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Geolens

Teachers

A lesson for your classroom or geography club

Geologic time

All teacher notes · Open the lab

Start projector

Younger gently dipping red sandstone overlies steep grey beds at Siccar Point, Scotland.
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Siccar Point: Devonian sandstone overlies much older, tilted Silurian beds. The unconformity records deformation, erosion and missing time.

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

    Start at the oldest end of the full Earth-history window.

    Open Observe

  2. 02

    Tinker

    Move towards the present, then enlarge Phanerozoic or Quaternary time.

    Open Tinker

  3. 03

    Explain

    Choose two intervals and compare their durations. How does enlarging a time window change which details you can see?

    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

Read dates in millions of years and compare the unequal durations of major intervals in Earth’s history.

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.

Ask students to predict the result before you change a setting. Compare the new view with the starting state.

Guide the demonstration

Start at the oldest end of the full Earth-history window.

Move towards the present, then enlarge Phanerozoic or Quaternary time.

Discuss a common misconception

“The Grand Canyon is as old as the rocks exposed in its walls.” The rocks formed before erosion exposed them in the canyon. Rock formation, deformation, uplift and river incision belong to different parts of the geological history and require different evidence to date. An old rock can therefore form the wall of a much younger landform.

Build the explanation

Earth’s history spans about 4.54 billion years. Placing events on a proportional timeline shows how recent human history is and why a short interval near the present can hide much older events.

Move towards the present, then enlarge Phanerozoic or Quaternary time.

The timeline is a scale for comparing ages; event dates have different uncertainties and should not all be read as equally precise.

Teaching notes

Read the numerical dates rather than the width of an event marker to judge its duration.

Distinguish the coiled time display from a column showing the thickness of real rocks.

Explain that enlarging a time window changes the display scale without changing how long the events lasted.

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.

ka

Green crystalline mantle peridotite enclosed by darker volcanic host rock at Peridot Mesa, Arizona.
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The volcanic host of this Peridot Mesa xenolith is described by its source as about 580 ka old: roughly 580,000 years. This dates the host eruption, not the formation of the older mantle fragment.
Read the definition

Pangaea

A dark Glossopteris fossil leaf from Permian rocks in Antarctica, with a branching vein network.
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Permian Glossopteris leaf from Antarctica. Related fossils on separated southern continents are evidence that these lands were once joined within Gondwana, which formed part of Pangaea.
Read the definition

Sources to keep beside the lesson

Same realm