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Section B is on natural systems.
Here, you are given a total of three questions numbered two, three and four. You are to
select only one from this section. Here is a question
2.
(a)
Study figures 1 and 2 which shows the global distribution of
tectonic plates and of active volcanoes respectively, and answer the
questions that follow.
For part (a) of the question, you have been given two maps from which you must
deduct your answers to the four short questions given below the map. You are instructed to
study or look at what is given to find all the information you need to answer the question.
(i)
Name the continent with no active volcanoes?
(1mark)
(ans.) The continent with no active volcanoes is Australia.
(ii)
How many active volcanoes are in South America? (1mark)
(ans.) There are 12 active volcanoes in South America.
(iii)
State ONE difference between the distribution of volcanoes in
North America and Africa.
(1mrk)
(ans.) One difference between the distribution of volcanoes in
North America and Africa is that in North America they
are concentrated along the west coast while, in Africa,
they are found in the east and central (middle) areas of
the continent.
(iv)
(b)
(i)
How is the distribution of volcanoes related to plate tectonics?
(1mark)
(ans.) The distribution of volcanoes is related to plate
boundaries as the volcanoes are located close to or on
the plate boundaries.
With the aid of a diagram, describe the MAIN features of the
trellis drainage pattern.
(4 marks)
(Please note that you are being asked for both a diagram and a written
account. Make sure to do both. However, a properly drawn, welllabelled diagram is acceptable. Here is a diagram of the trellis drainage
pattern)
(ans.) The trellis drainage pattern
develops in areas of alternate layers of
hard and soft rocks.
The main river develops along the main
slope, cutting across the bands of hard
rock.
The tributaries develop on soft rocks and
join the main river at right angles.
(ii)
Describe TWO ways in which rivers transport their load.
( 4 marks)
(There are four processes by which a river can transport its load,
however, you are asked to describe only two. Here are the processes)
(ans.) TRACTION: The stones are rolled along the river bed.
SALTATION: Sand-sized particles bounce along the bed in a 'leap
frog' movement (jerks and jumps).
SUSPENSION: Silt and clay-sized particles are carried within the
water flow.
SOLUTION: Some minerals dissolve in the water.
(c)
(i)
Explain how volcanoes are formed at convergent plate
boundaries.( 6 marks)
(ans.) Two plates, an oceanic and a continental, move towards one
another and converge. The heavier (or denser) oceanic plate sinks
below the continental one. This area of downward movement is known
as the subduction zone. The sinking plate melts due to the heat in the
mantle. The magma rises and reaches the surface through vents,
fissures or cracks in the continental plate forming volcanoes.
(ii)
Account for the formation of Fold Mountains at
a) collision zones
( 3 marks)
(ans.) Sometimes two continental plates moving towards each other
collide. There is no subduction as the density is the same.
Sedimentary rocks lying on the plates are pushed and folded forming
mountain chains. One example is the Himalayas, which have been
formed by the meeting of the Eurasian and Australian plates.
b) subduction zones.
(3 marks)
(ans.) Subduction zones - Two plates collide. The leading edge of one
(the oceanic) is subducted or forced under the edge of the other plate
(the continental). The area of subduction is called the subduction zone.
However, since the crust of the continental plate is too buoyant to be
forced down into the mantle, the continental crust has a lower density
and so is buoyant. Mountains are formed along its edge by the
crumpling of the marginal rocks and their upthrusting by the sudsiding
plate. One example is the Rockies Mountain.
(Include examples in your answer.)
Total 24 marks