A slope fails when the force pulling material downhill becomes larger than the slope’s resistance. Each factor in the diagram either adds to the pull or reduces the resistance.
This lesson is part of weathering and mass movement. It builds on explaining physical and chemical weathering under stated conditions, since weathered rock is easier to move.
How does each factor change stability?
| Factor | Effect on the slope | Direction |
|---|---|---|
| Steeper angle | More gravity pulls along the slope | Less stable |
| Heavy rain | Soil fills with water, heavier and slipperier | Less stable |
| Vegetation removed | No roots bind the soil | Less stable |
| Weak layered rock | Layers slide along each other | Less stable |
| Gentle angle, forest cover | Roots bind, water is taken up | More stable |
Worked example: an invented hillside
An original diagram shows a hillside above a village. Labels read: slope angle 35 degrees; top half bare, bottom half forested; a clay layer below the soil; a heavy rainfall arrow; a road cutting at the base.
Factor by factor.
- The 35 degree angle is steep, so gravity pulls strongly downslope.
- The bare top half has no roots, so rain soaks in and the soil loosens.
- The clay layer stops water sinking, so the soil above it becomes saturated and heavy.
- The road cutting removes support at the base of the slope.
Combine them. Heavy rain on the bare, steep top half saturates the soil above the clay. The wet soil gets heavier and slides along the clay, and the road cutting has removed the support below. The slope is most likely to fail at the top half, where more factors overlap.
The mistake that costs marks
The slip is to write a list, such as “angle, rain and vegetation cause landslides”. It names factors but never explains how they work together.
A stronger answer says which factors overlap and what each one does: “Bare soil cannot take up rainfall, so the clay layer becomes saturated, and the steep angle lets gravity move it.”
Check yourself
A diagram shows a gentle 10 degree slope fully covered by forest, with heavy rain and no road cutting. Is this slope as likely to fail as the hillside above? Explain.
Answer
It is less likely to fail. The gentle angle reduces the pull of gravity, and the forest roots bind the soil and take up water. Heavy rain still adds weight, but the other factors keep resistance high. Fewer factors overlap, so the risk is lower.
What to study next
Move on to comparing management measures using the supplied geographical context, where you decide how to reduce these risks. The graph evidence and fair-comparison lab helps with comparing slopes from data.
If you want a teacher to question you on slope diagrams, see online one-to-one Geography tuition.