Earth's Internal Structure — Practice · Grade 9 Science

Chapter: Earth’s Interior & Plate Tectonics

What this practice covers

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

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

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

Problem 1

Which is the thin, solid, outermost layer of the Earth, where we live?
  1. The crust
  2. The mantle
  3. The outer core
  4. The inner core
Answer and explanation

The crust

The crust is Earth's thin, rocky, solid outer layer. Below it lie the thick mantle and the metallic core.

Problem 2

The asthenosphere is important to plate tectonics because it
  1. is mainly solid rock that can deform and flow slowly beneath the rigid lithosphere
  2. is the coldest layer of the Earth
  3. makes up the solid inner core
  4. is where all earthquakes stop
Answer and explanation

is mainly solid rock that can deform and flow slowly beneath the rigid lithosphere

The asthenosphere is mainly solid mantle material that can deform over long times. It is not a worldwide liquid ocean of magma. Partial melt can occur in some regions.

Problem 3

The mantle is about 2,900 km thick, and the outer core is about 2,200 km thick. How many km thicker is the mantle than the outer core?
Answer and explanation

700 km

2,900 − 2,200 = 700 km.

Problem 4

Which layer lets tectonic plates slide, because it can flow slowly like soft clay even though it's still part of the solid mantle?
  1. Lithosphere
  2. Asthenosphere
  3. Outer core
  4. Inner core
Answer and explanation

Asthenosphere

The asthenosphere is soft and flows slowly, letting the rigid lithosphere above it glide along on top of it.

Problem 5

Explain why the inner core is solid even though it is the hottest layer of Earth.
Answer and explanation

Even though the inner core is extremely hot, the immense pressure from all the layers above it presses down so hard that it squeezes the iron and nickel atoms too tightly to flow, forcing the inner core to stay solid.

Problem 6

Why are composition-based and behavior-based layer names both useful? Explain the reasoning in your own words.
Answer and explanation

They answer different questions; crust and lithosphere therefore are not interchangeable terms.

Composition identifies what material is present.

Mechanical layers describe how it responds to stress and moves over time.

Response criteria

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

Problem 7

Distinguish the liquid outer core from the solid inner core. Show the reasoning, not just the conclusion.
Hints

A seismic inference requires several paths and wave types.

Answer and explanation

Both are deep core regions, but seismic evidence supports liquid outer and solid inner core; state differs with pressure and temperature.

Problem 8

Why cannot a single drill hole establish the structure of Earth's entire interior?
Hints

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

Answer and explanation

Direct drilling reaches only a tiny fraction of depth; global seismic and other indirect evidence are needed.

Problem 9

Solid rock can deform over geological timeSolid rock can deform over geological timeShort interval: holds its shapeLong interval: slow deformationA solid can change shape gradually under sustained stress.Slow deformation does not imply a global ocean of magma.
The panels illustrate solid-state deformation over a much longer interval. Their spacing and apparent time rate are deliberately exaggerated and have no numerical scale. They represent a mechanism, not measurements that every mantle region flows at the same speed.
Interpret the supplied visual model. What extra information would distinguish slow solid deformation from actual melting in a rock model?
Answer and explanation

Temperature, pressure, composition and evidence of a liquid phase or melt textures help distinguish them. Shape change alone can occur in a solid and does not establish melting.

Compare arrangements across the two schematic snapshots. The particles are crystal-scale markers for deformation, not actual mantle atoms or a literal time-lapse measurement.

Temperature, pressure, composition and evidence of a liquid phase or melt textures help distinguish them. Shape change alone can occur in a solid and does not establish melting.

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 10

A student labels crust, mantle and lithosphere as three separate non-overlapping compositional layers and calls slow mantle convection proof of liquid rock. Correct both statements.
Answer and explanation

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

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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