Earth’s History — Chapter quiz · Earth & Space Science, Grade 9

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

Problem 1

Which lists the divisions of the geologic time scale from LARGEST to smallest?
  1. Eon, era, period
  2. Period, era, eon
  3. Era, period, eon
  4. Period, eon, era
Answer and explanation

Eon, era, period

Eons are the largest unit, then eras, then periods.

Problem 2

In an undisturbed sequence of rock layers, which layer is the youngest?
  1. The top layer
  2. The bottom layer
  3. The middle layer
  4. Whichever layer has the most fossils
Answer and explanation

The top layer

By the law of superposition, the youngest layer is always on top.

Problem 3

About what percentage of Earth's 4.6-billion-year history is made up of Precambrian time?
Answer and explanation

88 %

Precambrian time covers roughly the first 4 billion years, about 88% of Earth's total history.

Problem 4

Which of the following is the best example of a fossil?
  1. A polished river stone
  2. Preserved dinosaur footprints in hardened mud
  3. A vein of quartz crystal
  4. A puddle of rainwater
Answer and explanation

Preserved dinosaur footprints in hardened mud

A fossil is a preserved remain or trace of a once-living organism — footprints (trackways) count.

Problem 5

A rock layer is cut through by a solidified magma intrusion. What does this tell geologists about the intrusion's age?
  1. The intrusion is younger than the layer it cuts through
  2. The intrusion is older than the layer it cuts through
  3. The intrusion formed at the exact same time as the layer
  4. Its age cannot be compared to the layer at all
Answer and explanation

The intrusion is younger than the layer it cuts through

By cross-cutting relationships, anything that cuts through existing rock must be younger than that rock.

Problem 6

Explain why an index fossil is useful for comparing rock layers on two different continents.
Answer and explanation

Because an index fossil's species lived over a wide area but only for a short span of time, finding the same one in rock layers on different continents lets geologists match those layers to roughly the same age.

Problem 7

The most useful index fossil is a species that was
  1. long-lived and local
  2. short-lived and geographically widespread
  3. rare everywhere
  4. still alive today
Answer and explanation

short-lived and geographically widespread

A narrow time range gives precision; a wide distribution allows correlation between distant places.

Problem 8

Explain why the geologic time scale's boundaries are not evenly spaced in years.
Answer and explanation

The divisions were originally defined by changes in the fossil record rather than by arithmetic, so a boundary marks a major turnover in life — often a mass extinction — whenever it happened. Absolute dating later attached years to those boundaries, revealing intervals of very different lengths. The scale reflects the history of life rather than a convenient subdivision of time.

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

Independent chapter check: 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

A layer contains fossils of a species known to have lived 300–295 million years ago. The layer
  1. is exactly 300 million years old
  2. formed within that interval
  3. is younger than 100 million years
  4. cannot be dated at all
Answer and explanation

formed within that interval

The fossil constrains the layer to the species' time range rather than to a single date.

Problem 11

Explain how fossils in successive layers provide evidence of change in life over time.
Answer and explanation

Each layer samples the organisms living when its sediment was deposited, and superposition establishes their order. Comparing layers upward shows species appearing, persisting and disappearing, with later assemblages differing systematically from earlier ones. The pattern is consistent across sites worldwide, which is what makes it evidence of change through time rather than local variation.

Response criteria

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

Problem 12

Independent chapter check: 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 sample retains 25% parent isotope with a 50-million-year half-life. Find its model age.
Answer and explanation

100 million years

Two half-lives × 50 = 100.

Problem 14

Later heating allows isotopes to leave a mineral. Why might its apparent age be misleading?
Answer and explanation

The closed-system assumption may be violated or the clock partially reset. The date needs geological interpretation.

Problem 15

Independent chapter check: 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 16

Which strengthens an account of early Earth?
  1. One unexplained date
  2. Agreement among independent geological and planetary evidence
  3. Ignoring altered samples without explanation
  4. Assuming every rock is the same age
Answer and explanation

Agreement among independent geological and planetary evidence

Independent evidence helps constrain an explanation.

Problem 17

Why can Earth be older than its oldest currently known surviving rock?
Answer and explanation

Earlier rocks may have been recycled or altered; surviving samples provide an incomplete record.

Problem 18

Independent chapter check: 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.

Browse all courses