform) margins are where two plates slide past each other and lithosphere is neither made nor lost.

Geophysicists also talk of active margins (where colliding continental and oceanic plates spark off volcanic eruptions, earthquakes and mountain building) and passive margins (tectonically quiet boundaries between continental and oceanic crust).

(Lambert “The Field Guide to Geology” 1988, Cambridge University Press)

Discussion questions:

1. What comprises the Earth's surface?

2. What is a plate?

3. What do the plates make up?

4. What shifts the plates?

5. How many types of plate margins are there?

6. What is

- a constructive margin

-a destructive margin

-a conservative margin

7. What is the difference between active and passive margins?

8. The map shows major plates. Name them.