Earth's Internal Structure — Drills · Grade 9 Science

Chapter: Earth’s Interior & Plate Tectonics

What these drills covers

By the end of Earth's Internal Structure you should be able to:

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

Vocabulary used: crust, mantle, lithosphere, asthenosphere, P wave, S wave.

Problem 1

Earth's layers from the outside in are
  1. crust, mantle, outer core, inner core
  2. mantle, crust, core
  3. crust, core, mantle
  4. inner core, crust, mantle
Answer and explanation

crust, mantle, outer core, inner core

Density increases inward, so the order is crust, mantle, outer core, inner core.

Problem 2

The outer core is inferred to be liquid because
  1. it is hottest
  2. S-waves do not pass through it
  3. it is nearest the surface
  4. it contains water
Answer and explanation

S-waves do not pass through it

S-waves cannot travel through liquid, and they are absent beyond the outer core, producing a shadow zone.

Problem 3

The thinnest layer is the
  1. crust
  2. mantle
  3. outer core
  4. inner core
Answer and explanation

crust

The crust is a few to tens of kilometres thick against thousands for the mantle and core.

Problem 4

Earth's radius is about 6,370 km and the crust is about 35 km thick under continents. What percentage of the radius is the crust, to one decimal place?
Answer and explanation

0.5 %

35 / 6,370 × 100 ≈ 0.5 percent — the crust is a thin skin.

Problem 5

Most of what is known about Earth's interior comes from
  1. direct drilling to the core
  2. seismic waves from earthquakes
  3. satellite photographs
  4. samples from volcanoes only
Answer and explanation

seismic waves from earthquakes

No drill has reached the mantle; wave speeds and paths reveal the layers indirectly.

Problem 6

Temperature and pressure inside Earth generally
  1. both decrease with depth
  2. both increase with depth
  3. temperature rises, pressure falls
  4. are constant
Answer and explanation

both increase with depth

Both rise with depth, which is why the inner core stays solid despite being hotter than the outer core.

Problem 7

Explain how scientists can describe a layer no one has ever sampled.
Answer and explanation

Seismic waves from earthquakes travel through Earth and change speed or direction when they cross a boundary between materials, and some types fail to pass through liquid at all. Recording when and where waves arrive at stations worldwide shows where those boundaries lie and what state the material is in. The description is an inference from many independent measurements rather than a direct observation, which is why it is stated with the evidence attached.

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

The inner core is hotter than the outer core yet is solid. Explain how both can be true.
Answer and explanation

Whether a material is solid depends on pressure as well as temperature. The inner core sits under far greater pressure than the outer core, and that pressure is enough to hold the iron in a solid arrangement despite the higher temperature. Temperature alone does not determine state.

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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