Observation

This question was not popular among the candidates and their performance in the question was a bit above average. Majority of the candidates who attempted this question were not conversant with the procedure for the mode of formation of shield volcanic mountain and blow hole. They were expected to answer thus:

(a) Characteristics of shield volcanic mountain/dome
- it is also called basic lava dome
- a product of extrusive volcanic activity
- relatively low in height
- symmetrical in shape
- convex in shape
- very broad/large at the base
- gentle slopes
- angle of slope is between 2 - 6⁰
- rises up to about 10⁰ at the top of the slope
- flattens at the top of the slope/summit
- it has a built-up vent
- product of very fluid lava
- materials could be arranged in layers
- height varies depending on fluidity of lava
- lava is mainly basalt
- dark coloured
- rich in iron and magnesium
- poor in silica
- looks like the shield of a warrior lying in the ground
- examples include Mauna Loa in Hawaii, also found in islands of Fernando Po and Sao Tome

(a) Mode of formation of shield volcanic mountain/dome
- formed from quiet volcanic eruptions
- magma reaches the surface through fissures/vent
- the fluid nature of magma makes the magma to spread out in sheets over wide areas around the vent
- lava is extremely hot (about 10000c)
- flow speed of 16 – 48km per hour
- reduction in lava temperature makes it to cool and solidify
- cooling is slow
- eruption is successive/steady
- a buildup of basic lava over a long period of time produces a shieldlike volcanic
feature called the shield volcano

(c) Characteristics of blow hole
- found along the coast
- found on the landward side of a marine cave or headland
- an erosional feature
- also called gloup
- found on cliffs and headlands
- it is a vertical shaft
- a small opening on ground surface connected to a cave
- it links the roof of a sea cave to the surface
- sea water gushes out from the cave to the surface through blow holes
- found at the top of a sea cave
- it may be circular or irregular in shape
- sizes vary from about 10cm diameter to several metres
- examples at Holborn Head in Scotland, Amesfrane Cliff in Morocco, Le Souffleur Island of Mauritius
- diagram