Earth processes: plates, coasts, tides and currents
Connect plate evidence and boundary processes to ocean-floor features, coastal change, tides and global circulation.
Section Topic 2 · Earth processes: plates, coasts, tides and currents
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 2.1, 2.2, 2.3 for examinations in 2025–2030. AS candidates study Topics 1–5; A Level candidates study all nine topics and are expected to know the AS content. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or grades. The syllabus requires core practical activities and scientific-method experience. Online explanation and simulated data questions can rehearse methods but do not count as completing practical work or demonstrate safe, competent use of equipment. Learners need suitable teacher-supervised laboratory and field opportunities.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: What is the difference between a high-tide height and tidal range?
What you will learn
- Use evidence for plate tectonics and explain ocean-floor features and tsunamis
- Distinguish weathering, erosion, transport and sedimentation at coasts
- Interpret tidal height, range, spring-neap patterns and tide graphs
- Explain surface and deep currents, gyres, Coriolis deflection and upwelling
Tectonic plates and ocean floors
The crust, mantle and core form Earth’s internal structure. Evidence for moving plates includes matching rocks and fossils across continents, paleomagnetic stripes and the jigsaw fit of coastlines. At convergent margins, subduction can produce trenches, earthquakes and volcanoes; divergent boundaries form ridges and new crust; transform boundaries produce lateral motion and earthquakes. Undersea displacement can generate a tsunami. Hydrothermal vents and abyssal plains arise through distinct geological processes, so identify the mechanism that fits the feature.
Weathering, erosion and sedimentation
Weathering breaks rock down in place; erosion removes and transports material. Ice, water, wind and gravity can erode. Sedimentation deposits suspended particles when transport energy falls. Flow speed and particle size affect whether material is removed, carried or deposited. The littoral zone runs between the highest and lowest spring-tide marks and includes rocky, sandy and muddy shores, estuaries and deltas. Explain landforms by tracing process, transport and deposition rather than listing terms.
Tides, currents and upwelling
Tide range is shaped by Earth–Moon–Sun alignment and local coast geometry, wind, air pressure and the size of the water body. Spring tides have greater range; neap tides have smaller range. Read tide tables by matching time to height, identifying successive highs/lows and calculating range. Winds, temperature, density and Earth’s rotation help drive currents. Coriolis deflects currents clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Surface circulation and density-driven deep flow form the global conveyor belt; upwelling brings deeper water and nutrients upward. When discussing boundary currents, use the corrected 2025 Version 3 term seabed and explain how contact with the seabed can affect movement and mixing.
Sources for this lesson
Pause and explain
Which feature is most associated with a divergent plate boundary?
Worked example
A tide table gives a high tide of 3.4 m and a low tide of 0.8 m. Calculate the range. A nearby upwelling zone has high nutrients. Explain why this may increase productivity.
Show the worked solution
- Calculate tidal range as high height minus low height.
- Find 3.4 − 0.8 = 2.6 m.
- Upwelling supplies nutrients to sunlit surface water; if light and other conditions are suitable, producers can use them.
Answer The tidal range is 2.6 m. Upwelling can bring nutrient-rich deep water to the lighted surface, supporting producer growth when light, temperature and other requirements are adequate.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A current moves 48 km east over 6 hours. Calculate its average speed and state its direction.
Give me a hint
Convert hours to seconds only if your chosen speed unit requires it.
Compare my reasoning
- Use speed = distance ÷ time.
- Calculate 48 ÷ 6 = 8 km h−1.
- Keep the direction separate from the speed magnitude.
Average speed = 8 km h−1 east. If reporting m s−1, convert 8 km h−1 to approximately 2.22 m s−1 east.
Look for these in your work
- Uses distance ÷ time
- Gives a consistent unit
- States direction separately
A steep rocky coast has little loose sediment, while a nearby river delta has extensive deposits. Explain one process difference that could help account for this pattern.
Give me a hint
Think about sediment supply, transport energy and deposition.
Compare my reasoning
- Identify that the river supplies suspended particles to the delta.
- Explain that flow slowing near the coast can allow deposition.
- Contrast with a steep exposed coast where higher energy may remove material.
A river can supply sediment that settles as flow slows at its mouth, building a delta. A steep, exposed coast may have stronger waves and currents that transport sediment away. Local geology and human structures can also influence the pattern.
Look for these in your work
- Mentions a sediment source
- Links lower flow energy to deposition
- Qualifies the comparison
A tide graph has two larger ranges around a new Moon and smaller ranges around quarter Moons. Interpret the pattern and give one local factor that can alter the observed range.
Give me a hint
Relate lunar phase to spring/neap tides, then include a coastal or weather factor.
Compare my reasoning
- Associate the larger ranges with spring tides.
- Associate the smaller ranges with neap tides.
- Name a local influence such as coastal geomorphology, wind or air pressure.
The pattern is consistent with spring tides near new Moon and neap tides near quarter Moons. The observed range can be modified by coastal shape, wind, air pressure and the size of the water body.
Look for these in your work
- Identifies spring and neap patterns
- Uses the graph evidence
- Includes a local influence
Common mistakes
- Using 'weathering' for material removal
- Treating all plate boundaries as subduction zones
- Confusing tide height with tidal range
- Assuming nutrient supply alone guarantees high productivity
What can you explain now?
Which feature is most associated with a divergent plate boundary?
Compare with the explanation
A mid-ocean ridge where new crust forms
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Why can a tide graph and a current measurement answer different questions about moving seawater?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.