Practical 11 — Plate Tectonics

Total: 87 marks — Investigation 1: 32 · Investigation 2: 29 · Investigation 3: 26.

This memo does not print with the worksheet.

Investigation 1 — the block models

• Q1 (9): Three sketches, 3 each. Divergent — blocks apart, gap filling with magma, ridge and rift valley labelled. Convergent — one block descending beneath the other, trench and volcanoes labelled. Transform — blocks sliding in opposite directions, fault line labelled. Each needs an underlined heading and ruled label lines.
• Q2 (12): Divergent — plates move apart · crust created · mid-ocean ridge, rift valley, hydrothermal vents, shallow earthquakes · Mid-Atlantic Ridge / East African Rift. Convergent — plates move together · crust destroyed · trench, volcanoes, island arc, deep earthquakes, mountains · Peru–Chile Trench / Himalayas. Transform — plates slide past · neither created nor destroyed · fault zone, severe shallow earthquakes · San Andreas Fault.
• Q3 (3): The snag-and-jerk models friction locking the plates while stress builds, then releasing suddenly. The stored elastic energy is released in one movement — an earthquake. They are shallow because the movement happens near the surface where the plates are in contact, and severe because that shallow energy reaches the surface with little attenuation.
• Q4 (3): Oceanic crust is denser than continental crust, so where they meet the oceanic plate subducts — forming a trench, and melting at depth to feed volcanoes. Where two continental plates meet, neither is dense enough to sink, so the crust has nowhere to go but up: it buckles and thickens into mountains.
• Q5 (3): Good (2): it shows the plates as rigid slabs moving over a softer layer beneath; it shows all three relative motions correctly; it shows crust being consumed at one edge. Misleading (1): accept any of — the cloth is not hot or convecting, and convection is the actual driver; the blocks do not melt, deform or create new material; the scale and speed are utterly unlike reality (cm per year); the real boundaries are not straight lines.
• Q6 (2): Crust created at divergent boundaries is balanced by crust destroyed at convergent boundaries through subduction. The total surface area stays constant — it is a recycling system, not a growth one.

Investigation 2 — vents vs land

• Q7 (8): One mark per row. Energy: chemical (H₂S) / sunlight · Producers: chemosynthetic bacteria / grasses and succulents · Process: chemosynthesis / photosynthesis · Primary consumers: tube worms, clams, mussels, shrimp / springbok, insects, tortoises · Predators: vent crabs, octopus, zoarcid fish / caracal, jackal, raptors · Light: none at all / full sunlight · Temperature: 2 °C to 400 °C within metres / roughly −5 °C to 38 °C · Duration: decades, then the community dies / thousands of years, stable.
• Q8 (4): The energy source. Every other ecosystem on Earth is ultimately powered by sunlight captured through photosynthesis. A vent community is in total darkness and is powered by the chemical energy in hydrogen sulphide, captured through chemosynthesis. It is the only major ecosystem not dependent on the sun.
• Q9 (4): Any two, well explained: both have the same trophic structure — producers, primary consumers, predators, decomposers; both have energy flowing one way through the web with losses at each level; both recycle nutrients; both show organisms adapted to their specific conditions; both have comparably high biomass density.
• Q10 (3): Chemosynthesis is the process by which organisms build sugar (organic food) using energy released by chemical reactions rather than light (2). The chemical is hydrogen sulphide (H₂S) in the vent fluid (1).
• Q11 (3): The worm houses chemosynthetic bacteria inside its own tissues (in an organ called the trophosome) and supplies them with the hydrogen sulphide, oxygen and CO₂ they need, absorbed through its plume. The bacteria produce sugars, which nourish the worm. It is a mutualistic symbiosis: the worm gets food, the bacteria get raw materials, protection and a stable position in the flow.
• Q12 (3): Vents need magma close to the seafloor and freshly cracked crust for seawater to seep into. Both occur at a spreading centre, where plates separate and new hot crust is forming. At a convergent boundary the crust is old, cold and being destroyed rather than created, so the heat source and the plumbing are both absent.
• Q13 (4): For (1): vent deposits are unusually concentrated in copper, zinc and gold; these metals are needed for renewable-energy technology; seabed mining may disturb fewer people than terrestrial mining. Against (1): it destroys an ecosystem found nowhere else on Earth, containing species still undescribed; recovery would take centuries if it happens at all; the deep sea is almost impossible to monitor or police. Position (2): any clear stance with a reason that draws on the candidate's own two points.

Investigation 3 — identification drill

• Q14 (18): 3 marks each. (a) Iceland — divergent · created · mid-ocean ridge, rift valley, volcanoes, geothermal activity. (b) Peru–Chile — convergent (oceanic–continental) · destroyed · ocean trench, Andes volcanoes, deep earthquakes. (c) San Andreas — transform · neither · fault zone, severe shallow earthquakes. (d) Himalayas — convergent (continental–continental) · neither created nor subducted; crust thickened · fold mountains, shallow earthquakes, no volcanoes. (e) East African Rift — divergent (continental) · created · rift valley, volcanoes, lakes. (f) Tristan da Cunha — not a boundary — a hotspot · neither · volcanic island, ageing chain of seamounts (the Walvis Ridge).
• Q15 (3): A stationary mantle plume burns through the plate moving over it. Because the plume stays put while the plate slides, it produces a chain of volcanoes that gets progressively older in the direction of plate motion — here, the Walvis Ridge running back toward Namibia.
• Q16 (3): Sitting in the middle of the African Plate, far from any boundary, means low earthquake and volcano risk (2) — most SA tremors are mining-induced, not tectonic. The exception is the Prince Edward Islands, specifically Marion Island, which is volcanically active and last erupted in 2004 (1). Accept Tristan da Cunha / Gough if identified as SA-administered research territory.
• Q17 (2): A continent begins to rift apart along a divergent boundary, dropping a valley. As the rift widens and the floor drops below sea level, seawater floods in — the Red Sea stage. Continued spreading widens it into a full ocean basin with its own mid-ocean ridge. The East African Rift is the first stage of that sequence, happening now.