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

Chapter: Earth’s History

What these drills covers

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

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

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

Problem 1

The principle of superposition states that in undisturbed layers
  1. the oldest is on top
  2. the oldest is at the bottom
  3. all layers are the same age
  4. age cannot be determined
Answer and explanation

the oldest is at the bottom

Later sediment is deposited on top of earlier sediment.

Problem 2

A fault cutting through several layers must be
  1. older than all of them
  2. younger than the layers it cuts
  3. the same age
  4. undatable
Answer and explanation

younger than the layers it cuts

A feature must exist before it can be cut, so the cutting event is younger.

Problem 3

Earth is about 4.6 billion years old. If that were compressed into 24 hours, roughly how many hours would 1 billion years occupy, to one decimal place?
Answer and explanation

5.2 hours

24 / 4.6 ≈ 5.2 hours per billion years.

Problem 4

An unconformity in a rock sequence represents
  1. continuous deposition
  2. a gap in the record where rock was eroded or never deposited
  3. a fossil bed
  4. a lava flow
Answer and explanation

a gap in the record where rock was eroded or never deposited

It marks missing time between the layers above and below.

Problem 5

The geologic time scale is divided mainly on the basis of
  1. round numbers of years
  2. major changes in the fossil record
  3. the colour of rocks
  4. continental positions
Answer and explanation

major changes in the fossil record

Boundaries mark substantial changes in life, many of them mass extinctions.

Problem 6

Layer A lies beneath layer B, which lies beneath layer C, all undisturbed. How many layers are younger than A?
Answer and explanation

2

B and C were both deposited after A.

Problem 7

Explain the difference between relative and absolute dating and why both are used.
Answer and explanation

Relative dating establishes the order of events — which layer is older — using principles such as superposition and cross-cutting relationships, but gives no ages in years. Absolute dating uses radioactive decay to give a numerical age with an uncertainty. Both are used because relative methods work on almost any sequence and are cheap, while absolute methods anchor that sequence to actual dates; together they produce a calibrated history.

Response criteria

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

Problem 8

Why is the rock record described as incomplete, and what follows for interpreting it?
Answer and explanation

Deposition is intermittent and erosion removes material, so many intervals left no rock or had their rock destroyed, producing unconformities. What follows is that absence of evidence for an event is weak evidence it did not happen, and that correlating sequences from several places is necessary to piece together a fuller history than any single site preserves.

Response criteria

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

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