Biology & life sciences · 09 / 12
Plate tectonics: read the evidence
High school · intermediate · Worked example and 4 exercises with solutions, at your pace.
The principle, simply
Lithospheric plates move relative to their neighbours. Divergent boundaries separate plates and create oceanic lithosphere at ridges. Convergent boundaries may involve subduction; transform boundaries slide past each other. Earthquakes, volcanism, seafloor ages and geodetic measurements provide complementary evidence.
Simplified divergent boundary: rocks become older away from the axis on both sides. Arrows show relative plate separation.
- 01Locate evidence
- 02Compare ages
- 03Draw a cross-section
- 04Explain movement
WORKED EXAMPLE
From principle to reasoning
At a ridge, rocks are youngest near the axis and become older away from it on both sides. Symmetrical magnetic bands provide further evidence of spreading.
Solve a problem in this subject
- Frame a question
State the relationship to explain and its scale: molecule, cell, organism, population or geological time.
- Describe evidence
Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.
- Connect a mechanism
Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.
- Answer with a limit
Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.
YOUR TURN
01 · Try before checking
Why can earthquakes deepening towards a continent suggest subduction?
One hint
Draw earthquake foci in cross-section, not only on a map.
Reveal and understand the solution
Foci may trace an inclined plane where an oceanic plate descends beneath another. Compare this with relief, volcanism and other observations.
Compare method, conditions and answer; identify the exact part to revisit.
IMPROVE THROUGH PRACTICE
Three additional challenges with solutions
Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.
Do plates converge or diverge at an ocean ridge?
A hint to get started
Connect lithosphere formation to motion.
Step-by-step solution
- They move apart: divergence.
- Hot material rises and oceanic lithosphere forms.
- A ridge is not a subduction zone.
Which step did you miss? Retry without help, then explain why the method works.
Seafloor age increases away from both sides of a ridge. What does this support?
A hint to get started
Compare a spreading prediction with observations.
Step-by-step solution
- The youngest lithosphere is near the ridge.
- Greater distance corresponds to greater age.
- This supports seafloor spreading, alongside other evidence.
Which step did you miss? Retry without help, then explain why the method works.
Why do some ocean trenches have deep earthquakes nearby?
A hint to get started
One plate descends beneath another.
Step-by-step solution
- In subduction, oceanic lithosphere sinks.
- Earthquake foci outline an inclined zone associated with the slab.
- Their depths help infer slab geometry.
Which step did you miss? Retry without help, then explain why the method works.
The trap to avoid
Plates are not merely continents; many include both continental and oceanic lithosphere.
Without notes, can you…
- Explain the principle in your own words?
- Rebuild the diagram and its connections?
- Solve the exercise and justify the method?
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