Natural systems (cont'd)
Marjorie Henry, Contributor
The discussion on the questions given in the last examinations to test natural systems continues this week, with the focus being on question 3.

In reference to the geography syllabus, the specific objectives tested in this question include:

6 - Define denudation, weathering, mass wasting and erosion;

7 - Explain the processes of weathering.

14 - Describe the characteristics of limestone.

15 - Explain the processes operating in limestone landscapes and the formation of Karst landforms in the Caribbean.

Clothing and textile is one of the practical subjects offered at some high schools. Here, these students of Ascot High School set bases for the material they will stitch later. -Anthony Minott/Freelance Photographer

A diagram showing 'mass movement' is the stimulus material in section (a). This you must study carefully to answer the three questions below the diagram. These are the answers to those questions:

(i) The type of mass movement shown is soil creep.

(ii) Indicators to support this are tension gashes in the road; utility pole tilted; base of tree turned downslope; soil piled up behind wall, forcing wall to bulge and break.

(iii) ONE factor that would influence this type of movement is the gradient of the slope.

Other answers could be the sparse vegetation or the influence of gravity.

For section (b) (i), you are asked for definitions. Certain key words must be included in giving these meanings.

(i) (a) Denudation: The wearing away of a land surface by chemical and mechanical weathering. The weathered material is then carried away by natural agents, such as wind, ice or water.

Another definition is - The lowering of the Earth's surface due to weathering, erosion and transport.

It is important to mention that the Earth's surface is being sculptured and the processes responsible for that.

(i) (b) Weathering: The processes whereby rocks exposed to the weather are changed, by decay and disintegration, into soil.

In your definition, you must include the breakdown of rocks in situ.

Section (b) (ii) is asking for a description of the physical weathering process of freeze-thaw. The expected response is as follows:

Rain water percolates into the pores, cracks and crevices of rock and, when this moisture freezes, the consequent expansion causes stresses to occur in the openings, widening them or loosening individual mineral grains. Such continued stressed ultimately results in the breakage of the rock. Slowly, angular fragments of rock are spilt off from the bedrock, while even great blocks may be disrupted and dislodged. This mechanical action is one of repeated freeze-thaw process.

Section (c) of the question will be discussed in the next lesson.

Question c (i) is asking you to explain the susceptibility of limestone to chemical weathering. In your answer, you will need to mention (a) the composition of limestone and (b) the agent responsible for bringing about the decomposition of the minerals making up that rock. Your answer should be as follows:

Two heads are always better than one when it comes to studying. Do you think these Manchester High School students understand that? -Anthony Minott/Freelance Photographer

Limestone is susceptible to corrosion by rain water containing dissolved carbon dioxide. Rain water, which has absorbed carbon dioxide gas from the air, is turned into a very weak carbonic acid. This diluted acid reacts upon such calcareous rocks, as chalk and limestone, converting the calcium carbonate of which they are composed into a much more soluble form called calcium bicarbonate.

In subsection (c) (ii), note that you are asked to explain the formation of stalactites and stalagmites. You are not asked to describe them. Your answer must include (a) the role of the rain water on limestone; (b) the evaporation of moisture; (c) the very slow build up of deposits of calcium carbonate to form the features; (d) that the features are formed in caves. Take care that you do not confuse both features as is the tendency of so many students.
Your response should be as follows:-

Rain water containing carbon dioxide permeates the cracks and fissures in a limestone mass and dissolves the calcium carbonate. The water may then seep through cracks in the roof of the cave and partially evaporates, leaving behind a small deposit of calcium carbonate on the roof of the cave. This extends downwards progressively over time to form a stalactite. Some of the water drips down from the roof of the cave onto the floor. The water evaporates and leaves behind a deposit of calcium carbonate. This accumulates over time and builds upwards from the floor to form a stalagmite.

Although you were not asked for it, you may draw a diagram to assist you with your answer. This must be done properly, clearly and well annotated. The answer for subsection (c) (iii) is clearly outlined for you in the new edition of The Caribbean Environment for CXC Geography written by Mark Wilson. It states:-

Cockpits are formed like this:

1. The limestone has a criss-cross pattern of joints.

2. The rock nearest to the joints is dissolved fastest, because this is where water collects.

3. A deep, star-shaped depression is formed where two joints meet. Cockpits form at the intersection of joints where erosion is fastest.

4. Small hills remain away from the joints. Where the jointing is regular, the hills are arranged in rows.

All of this information is needed in order to be awarded full marks for this section of the question.

I will continue to discuss the question paper after the Christmas break. Do enjoy your holidays, but make sure to set aside some time to review some lessons, especially those of you who will be writing the examination in June 2008.

Marjorie Henry is an independent contributor.