Earth’s History — Problem set, Grade 9

Use relative order, isotope evidence, and planetary records to reconstruct Earth’s past.

Review the key ideas

Geologic Time & the Rock Record

Geologic time has nested divisions, including eons, eras, periods, epochs, and ages. Superposition gives relative order in an undisturbed sequence; numerical ages require additional evidence.

Fossils & Relative Dating

Relative dating uses clues like superposition, original horizontality, and cross-cutting relationships to figure out the order rock layers and fossils formed in — not their exact age in years.

Radiometric Dating & Half-Life

A half-life model estimates time since a defined event under stated assumptions about the isotope system.

Reconstructing Early Earth

Reconstructing early Earth requires multiple records because Earth has recycled much of its oldest surface.

Problem 1

Put these units of the geologic time scale in order from LARGEST to smallest: period, eon, era.
Answer and explanation

Eon, era, period

The geologic time scale is organized from biggest to smallest as eon → era → period, similar to how a book is divided into volumes, then chapters, then pages.

Problem 2

If Earth's 4.6-billion-year history were compressed into a single 365-day year starting January 1, about how many days before the end of the year (Dec 31) would an event from 46 million years ago fall? Round to the nearest whole day.
Answer and explanation

About 4 days before Dec 31 (around December 27).

Set up the ratio: 46,000,000 ÷ 4,600,000,000 = 0.01. Multiply by 365 days: 0.01 × 365 = 3.65 ≈ 4 days. So the event would fall about 4 days before December 31, around December 27.

Problem 3

Explain why the Mesozoic era is often called the 'age of reptiles,' and describe the event that ended it.
Answer and explanation

Dinosaurs and other reptiles dominated life during the Mesozoic; it ended about 66 million years ago with a mass extinction widely linked to an asteroid impact.

Throughout the Mesozoic era, dinosaurs and other large reptiles were the dominant life forms on land, which is why the era earned the nickname 'age of reptiles.' The Mesozoic ended abruptly about 66 million years ago with a mass extinction event that wiped out the non-avian dinosaurs — an event widely linked by scientific evidence to a massive asteroid impact.

Problem 4

A hiker finds a fossilized leaf imprint in a layer of shale (a sedimentary rock). Explain why sedimentary rock is the most common place to find fossils.
Answer and explanation

Sedimentary rock forms from layers of sediment that can quickly bury and preserve remains before they decay.

Fossils need to be buried and protected before decay or scavengers destroy them. Layers of sand, mud, or silt settling on top of a dead organism can bury it relatively quickly, and as those sediment layers compact and harden into sedimentary rock, the preserved remains or trace (like a leaf imprint) become a fossil.

Problem 5

A road cut shows five horizontal, undisturbed rock layers. A geologist also notices the layers are perfectly flat, not tilted. Which relative dating principle explains why she expects them to be flat, and what would it mean if she found them tilted instead?
Answer and explanation

Original horizontality; tilted layers would mean they were disturbed after they formed.

Original horizontality states that sediment is deposited in flat, horizontal layers. Since these layers are flat, they likely haven't been disturbed since forming. If they were tilted or folded, it would mean some geologic force (like tectonic activity) bent them out of their original flat position sometime after they formed.

Problem 6

A cross-section shows: Layer A (bottom), Layer B, Layer C (top), all horizontal, cut by Fault X, which cuts through Layers A and B but does NOT cut through Layer C. Put A, B, C, and X in order from oldest to youngest, and explain your reasoning.
Answer and explanation

Oldest to youngest: A, B, X, C.

By superposition, Layer A is older than Layer B, which is older than Layer C (bottom to top = oldest to youngest). By cross-cutting relationships, Fault X must be younger than every layer it cuts through — so X is younger than both A and B. But since X does NOT cut through Layer C, Layer C must have been deposited after the fault already formed (otherwise the fault would cut through it too). So the full order, oldest to youngest, is: A, B, X, C.

Problem 7

An igneous intrusion cuts across sedimentary layers. The intrusion is
  1. older than the layers
  2. younger than the layers it cuts
  3. the same age
  4. impossible to order
Answer and explanation

younger than the layers it cuts

Cross-cutting relationships: the layers had to exist before being intruded.

Problem 8

Explain how rock layers in two distant places can be matched in time.
Answer and explanation

By correlation: matching distinctive marker beds such as a volcanic ash layer, which was deposited everywhere in a geological instant, and matching index fossils of species that existed worldwide for only a short interval. Absolute dates from ash layers can then anchor both sequences, so the two records can be aligned even where the rock types differ completely.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 9

Chapter review: A sediment layer lies above ash dated 72 million years and below ash dated 68 million years in an undisturbed sequence. State its age bracket and explain why equal layer thickness does not select one exact age.
Answer and explanation

Its deposition is younger than 72 and older than 68 million years, subject to valid ash-event dates and order. Thickness does not imply constant accumulation; gaps and variable deposition can prevent an exact midpoint age.

Its deposition is younger than 72 and older than 68 million years, subject to valid ash-event dates and order. Thickness does not imply constant accumulation; gaps and variable deposition can prevent an exact midpoint age.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 10

Two rock sequences 500 km apart contain the same distinctive fossil. Explain what this allows and what it does not.
Answer and explanation

It allows correlation: the two layers formed during the same interval, so the sequences can be aligned in time. It does not establish a numerical age, nor that the two places had identical environments, since the same species can live across a range of settings. Absolute dating of an associated ash layer would be needed to attach years.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 11

An index fossil existed from 420 to 415 million years ago. What is the width of its range, in million years?
Answer and explanation

5 million years

420 − 415 = 5 million years, a narrow range that makes it useful for dating.

Problem 12

Chapter review: Beds A then B were deposited, a fault cuts both, and later bed C covers the fault without being cut. Order these four events and state what remains unknown.
Answer and explanation

A deposition precedes B deposition; faulting follows both; C deposition follows faulting. Exact ages, durations and any unrelated event history remain unknown without additional evidence.

A deposition precedes B deposition; faulting follows both; C deposition follows faulting. Exact ages, durations and any unrelated event history remain unknown without additional evidence.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 13

A sandstone contains a 500-million-year-old mineral grain. Must the sandstone have formed 500 million years ago? Explain.
Answer and explanation

No. The grain could have formed in an older rock and later been eroded and deposited. Its crystallization age can predate the sandstone.

Problem 14

Chapter review: A closed-system model begins with 96 parent units and now contains 12. Half-life is 8 million years and initial daughter assumptions are satisfied. Find elapsed time and name a condition that could invalidate the interpretation.
Answer and explanation

12/96 = 1/8, or three half-lives. Time is 24 million years. Parent/daughter gain or loss, unsuitable initial conditions, or dating a different closure event could invalidate the stated interpretation.

Compute remaining fraction: 12/96 = 1/8.

Count three halving intervals and multiply 3×8 = 24 million years.

Check closure, initial assumptions and the actual event recorded.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

A student uses one recent volcanic-rock date as Earth’s age. Explain the error and suggest better evidence.
Answer and explanation

That date records the volcanic material’s history, not Earth’s formation. Compare appropriate ancient minerals, meteorites, and other planetary evidence.

Problem 16

Chapter review: An ancient grain dated 4.2 billion years occurs inside a rock whose formation is dated 2.8 billion years. Explain a possible history and why neither date alone maps global early-ocean conditions.
Answer and explanation

An older grain could survive and be incorporated into a younger host during later geological processes. The dates concern different material events. Global ocean conditions require additional dated, interpretable records across locations, not these ages alone.

An older grain could survive and be incorporated into a younger host during later geological processes. The dates concern different material events. Global ocean conditions require additional dated, interpretable records across locations, not these ages alone.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
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