The use of knowledge
Marjorie Henry, Contributor

In the last lesson, I started to discuss Section B, Natural Systems, of the question paper. Sections (a) and (b) of Question 2 were done. Today, I will continue the discussion by going through section (c). This section is Profile 3 of the question, that is, 'use of knowledge'. More is required of you in this section when compared with the other sections of the question. Again, I refer you to the key verbs here, namely, EXPLAIN; ACCOUNT FOR. You must write fully about the matter. You must use complete sentences, as well as paragraphs, where necessary. In essence, you are now engaged in a discussion.

Most Marks

Clothing and textile teacher at Ascot High School, Luna Bailey (left), demonstrates to Chantel Graham how a design for a skirt is drafted, during a class, recently. -Anthony Minott/Freelance Photographer

You may have noticed that section (c) is allotted the most marks in the question, that is, 12 out of the total mark of 24. Let us now look at the sub-sections of section (c).

(i) Explain how the shape of a volcano is influenced by the type of lava ejected.

Begin by referring to the types and characteristics of the lava. Two main types of lava are distinguished. These are ACID LAVA and BASIC LAVA. Acid lava is stiff and viscous, has a high silica content and flows slowly. Basic lava is more fluid and has a lower silica content. It flows rapidly and travels much farther, before hardening, than acid lava. Each type of lava will, in turn, produce slopes of varying heights and shapes. Your full answer should be as follows:-

Acid lava is highly viscous. It flows slowly and seldom travels far before solidifying. It produces steep-sided convex cones. Basic lava is highly fluid. Due to its high fluidity, it flows readily with a speed of 16-48 km per hour (10-30 miles per hour). It spreads out as a thin sheet over great distances before it solidifies. The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.

Sketches

Please note that you can do sketches to show the difference in the shape of the volcanic cones formed, although the question did not ask for them specifically.

Diagrams that are properly done and labelled often allow you to be given full marks for your answers. It may show some feature that was missed in your discussion.

Sub-section (ii) of the question states:-

(ii) Account for the occurrence of earthquakes along transform plate boundaries.

Begin by distinguishing transform plate boundaries then state how earthquakes occur along them. Your response could be as follows:-

Transform plate boundaries are moving at different speeds. The plates move in parallel, that is, sideways, but in opposite directions passing each other, without causing destruction or adding new materials to the ocean floor. The plate boundaries are characterised by numerous earthquakes but little or no volcanic activity. Friction prevents the plates from moving smoothly. They do not move smoothly, but jam and grate along as they slide past. The forces are built up so strongly that the plates suddenly jerk or snap.

Sub-section (iii) concludes this question.

(iii) Explain how explosive volcanoes are formed.

The answer to this is as follows:- Explosive volcanoes are formed at subduction zones. The subducting plate melts as a result of high temperatures and high pressure. The melted plate material is forced upwards since the newly formed magma is less dense. The rising magma sometimes blocks the vent or pipe of the volcano so that pressure is built up. This is released with great force as the gases around expand. They erupt violently as their conduit or vent system extend deep into the Earth's crust.

(Source: New Caribbean Geography - by Vohn A. M. Rahil)

Marjorie Henry is an independent contributor.