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Teachers

A lesson for your classroom or geography club

Mass movement

All teacher notes · Open the lab

Start projector

An aerial view of the Oso landslide's bare source scar, debris path and deposits across the valley.
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Oso, Washington, 2014: an aerial photograph of the landslide source, travel path and deposit. The displaced material crossed the valley floor.

A suggested lesson sequence

Use these stages at your own pace. Pause for students to sketch, compare observations or lead part of the demonstration.

  1. 01

    Observe

    Locate crown, curved shear surface and toe. Read the assumed strength and initial factor of safety.

    Open Observe

  2. 02

    Tinker

    Increase Pore pressure and relate falling resistance to effective stress. At the highest setting, play the prescribed rotational movement.

    Open Tinker

  3. 03

    Explain

    How does increasing pore pressure reduce the resistance to sliding along the selected surface?

    Open Explain

  4. 04

    Check

    Choose an answer and explain your reasoning. Each question includes feedback.

    Open Check

Rehearse with the model

The same scene and controls as the lab. Load it when you are ready; use Start projector for the full classroom view.

Before the lesson

Identify the main parts of a rotational slump and explain how rainfall can reduce the stability of a slope.

Open the model before the lesson and choose a view that makes the important feature easy to see. Drag to rotate, and use the on-screen controls to change one variable at a time. The Controls panel explains the available keyboard shortcuts.

Pause the animation at a useful moment. Ask students what should happen next, then play it to test their explanation.

Guide the demonstration

Locate crown, curved shear surface and toe. Read the assumed strength and initial factor of safety.

Increase Pore pressure and relate falling resistance to effective stress. At the highest setting, play the prescribed rotational movement.

Discuss a common misconception

“Rain can destabilise a slope only by making the material heavier.” Added water can increase weight, but rising pore-water pressure can also reduce effective stress and frictional resistance along a potential slip surface. Drainage, cohesion, slope geometry and material properties influence stability, so the effect is not explained by added mass alone.

Build the explanation

Gravity drives a slope downhill while friction and cohesion resist movement. Rising pore-water pressure can reduce effective stress and resistance, making a chosen slip surface less stable.

Increase Pore pressure and relate falling resistance to effective stress. At the highest setting, play the prescribed rotational movement.

The factor of safety describes the initial slope and selected surface; the later rotation is an illustration of movement, not a calculated runout.

Teaching notes

Consider slope material, gravity and water pressure together rather than attributing every failure to wave erosion.

Distinguish a rockfall from a slide or flow by describing how the material moves.

Explain how saturation can reduce slope stability, including on slopes that are not beside the sea.

Check understanding

Open Check for 4 questions. Ask students to explain their choice before revealing the answer.

Return to the opening photograph. Ask students to identify one visible feature and explain which part of the model helps account for it.

Look more closely

Use these observations to connect the vocabulary to features students can identify.

Rotational slump

Uneven displaced ground and scarps above the coast at Redcliff Point, Dorset.
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Slumping above Redcliff Point, Dorset. Rotational movement displaces blocks along a curved shear surface; the surface scars and tilted ground provide evidence.
Read the definition

Till

An exposed bank of poorly sorted glacial sediment containing stones of several sizes.
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A natural exposure of glacial till. Ice deposits a mixture of grain sizes directly, often without the sorting produced by flowing water.
Read the definition

Sources to keep beside the lesson

Same realm