The Earth Contents

2. Plate Tectonics

Conclusion

Constructive margins are where plates diverge. Destructive margins are where plates converge. Conservative margins are where plates slip past each other. We have noted that oceans are created where plates move apart, and that volcanoes are characteristically found where plates converge.

Questions and answers

1.Plate composition approximates that of the underlying rock; which two types of rock are the most common?

question 1 answers

2.What are the three different types of plate motion?

question 2 answers

3.What type of plate motion would you associate with the formation of mid-ocean ridges?

question 3 answers

4.What type of plate motion would you associate with the formation of trenches?

question 4 answers

5.What type of plate motion would you associate with faults?

question 5 answers

6.Why is plate motion relatively insignificant in altering the global crustal mass?

question 6 answers

7.Why is plate movement of interest to oceanographers?

question 7 answers

8.Give an example of an expanding ocean and a contracting ocean?

question 8 answers

9.Name some other examples of volcanoes formed in a similar manner to those in the so-called Pacific 'Ring of Fire'.

question 9 answers

10. What is bathymetry and how does it develop?

question 10 answers

11.Why is the continental shelf important to civilisation?

question 11 answers

12.What are some of the characteristics of the abyssal plain?

question 12 answers

Question 1 answer

Basalt and granite.

Question 2 answer

The three different types of relative motion are constructive, destructive and conservative.

Question 3 answer

Constructive plate motion is associated with the formation of mid-ocean ridges.

Question 4 answer

Destructive plate motion is associated with the formation of trenches.

Question 5 answer

Conservative plate motion is associated with faults, where two plates slip past each other and material is neither created nor destroyed.

Question 6 answer

Whilst the amounts of material added or removed from the crust at plate boundaries are locally significant, the additions are insignificant except over very long time-scales. At the present rate of production of new oceanic crust, it would take 100 million years to replace the existing ocean crust entirely. On an environmental time-scale, the crust therefore has a fixed composition.

Question 7 answer

The regions where two plates move apart are of special interest to oceanographers because they represent the sites where new oceans are created. One such example is the Atlantic Ocean, which first opened up when Africa and Europe began to split from the Americas some 135 million years ago. As the two oceanic plates separated, a long rift valley formed at the junction, and basalt welled up to the surface from a series of volcanies. At present, there is a series of volcanic islands on the 'Mid-Atlantic ridge' including Iceland, Helena and Tristan. There is also a series of volcanoes, hot springs and vents on the Mid-Atlantic ridge which do not reach the surface.

Question 8 answer

The Atlantic Ocean is an example of an expanding ocean, whilst the Pacific Ocean is an example of a contracting one.

Question 9 answer

Vesuvius, Etna, Stromboli and Santorini were all formed as the Mediterranean plates sank under Europe. As the oceanic crust sinks down, the friction of the descending plate causes intense heat to be generated, and volcanoes are typically found above the subducting plate.

Question 10 answer

Bathymetry is the measurement of the depth of the ocean floor from the water surface; the oceanic equivalent of topography. The bottom topography is as a result of plate tectonic processes causing ridges and trenches in the ocean floor.

Question 11 answer

The continental shelf is an increasingly important resource, containing many important mineral resources and oil reserves. The continental shelf is also the location of many of world's most important fisheries.

Question 12 answer

The Abyssal plain;

- is underlain by basaltic oceanic crust,

- is covered by a layer of sediment which becomes deeper and older away from the mid-ocean ridge, towards the edge of the continents,

-ocean deeps represent about 30% of the 'surface' of the Earth, more than the entire area of the continents,

- are not entirely flat, but can consist of a number of large hills, the largest of which penetrate above the ocean surface as islands.

- coral grows on islands which reach or approach the surface of tropical seas.