Earth’s History — Chapter Practice, Grade 9

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

Problem 1

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

Eon, era, period

Eons are the biggest divisions, then eras, then periods — like books, chapters, and pages.

Problem 2

According to the law of superposition, in an undisturbed sequence of rock layers, which layer is the oldest?
  1. The layer on the very bottom
  2. The layer on the very top
  3. The middle layer
  4. It's impossible to tell
Answer and explanation

The layer on the very bottom

Superposition says undisturbed layers get younger as you go up, so the bottom layer formed first and is the oldest.

Problem 3

Which era/nickname pairing is correct?
  1. Mesozoic — age of reptiles
  2. Cenozoic — age of reptiles
  3. Paleozoic — age of mammals
  4. Precambrian — age of dinosaurs
Answer and explanation

Mesozoic — age of reptiles

The Mesozoic era is nicknamed 'the age of reptiles' because dinosaurs and other reptiles dominated it.

Problem 4

About what percentage of Earth's history does Precambrian time make up?
Answer and explanation

88 %

Precambrian time covers roughly the first 4 billion years of Earth's 4.6-billion-year history, which is about 88%.

Problem 5

Using the 'Earth's history = 1 year' model (4.6 billion years = 365 days), about how many days before December 31 would an event from 46 million years ago fall? Round to the nearest whole day.
Answer and explanation

4 days

46,000,000 ÷ 4,600,000,000 = 0.01. Multiply by 365 days: 0.01 × 365 = 3.65, which rounds to about 4 days before the end of the year.

Problem 6

Why is evidence from early Earth incomplete, and why is the Precambrian still important?
Answer and explanation

Many ancient rocks were altered, eroded, or recycled, and much early life left subtle traces. Preserved evidence records the formation of crust and oceans, changes in the atmosphere, and early life.

Problem 7

Horizontal beds A, B and C lie bottom to top; a fault cuts all three. Order their formation and faulting. Explain the reasoning in your own words.
Answer and explanation

A, then B, then C, then faulting, assuming the beds were not overturned.

Superposition gives A before B before C.

The fault is younger than every bed it cuts.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

An intrusion cuts layers A and B. Which is younger? Show the reasoning, not just the conclusion.
Hints

Relative order does not provide numerical ages.

Answer and explanation

The intrusion is younger than the layers it cuts, by cross-cutting relationships.

Problem 9

What does an unconformity indicate about the record?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

A gap from nondeposition and/or erosion; the preserved sequence does not contain every interval.

Problem 10

An unconformity records a missing interval in the preserved sequenceAn unconformity records a missing interval in the preserved sequenceYounger horizontal depositionover an eroded older surfaceOlder deposition → tilting → erosion/gap → younger deposition
Original hypothetical angular-unconformity section. The wavy red surface denotes the record gap/erosional contact; older bedding is truncated there. Shapes are schematic and layer thickness does not set duration. The example establishes relative relationships before dates are attached.
Interpret the supplied visual model. What sequence is established before any numerical dates are supplied?
Answer and explanation

Older deposition preceded tilting; erosion/nondeposition preceded the younger overlying deposit. The interval length and exact dates remain unconstrained without further evidence.

Follow the order required by tilted layers cut at the wavy surface and younger material deposited above it.

Older deposition preceded tilting; erosion/nondeposition preceded the younger overlying deposit. The interval length and exact dates remain unconstrained without further evidence.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 11

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 12

Which of these is the best example of a fossil?
  1. A dinosaur footprint preserved in hardened mud
  2. A shiny piece of quartz
  3. A smooth river pebble
  4. A cloud formation
Answer and explanation

A dinosaur footprint preserved in hardened mud

A fossil is a preserved remain or trace of a once-living organism, and a footprint (trackway) counts even without any body parts.

Problem 13

Relative dating tells you...
  1. The exact age of a rock in years
  2. Whether a rock layer is older or younger than another layer
  3. The chemical composition of a rock
  4. The temperature at which a rock formed
Answer and explanation

Whether a rock layer is older or younger than another layer

Relative dating only compares the order of events — older versus younger — not exact ages in years.

Problem 14

A geologist finds rock layers tilted at a steep angle. Which principle tells her the layers must have been disturbed after they formed?
  1. Original horizontality
  2. Superposition
  3. Cross-cutting relationships
  4. Index fossil correlation
Answer and explanation

Original horizontality

Original horizontality says layers are deposited flat, so tilted layers were bent or tipped sometime after they formed.

Problem 15

A drill core pulls up 6 distinct horizontal, undisturbed rock layers labeled 1 (top) through 6 (bottom). Which layer number is the oldest?
Answer and explanation

6

Undisturbed layers follow the law of superposition — the deepest, bottom layer (6) is the oldest.

Problem 16

A volcanic dike (a wall of hardened magma) cuts straight across five sedimentary rock layers. What can you conclude?
  1. The dike is younger than all five layers
  2. The dike is older than all five layers
  3. The dike formed at the same time as the layers
  4. No conclusion can be drawn
Answer and explanation

The dike is younger than all five layers

By cross-cutting relationships, anything that cuts through existing rock must have formed after that rock existed.

Problem 17

Explain why an index fossil needs to have lived over a wide geographic area but only for a short span of geologic time.
Answer and explanation

A wide range means the same species could get fossilized in many different places, so geologists can find it in rock layers far apart; a short time span means finding that fossil narrows down the age of the layer very precisely, since the species wasn't around for very long.

Problem 18

Species X was widespread for 1 million years; Y was local for 100 million years. Which is a better potential index fossil? Explain the reasoning in your own words.
Answer and explanation

X is more useful, provided it is identifiable and occurs in the sampled layers.

A short time range narrows the possible age.

Wide distribution permits correlation across many sites.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 19

A fault cuts a fossil-bearing bed. Which event is later? Show the reasoning, not just the conclusion.
Hints

Reworking and identification errors must be considered.

Answer and explanation

The faulting is later than deposition of the bed it cuts.

Problem 20

Why are widespread, short-lived species useful index fossils?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Their limited time range constrains age and broad distribution permits comparison across locations.

Problem 21

The same time marker can occur in different rock typesThe same time marker can occur in different rock typesYoungerIndex group XOlderYounger, different rockIndex group XOlder, different rockSite ASite BRestricted time range + broad distribution + reliable identification
Original hypothetical two-site correlation. Fossil group X is invented; its restricted range is a supplied assumption, not a claim about a real named species. Rock color and vertical position are not the time marker by themselves.
Interpret the supplied visual model. Why might identical sandstone appearance be weaker correlation evidence than a well-established short-range fossil marker?
Answer and explanation

Similar sedimentary rock can form at many times. A suitable reliably identified fossil with a narrow known range constrains time more specifically, with habitat and preservation qualifications.

The dashed connection follows the supplied group X interval across sites with different heights and material types. It is a time relation, not a continuous physical layer drawn between them.

Similar sedimentary rock can form at many times. A suitable reliably identified fossil with a narrow known range constrains time more specifically, with habitat and preservation qualifications.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 22

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 23

A sample retains 12.5% parent isotope. How many half-lives have elapsed?
Answer and explanation

3 half-lives

100 → 50 → 25 → 12.5.

Problem 24

Two half-lives of 30 million years have elapsed. Find the age.
Answer and explanation

60 million years

2 × 30 = 60.

Problem 25

Why should a date be checked against field relationships?
Answer and explanation

The isotope system may date a particular mineral event or have been altered; field evidence helps interpret it.

Problem 26

A closed sample retains 25% of its original parent isotope. Half-life is 8 million years. Find elapsed time. Explain the reasoning in your own words.
Answer and explanation

16 million years under the supplied closed-system assumptions.

100%→50%→25% is two half-lives.

2×8 million=16 million years.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 27

A sample retains 25% parent with half-life 6 million years. Find elapsed time. Show the reasoning, not just the conclusion.
Hints

The fraction is parent remaining, not daughter produced.

Answer and explanation

Two half-lives have elapsed: 2×6=12 million years.

Problem 28

Why can leakage of parent or daughter make a radiometric age unreliable?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The closed-system accounting no longer holds; measured ratios can reflect movement as well as decay.

Problem 29

A mineral date is tied to a particular clock eventA mineral date is tied to a particular clock eventOlder grain formsTransport / burialLater heatingGrain crystallizationSediment depositionPossible clock disturbanceName the dated material and the event its isotope system records.
Original hypothetical mineral-event timeline with no fixed interval scale. The diagram distinguishes possible clock-start and disturbance events rather than asserting that every heating episode resets every isotope system completely.
Interpret the supplied visual model. Why can a 200-million-year grain inside a younger sediment fail to date that sediment’s deposition?
Answer and explanation

The grain may have formed earlier and retained its isotope record through later transport and burial. Its age records the older grain event, not automatically the younger deposition.

The timeline separates grain formation, sediment deposition and later disturbance. Each is an event that could leave different evidence.

The grain may have formed earlier and retained its isotope record through later transport and burial. Its age records the older grain event, not automatically the younger deposition.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 30

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 31

Why is early Earth’s surface record incomplete?
  1. Earth never changed.
  2. Recycling and alteration removed or changed old rocks.
  3. All rocks formed yesterday.
  4. Meteorites stop erosion.
Answer and explanation

Recycling and alteration removed or changed old rocks.

Earth processes continually modify the record.

Problem 32

How does density help explain differentiation?
Answer and explanation

Under conditions permitting movement, denser materials can sink relative to less dense materials.

Problem 33

Why combine lunar and meteoritic evidence with Earth rocks?
Answer and explanation

They preserve records that Earth’s surface processes may have erased.

Problem 34

Why do researchers use meteorites as well as Earth's oldest rocks to study early solar-system history? Explain the reasoning in your own words.
Answer and explanation

Independent records help reconstruct events that surviving Earth rocks alone cannot fully record.

Earth's earliest record has been altered and recycled.

Some meteorites preserve material from early solar-system processes that complements terrestrial evidence.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 35

Why combine fossil and geochemical records when reconstructing early life? Show the reasoning, not just the conclusion.
Hints

State the proxy's assumptions.

Answer and explanation

Independent evidence can constrain biological timing and environmental context and reveal inconsistent interpretations.

Problem 36

A rock has been strongly heated since formation. How could this affect its historical signal?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Later alteration can modify minerals or isotope signatures; distinguish original records from later changes.

Problem 37

A local water indicator constrains a local historyA local water indicator constrains a local historyA local mineral or sediment can preserve a condition indicatorLocal water evidence ≠ a complete global ocean map.Compare chemistry, ages, setting and alternative explanations.
Original hypothetical local water-related record. No specific mineral’s isotope ratio or numerical age is supplied, so the figure illustrates inference scope rather than establishes an actual dated global ocean.
Interpret the supplied visual model. What evidence would be needed to extend a local water inference into a global reconstruction?
Answer and explanation

Comparable dated records across locations, appropriate chemical/geological mechanisms and tests of preservation/alteration would be needed. One local indicator is insufficient for global extent or composition.

The selected grain and layer are a local record. The blue line illustrates a water-related interpretation, not a surveyed global coastline.

Comparable dated records across locations, appropriate chemical/geological mechanisms and tests of preservation/alteration would be needed. One local indicator is insufficient for global extent or composition.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 38

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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