Observation

This was a fairly popular question and the candidates performed averagely. Many of the candidates who answered the question correctly named the landforms associated with extrusive volcanic activities such as basic lava cone, acid lava cone, lava plateau, lava plain, ash and cinder cone, volcanic plug, crater lake and caldera. Quite a good proportion of the candidate who answered the question could not describe the characteristics of sills well. The expected answers included:

• a volcanic feature;
• has fairly parallel sides;
• lies horizontally between bedding planes;
• an igneous rock;
• thickness ranges from a few metres to hundreds of kilometres;
• a hard rock;
• may trap water underground;
• may give rise to waterfalls when exposed along a river valley;
• examples are Sekondi sill, Fouta Djallon sill, sills at the west of Jos Plateau, Saionya scarp of Sierra Leone.

The points which were answers to the formation of sills included:

• formed due to volcanic activity;
• development of cracks in crustal rocks lead to its formation;
• cracks lead to reservoirs of magma in the interior part of the earth
• magma flows up;
• magma forces itself into lines of weakness along bedding planes;
• magma cools and solidifies within the bedding planes;