Geologic Time & the Rock Record — Practice · Grade 9 Science

Chapter: Earth’s History

What this practice covers

By the end of Geologic Time & the Rock Record you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: geologic time scale, eon, era, superposition, Ma.

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.

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