Interior of the Earth

Total: 120 marks — Investigation 1: 37 · Investigation 2: 33 · Investigation 3: 28 · Investigation 4: 22.

Investigation 1 — the diagram

• Q1 (14): Working outward: inner core (solid, iron–nickel, ~1 200 km radius) · outer core (liquid, iron–nickel, ~2 200 km thick — CAPS quotes 2 180 km) · mantle (semi-fluid/plastic silicate rock, high magnesium and iron, to ~2 900 km below the surface) · asthenosphere (plastic, upper mantle) · lithosphere (solid, 60–100 km, silicon, aluminium, calcium, magnesium, potassium, sodium). Mark the convention marks strictly — they are free marks in an exam and learners routinely throw them away.
• Q2 (3): One mark for a recorded time, two for an honest list of first-attempt errors. There is no target time; the point is the comparison.
• Q3 (3): Inner core, mantle, lithosphere.
• Q4 (5): Inner core solid · outer core liquid · mantle semi-fluid / plastic · asthenosphere plastic · lithosphere solid.
• Q5 (4): They are mechanical divisions — defined by how the rock behaves (rigid vs able to flow) rather than by what it is made of (2). The mantle is a compositional layer, so the lithosphere and asthenosphere cut across the top of it rather than sitting outside it (2) — which is why a simple ring diagram cannot show both schemes at once without a note.
• Q6 (4): The crust is the outermost rocky shell; the lithosphere is the crust plus the rigid uppermost mantle, so the lithosphere is thicker (2). If a question uses CAPS's wording, answer it as asked — but you can note the distinction, and a good marker will credit it (2).
• Q7 (4): 100 ÷ 6 371 × 100 = 1.6% (2). At the size of a page, 1.6% of the radius is around a pixel — thinner than the pen line drawing it, and impossible to label (2).

Investigation 2 — the eight methods

• Q8 (16): Drilling — actual rock samples — direct. Seismic waves — layering, boundaries, state — indirect. Active volcanic material — upper mantle composition — indirect (it came to us, we did not go to it). Dormant/extinct volcano remnants — exposed once-deep rock — indirect. Meteorites — likely core composition — indirect. Olivine and pyroxene — lithosphere composition — indirect. Dimensions, density, rotation, gravity, magnetic field — total mass and its distribution, and evidence for a liquid metal core — indirect. Behaviour of sound in rock — material and layer thickness — indirect. (2 per row.)
• Q9 (3): One — drilling (1) — reaching 12 262 m at Kola (2).
• Q10 (4): 12.262 ÷ 6 371 × 100 = 0.19%, about 0.2% (3). So direct evidence covers a fifth of one percent of the way to the centre, and every statement about the other 99.8% is inference (1).
• Q11 (3): The rock was over 180 °C (1), hotter than predicted (1), and at that temperature it behaved plastically rather than as brittle rock, closing the hole around the drill (1).
• Q12 (4): Meteorites are thought to be leftover material from the same cloud that formed the planets, and iron ones resemble what a differentiated core should contain (2). Weakness, any one (2): a meteorite is not from the Earth, so the resemblance is an assumption; and material that survived a fall through the atmosphere may not be representative.
• Q13 (3): Volcanic material originates in the upper mantle, at most a few hundred kilometres down (2). Nothing from the core, ~2 900 km down, ever reaches the surface — so the core remains entirely inferred (1).

Investigation 3 — the rocks

• Q14 (8): Two marks per sample, marked for internally consistent reasoning rather than against a fixed answer, since the learner's own rocks will vary.
• Q15 (6): A = granite — large interlocking crystals mean slow cooling deep underground (3). B = basalt — no visible crystals means fast cooling at or near the surface, and the small round holes are gas bubbles frozen in (3).
• Q16 (3): C (1) — it is layered, and made of rounded grains that rub off, so it is cemented sediment rather than cooled melt (2).
• Q17 (4): Greater density for the same volume means more heavy elements, especially iron and magnesium (2). That makes it more like mantle material than like continental crust — olivine and pyroxene are exactly this kind of dense, dark, iron-and-magnesium-rich mineral (2).
• Q18 (4): Igneous rock is solidified molten material from inside the Earth (2). It is a sample of the interior that travelled to you, so its composition and crystal size are evidence about conditions at depth — composition tells you what is down there, crystal size tells you how fast it cooled and therefore how deep (2).
• Q19 (3): If minerals found at the surface match what the model predicts the lithosphere is made of (2), the model is supported by an independent line of evidence rather than resting on seismic waves alone (1).

Investigation 4 — the shadow zone

• Q20 (2): Roughly 103°–140° — from the data, between stations 4 (95°, both arrive) and 8 (140°, P returns). Accept any answer consistent with the table.
• Q21 (5): S waves cannot travel through a liquid (2). They are absent everywhere beyond the shadow zone, which means they have been stopped by something they cannot cross (2) — so the outer core must be liquid (1).
• Q22 (4): They have been refracted — bent — on entering and leaving the core, because the wave speed changes sharply at the boundary (2). The bent path is longer than a straight one, so they arrive late (2).
• Q23 (4): Because a whole band of the Earth's surface receives no direct arrivals — a "shadow" cast by the core (2). A gradual change in the rock would bend waves smoothly and blur the effect; a sharp-edged shadow requires a sharp boundary, which is how we know the core has an edge rather than fading in (2).
• Q24 (3): Thousands of earthquakes are recorded every year by a worldwide network of seismometers (2), so the same pattern has been reproduced from countless different source locations — the strongest form of repeatability there is (1).
• Q25 (4): Seismic waves travel faster through denser, more rigid material (2). So measuring how the speed changes with depth reveals where the density changes, and by how much — which is how the layers were not only located but weighed (2).

Marking note. Q1 carries 14 of the sheet's marks on a single diagram, deliberately. It is the one output CAPS names for this block, and a learner who can talk about the layers but cannot draw them has not met the outcome.