Geological Time

Total: 189 marks — Investigation 1: 47 · Investigation 2: 39 · Investigation 3: 31 · Investigation 4: 36 · Investigation 5: 36.

Note on the supplied roll. All Investigation 2 answers below assume the supplied roll: 240 sheets × 11.0 cm = 26.4 m, so 1 sheet = 4600 ÷ 240 = 19.17 million years. If the learner measured their own roll, mark for correct method and internal consistency, not against these numbers.

Investigation 1 — the table and the wall chart

• Q1 (8): Hadean 4600–4000, 600 my, Earth molten, no solid surface · Archean 4000–2500, 1500 my, crust cools, oceans form, first life · Proterozoic 2500–541, 1959 my, oxygen appears, first complex and multicellular life · Phanerozoic 541–today, 541 my, abundant life with hard parts. (2 each.)
• Q2 (9): Palaeozoic 541–251, "ancient life", ends with the Permian–Triassic extinction · Mesozoic 251–65, "intermediate life", ends with the Cretaceous–Tertiary extinction · Cenozoic 65–today, "recent life", ongoing. (3 each.)
• Q3 (12): Palaeozoic: Cambrian 541–488 · Ordovician 488–443 · Silurian 443–416 · Devonian 416–359 · Carboniferous 359–299 · Permian 299–251. Mesozoic: Triassic 251–199 · Jurassic 199–145 · Cretaceous 145–65. Cenozoic: Palaeogene 65–23 · Neogene 23–2 · Quaternary ~2–today. One mark each, awarded only if the order and the parent era are both right.
• Q4 (4): Hadean, Archean and Proterozoic (3). The Phanerozoic is excluded because the grouping is defined by what came before the Cambrian — before animals had hard parts, so before the fossil record properly begins (1).
• Q5 (5): Durations 600 / 1500 / 1959 / 541 my (2). Proterozoic is longest (1). Phanerozoic = 541 ÷ 4600 = 11.8%, so roughly one-ninth of Earth's history holds almost the entire fossil record (2). Accept "about 12%".
• Q6 (6): Marked as stated. The commonest failure is drawing the four eons as four equal blocks — check the Phanerozoic band really is about a ninth of the strip.
• Q7 (3): Any defensible answer, e.g. the nesting only becomes obvious when you have to decide which column a name goes in (1); the gaps show you what you did not know (1); calculating the durations shows how uneven they are, which reading a finished chart hides (1).

Investigation 2 — the toilet roll

• Q8 (3): 4600 my ÷ 240 sheets = 19.17 million years per sheet (2), working shown (1). Equivalently 174 my per metre, or 1.74 my per centimetre.
• Q9 (12): Sheet = (4600 − age) ÷ 19.17; distance = sheet × 11 cm. Earth forms 0 · oldest crust 15.7 (1.72 m) · Archean 31.3 (3.44 m) · Barberton 57.4 (6.31 m) · Proterozoic 109.6 (12.05 m) · Great Oxygenation 114.8 (12.63 m) · Vredefort 134.6 (14.81 m) · Cambrian explosion 211.8 (23.30 m) · Table Mountain sandstone 213.4 (23.47 m) · Permian–Triassic 226.9 (24.96 m) · Pangaea breaks up 229.6 (25.25 m) · K–T 236.6 (26.03 m) · australopithecines 239.8 · H. sapiens 239.98 · recorded history 240.0. Allow ±0.5 of a sheet.
• Q10 (4): The Precambrian ends at sheet 211.8, so it occupies 211.8 of 240 sheets ≈ 88% (3) — about 23.3 m of the 26.4 m roll (1). The topic page quotes 87%, which comes from rounding the Phanerozoic to 600 my. Accept 87% or 88%; do not penalise either.
• Q11 (4): True (1). Sheet 240 covers 19.17 mya to today, and H. sapiens at 0.3 mya falls at (19.17 − 0.3) ÷ 19.17 = 98.4% of the way along the final sheet (2) — about 1.7 mm from the end of the roll (1).
• Q12 (5): 0.005 my ÷ 19.17 my per sheet = 0.000261 sheets; × 110 mm = 0.029 mm, i.e. about three hundredths of a millimetre (3). A pencil line is roughly 0.5 mm wide, so the line you draw to mark recorded history is about 17 times thicker than the thing it marks (2). This is the single best number on the sheet — the act of recording it destroys the scale.
• Q13 (4): Barberton (1), sheet 57.4 (1). It carries 3.5-billion-year-old stromatolite fossils, so life already existed less than a quarter of the way along the roll (1) — life is not a late addition to Earth's history but an early one, and it was marine (1).
• Q14 (3): Walking makes the duration physical: you cover more than 20 m of empty Precambrian before anything with a shell appears (2), and the entire part of the story you have ever been taught happens in the last two paces (1).
• Q15 (4): Any two, 2 each: the roll is uniform so it implies a constant rate of change, when the pace of geological and biological change is wildly uneven; it is linear, so everything recent is unreadable; it has a hard start, but Earth's formation took tens of millions of years and was not an instant; the dates themselves carry uncertainty that a sheet boundary hides.

Investigation 3 — the scaled timeline

• Q16 (8): 230 my/cm. Hadean 0–2.6 cm · Archean 2.6–9.1 cm · Proterozoic 9.1–17.6 cm · Phanerozoic 17.6–20 cm. Marks: correct scale (2), four bands correctly proportioned (4), labelled with dates (2).
• Q17 (6): (a) Phanerozoic 541 ÷ 230 = 2.35 cm = 23.5 mm (2) · (b) Cenozoic 65 ÷ 230 = 0.283 cm = 2.8 mm (2) · (c) Quaternary 2 ÷ 230 = 0.0087 cm = 0.087 mm (2), which is under a tenth of a millimetre.
• Q18 (4): The K–T extinction (65 mya) and the arrival of H. sapiens (0.3 mya) are 2.8 mm and 0.013 mm from the right-hand edge respectively (2), so they fall within one pencil line of each other and of the end of the timeline (2). The answer must contain a number.
• Q19 (6): The technique is an inset, blow-up or expanded section — accept "log scale" as an alternative with reasoning (2). Redrawn timeline with a to-scale main strip plus an expanded Phanerozoic beneath it, clearly linked (4).
• Q20 (3): The topic page's eon ribbon draws the four eons to scale, so the Phanerozoic is a thin strip at the right — and then expands its three eras into separate cards below (2). It has to, because at true scale the eras are a few pixels wide and could not be labelled or tapped (1).
• Q21 (4): The to-scale version is honest about proportion but hides content (1): almost everything with a name — every period, every extinction, every fossil group — is compressed into the last centimetre and becomes invisible (2). A chart nobody can read is not more honest, it is just unusable; the blow-up is a labelled exception rather than a distortion, which is why real charts say so on the diagram (1).

Investigation 4 — the 24-hour day

• Q22 (4): 1 hour = 4600 ÷ 24 = 191.7 my (1) · 1 minute = 3.19 my (1) · 1 second = 4600 ÷ 86 400 = 0.0532 my = about 53 000 years (2).
• Q23 (12): Oldest crust 01:33:55 · Archean 03:07:50 · Barberton 05:44:21 · Proterozoic 10:57:23 · Vredefort 13:27:39 · Cambrian explosion 21:10:39 · Permian–Triassic 22:41:26 · Pangaea 22:57:23 · K–T 23:39:39 · australopithecines 23:59:00 · genus Homo 23:59:07 · H. sapiens 23:59:54 · recorded history 23:59:59.9. Allow ±2 s.
• Q24 (4): Barberton stromatolites at 05:44:21 (2) — under a quarter of the day gone (1). The comment should note that life appears early, and that everything after it is not the arrival of life but its elaboration (1).
• Q25 (6): CAPS must mean the hominin lineage, not our own species (2). One minute before midnight is 3.19 mya, which is the time of the australopithecines — Australopithecus (2). The site is the Cradle of Humankind in Gauteng, with Sterkfontein its best-known cave (2). Marking note: full marks require the learner to say that neither figure is wrong — the word "humans" is doing the work. This is the best question on the sheet, and it is a real disagreement in a real syllabus, not a trick.
• Q26 (3): 0.005 my ÷ 0.0532 my per second = 0.094 seconds — under a tenth of a second (2), less time than it takes to blink (1).
• Q27 (4): Any reasonable pair, 2 each. The roll is better at duration — you physically walk it, and boredom is part of the lesson; it also survives on a wall. The day is better at the very recent end, because seconds are a unit we have intuition for, where 1.7 mm of paper is not; it also needs no equipment at all.
• Q28 (3): One sentence containing one figure, e.g. "On a clock where Earth forms at midnight and it is now midnight again, our species appears at 23:59:54 — we have been here for six seconds of a 24-hour day." (3)

Investigation 5 — the decay curve

• Q29 (8): 2 half-lives → 10.52 y, 2.00 g, 25% · 3 → 15.78 y, 1.00 g, 12.5% · 4 → 21.04 y, 0.50 g, 6.25% · 5 → 26.30 y, 0.25 g, 3.125% · 6 → 31.56 y, 0.125 g, 1.5625%. (Roughly 1.5 per row; be generous on the recurring percentages.)
• Q30 (8): As allocated. The commonest error is joining the points with straight lines — decay is exponential and the curve must be smooth and always falling, never reaching the axis.
• Q31 (4): 0.50 g is 1/16 of 8.00 g = 2⁻⁴, so 4 half-lives = 4 × 5.26 = 21.04 years (3), and the graph should be read at the same point (1).
• Q32 (4): Reading off gives roughly 7.4 years (2); accept 7–8 years. You cannot count half-lives because 3.0 g is not a simple fraction of 8.00 g — 3/8 is not a power of one-half, so the age falls between whole half-lives and only the curve can give it (2). This is precisely why the exam gives you a graph rather than a table.
• Q33 (4): Each half-life removes half of what is left, so there is always something remaining — mathematically it approaches zero without arriving (2). For very old rock the surviving parent becomes so small that measurement error swamps it, which is what sets the practical upper limit of any isotope, and why different isotopes are used for different age ranges (2).
• Q34 (4): Carbon-14's half-life is only 5 730 years, so after 200 million years — some 35 000 half-lives — no measurable ¹⁴C remains at all (2). Use Potassium-40 (1.25 by) or Uranium-238 (4.5 by) (2). Both also require igneous rock, which is the Investigation-2 link back to bracketing.
• Q35 (4): Use the number the question gives you, because the marks are for the method — counting half-lives and reading the graph — not for the constant (2). The booklet has confused Potassium-40 with Uranium-238, whose half-life genuinely is 4.468 billion years and which is quoted correctly one line earlier in the same passage (2).

Marking note. Investigations 2, 3 and 4 all model the same 4 600 million years, and a learner who has done all three should be able to move between them. If the three sets of answers are internally inconsistent — a Cambrian explosion at 88% of the roll but at 15:00 on the clock — that is worth flagging even though no single question asks for it.