Question 6 – Alluvial Fan

(6a) An alluvial fan is a fan-shaped deposit of sediments such as sand, gravel, and silt formed when a river or stream flows from a steep slope onto a flat plain and loses its velocity, causing it to deposit its load.

(6b) Diagram requirement: Candidates are required to draw a neat, well-labelled diagram of an alluvial fan in the exam.

Characteristics of an alluvial fan:

• It is fan-shaped or cone-shaped in appearance.
• It is formed at the foot of a mountain or steep slope.
• It consists of deposited materials such as sand, gravel, pebbles, and silt.
• It has a gentle outward slope from the apex to the edges.
• The sediments are thickest near the apex and thinner towards the margins.
• It is usually crossed by several distributary channels.

Mode of formation of an alluvial fan:

• A river or stream flows down a steep mountain slope carrying large quantities of sediments.
• On reaching a flat plain or valley floor, the gradient suddenly decreases.
• The river loses velocity and transporting power.
• Coarse materials such as gravel and pebbles are deposited first near the mouth of the slope.
• Finer materials such as sand and silt are carried farther and deposited towards the outer edges.
• Repeated deposition over time causes the sediments to spread outward in a fan-like pattern, resulting in the formation of an alluvial fan.

Question 7 – Rainfall and Latitude

(7a) Factors influencing the amount of rainfall received by a place (pick any four in the exam):

• Relief or topography influences rainfall because mountains force moist air to rise, cool, and condense, resulting in rainfall.
• Distance from the sea affects rainfall because areas closer to the sea receive more moisture-laden winds and therefore more rainfall.
• Prevailing winds influence rainfall depending on whether they carry moisture from the ocean or dry air from the land.
• Latitude affects rainfall because different latitudinal zones receive varying amounts of heat and atmospheric moisture.
• Ocean currents influence rainfall as warm currents increase evaporation and rainfall, while cold currents reduce rainfall.
• Vegetation cover affects rainfall because plants release moisture into the atmosphere through transpiration.
• Atmospheric pressure systems influence rainfall since low-pressure areas encourage rising air and precipitation, while high-pressure areas discourage rainfall.
• Temperature affects rainfall because higher temperatures increase evaporation and the amount of moisture available in the atmosphere.

(7b) Diagram requirement: Candidates are required to draw a diagram showing the relationship between latitude and temperature.

Explanation of the relationship between latitude and temperature:

• Latitude is the angular distance of a place north or south of the Equator.
• Temperature generally decreases from the Equator towards the Poles.
• Places near the Equator receive the sun’s rays almost vertically throughout the year.
• Vertical rays are concentrated on a smaller surface area and therefore produce higher temperatures.
• As latitude increases, the sun’s rays strike the Earth at a more oblique angle.
• Oblique rays spread over a larger surface area and provide less heat per unit area.
• The rays also pass through a thicker layer of the atmosphere, causing more heat to be absorbed and scattered.
• Consequently, places in low latitudes experience higher temperatures.
• Places in middle latitudes experience moderate temperatures.
• Places in high latitudes experience low temperatures due to the highly slanting rays of the sun.
• Therefore, there is an inverse relationship between latitude and temperature: as latitude increases, temperature decreases.