WAEC SSCE - Geography - 2019 (Practical and Physical Geography)

Chemically formed sedimentary rocks

(a) Mode of formation

Chemically formed sedimentary rocks are produced from minerals that were dissolved in water. When water containing these dissolved minerals evaporates, or when the water becomes over-saturated, the minerals are precipitated (deposited) out of solution. The precipitated material accumulates and, over time, is compacted and cemented to form solid rock. This process commonly takes place in seas, salt lakes, lagoons and hot springs where evaporation is high, and in limestone caves where deposition forms stalactites and stalagmites.

(b) Three examples

1. Rock salt (halite).
2. Gypsum.
3. Limestone (chemically formed) and potash.

(c) Four importance (uses)

1. Rock salt is a source of common salt used for food, and in the chemical industry.
2. Gypsum is used in making plaster of Paris, cement and fertilizer.
3. Limestone is used for the manufacture of cement and as a building and road-making material.
4. These rocks provide valuable raw materials that support industries and generate employment and revenue; some (potash) are used as fertilizers to improve agriculture.

(a) Formation of a stack

A stack is a pillar or column of rock standing in the sea, detached from the coast. It is formed by marine (wave) erosion of a headland:

1. Waves attack lines of weakness (joints or faults) at the base of a headland by hydraulic action, abrasion and solution.
2. Continued erosion widens the weakness into a sea cave on the side of the headland.
3. When two caves on opposite sides of the headland are eroded until they meet, a natural arch is formed through the headland.
4. Further erosion and weathering enlarge the arch until its roof becomes too heavy and collapses.
5. The collapse leaves an isolated column of rock standing in the sea, separated from the headland; this pillar is the stack. (When a stack is later worn down to a low stump it becomes a stump.)

Diagram should show, in stages: headland with cave, arch, and finally the detached stack.

(b) Formation of a fold mountain

Fold mountains are formed by the folding of sedimentary rocks under compression:

1. Over a long period, thick layers of sediment accumulate in a large shallow depression in the earth's crust called a geosyncline, usually on the floor of a sea.
2. The sediments are compacted and cemented into layers of sedimentary rock.
3. Powerful horizontal compressional forces (caused by the movement of crustal/tectonic plates towards each other) squeeze the rock layers from opposite sides.
4. The layers are crumpled and buckled into a series of up-folds called anticlines and down-folds called synclines.
5. These folded and uplifted rock layers form the fold mountains, for example the Alps, Himalayas and Atlas Mountains.

Diagram should show folded strata with labelled anticlines (upfolds) and synclines (downfolds).

(a) Four characteristics each

(i) Time Zone

1. A time zone is a belt or region of the earth that keeps the same standard time.
2. Each standard time zone covers \(15^\circ\) of longitude, because the earth turns \(15^\circ\) in one hour (\(360^\circ \div 24 = 15^\circ\)).
3. Places within one time zone use a common (standard) time based on a central meridian.
4. Time increases (is ahead) eastward and decreases (is behind) westward from the Greenwich Meridian.

(ii) International Date Line

1. It is an imaginary line drawn approximately along the \(180^\circ\) meridian.
2. It runs roughly north-south from the North Pole to the South Pole through the Pacific Ocean.
3. It is not perfectly straight; it bends to avoid dividing land areas and island groups into two different dates.
4. A traveller crossing it going westward gains a day (adds one day), while one crossing it going eastward loses a day (subtracts one day).

(b) Calculation of the longitude of town B

Time in A = 9.00 a.m.; time in B = 6.00 p.m. same day. B is ahead of A, so B lies to the east of A.

Time difference \(= 6.00\text{ p.m.} - 9.00\text{ a.m.} = 9\) hours.

Longitude difference \(= 9 \times 15^\circ = 135^\circ\).

Since B is east of A and A is at \(30^\circ\)W:

(Starting from \(30^\circ\)W and moving \(30^\circ\) east reaches the \(0^\circ\) meridian, leaving \(105^\circ\) to be counted eastward.)

The longitude of town B is \(105^\circ\)E.

(a) What is a lake?

A lake is a body of water, usually fresh but sometimes salty, that occupies a hollow or depression in the surface of the land and is completely surrounded by land. It is inland and standing (still) water.

(b) Formation of lakes by wind erosion (deflation)

In hot desert regions, strong winds pick up and blow away loose, dry sand and dust from the surface in a process called deflation. Where the wind removes the loose materials continuously from a soft or unconsolidated area, it scoops out a shallow, rounded depression called a deflation hollow. Wind erosion may continue downward until the floor of the hollow reaches the level of the underground water table. Water then seeps into the hollow, or rain water collects in it, forming a small lake known as a deflation-hollow lake or oasis. Such lakes are common in desert basins.

(c) Three benefits of lakes to man

1. Source of water: Lakes supply water for domestic use, irrigation and industry.
2. Fishing: Lakes support fishing, providing food and employment.
3. Transport, tourism and power: Lakes serve as inland waterways for navigation, attract tourists for recreation, and man-made lakes behind dams are used to generate hydro-electric power.