Earth’s Interior & Plate Tectonics — Chapter quiz · Earth & Space Science, Grade 9

Connect Earth’s interior, moving plates, crustal evidence, and natural hazards.

Problem 1

Which layer of Earth is liquid and generates Earth's magnetic field?
  1. Inner core
  2. Outer core
  3. Mantle
  4. Lithosphere
Answer and explanation

Outer core

The outer core is liquid iron and nickel, and its swirling motion generates Earth's magnetic field.

Problem 2

The mantle is about 2,900 km thick and the inner core has a radius of about 1,220 km. How many km thicker is the mantle than the inner core's radius?
Answer and explanation

1680 km

2,900 − 1,220 = 1,680 km.

Problem 3

At a transform boundary, plates mostly...
  1. Slide past each other sideways
  2. Pull apart to form new crust
  3. Collide, and one subducts under the other
  4. Melt into a hot spot
Answer and explanation

Slide past each other sideways

Transform boundaries involve plates sliding horizontally past one another, like the San Andreas Fault.

Problem 4

Which convergent boundary is most likely to build a chain of volcanic islands like Japan?
  1. Continental-continental
  2. Oceanic-oceanic
  3. Two transform plates sliding past each other
  4. Two divergent ridges meeting
Answer and explanation

Oceanic-oceanic

When two oceanic plates converge, one subducts beneath the other, melting and feeding a volcanic island arc such as Japan.

Problem 5

Why do continental-continental collisions build tall mountains but produce very little volcanic activity?
  1. Neither plate is dense enough to subduct, so the crust folds and crumples instead of melting
  2. Continental crust always melts at very low temperatures
  3. There is no magma anywhere near continental plates
  4. Continental collisions only happen at transform boundaries
Answer and explanation

Neither plate is dense enough to subduct, so the crust folds and crumples instead of melting

With neither plate dense enough to subduct into the mantle, the crust on both sides simply crumples upward — like in the Himalayas — instead of melting into magma.

Problem 6

What is the focus (hypocenter) of an earthquake?
  1. The point on the surface above the rupture
  2. The underground point where the rupture begins
  3. The instrument used to measure shaking
  4. The slowest-moving seismic wave
Answer and explanation

The underground point where the rupture begins

The focus, or hypocenter, is the underground point where the earthquake's rupture starts.

Problem 7

A magnitude 3 earthquake and a magnitude 5 earthquake occur. About how many times more ground-shaking does the magnitude 5 quake produce?
Answer and explanation

100

Each whole number on the magnitude scale is about 10× more shaking; from 3 to 5 is two steps, so 10 × 10 = 100.

Problem 8

Which type of volcano is built from thick, sticky magma erupting explosively in alternating layers of ash and lava?
  1. Shield volcano
  2. Composite (stratovolcano)
  3. Cinder cone
  4. Hot spot
Answer and explanation

Composite (stratovolcano)

Composite volcanoes (stratovolcanoes) like Mt. St. Helens and Mt. Fuji build steep cones from alternating ash and lava layers.

Problem 9

Explain how matching fossils on continents now separated by oceans support continental drift.
Answer and explanation

The same species (like Mesosaurus, a freshwater reptile) could not have crossed a wide, deep ocean, so finding its fossils on multiple continents shows those continents must have once been joined together.

Problem 10

What evidence finally gave continental drift a scientific mechanism in the 1960s?
  1. Wegener's original fossil evidence
  2. Photographs of matching coastlines
  3. Seafloor spreading and paleomagnetic stripes on the ocean floor
  4. The discovery of Pangaea
Answer and explanation

Seafloor spreading and paleomagnetic stripes on the ocean floor

Seafloor spreading, confirmed by symmetric paleomagnetic stripes on the ocean floor, finally explained how continents could move.

Problem 11

Which statement about the mantle is best supported?
  1. It is entirely molten rock
  2. It is solid rock that can flow very slowly
  3. It is liquid iron
  4. It is the thinnest layer
Answer and explanation

It is solid rock that can flow very slowly

The mantle is solid but deforms slowly over long timescales, which is what allows convection.

Problem 12

Explain why the mantle being solid does not prevent convection within it.
Answer and explanation

Over short timescales mantle rock behaves as a solid and transmits seismic waves, but over millions of years it deforms slowly under heat and pressure and can flow. Convection there happens at centimetres per year, far too slow to observe directly, which is why it is inferred from plate motion and heat flow rather than seen.

Response criteria

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

Problem 13

Independent chapter check: 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.

Problem 14

Two continental plates collide. The most likely result is
  1. a deep trench
  2. a mountain range
  3. new ocean floor
  4. a transform fault
Answer and explanation

a mountain range

Neither plate subducts readily, so the crust thickens and buckles upward.

Problem 15

Earth's surface area stays roughly constant although new crust forms continuously at ridges. Explain.
Answer and explanation

Crust is destroyed at subduction zones at about the rate it forms at ridges, so the total area remains roughly in balance. If creation happened without destruction the planet would have to expand, and there is no evidence of that; instead the ages of ocean floor show old crust being consumed at trenches while the youngest sits at the ridges.

Response criteria

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

Problem 16

Independent chapter check: West and east stations on adjacent plates both move east: west 6 cm/year and east 2 cm/year in one reference frame. Find the relative closing rate. Explain why this alone does not prove a subduction zone.
Answer and explanation

The gap closes at 6−2 = 4 cm/year. Relative convergence is established, but plate types, trench geometry, earthquake depths and geological evidence are needed to distinguish subduction from other convergent settings.

The gap closes at 6−2 = 4 cm/year. Relative convergence is established, but plate types, trench geometry, earthquake depths and geological evidence are needed to distinguish subduction from other convergent settings.

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 17

The epicentre is located by
  1. one station's reading
  2. combining distance circles from three or more stations
  3. the largest wave recorded
  4. the depth of the focus
Answer and explanation

combining distance circles from three or more stations

Each station gives a radius only; intersecting circles give the position.

Problem 18

Explain why hazard maps are based on past earthquake locations but cannot predict the date of the next one.
Answer and explanation

Past locations show where stress has repeatedly been released, so they identify which areas are at risk and roughly how often large events have occurred, which is what a hazard map conveys. They do not reveal when stress on a particular fault will exceed its strength, because that depends on conditions at depth that cannot be measured directly. The honest output is a probability over decades, not a date.

Response criteria

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

Problem 19

Independent chapter check: In a stated travel-time model, P arrives at 50 s and S at 86 s at one station. Calculate the S–P gap. Explain why neither that gap nor the epicenter alone establishes shaking damage at every town.
Answer and explanation

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

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 20

The oldest ocean floor is found
  1. at mid-ocean ridges
  2. near subduction zones
  3. in the middle of continents
  4. at transform faults
Answer and explanation

near subduction zones

Crust ages as it travels from the ridge and is oldest just before being consumed at a trench.

Problem 21

Explain why ocean floor is much younger than continental rock.
Answer and explanation

Ocean floor is continuously created at ridges and destroyed at subduction zones, so it is recycled on a timescale of roughly 200 million years and none survives longer. Continental crust is less dense and resists subduction, so it is not recycled in the same way and can persist for billions of years. The difference is in whether the crust can be pulled back into the mantle.

Response criteria

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

Problem 22

Independent chapter check: Oceanic crust is 80 km from a ridge and 4 million years old. Under steady symmetric spreading, find the half-spreading rate in cm/year and the full separation rate. State one independent check.
Answer and explanation

80/4 = 20 km per million years = 2 cm/year on one side; full symmetric separation is 4 cm/year. Check opposite-side age patterns and magnetic records; variable rates or asymmetric movement can challenge the assumption.

80/4 = 20 km per million years = 2 cm/year on one side; full symmetric separation is 4 cm/year. Check opposite-side age patterns and magnetic records; variable rates or asymmetric movement can challenge the assumption.

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

Which is best supported by a long earthquake record?
  1. The exact day of the next quake
  2. A probability estimate with uncertainty
  3. A guarantee no quake will recur
  4. Identical damage in all towns
Answer and explanation

A probability estimate with uncertainty

Records inform likelihood, not exact timing.

Problem 24

Why might a smaller nearby earthquake cause more damage than a larger distant one?
Answer and explanation

Local shaking, distance, ground conditions, building vulnerability, and exposure affect damage as well as magnitude.

Problem 25

Independent chapter check: One station gives a modeled epicenter distance of 120 km. A proposed building retrofit lowers susceptibility to shaking. Explain what the distance constrains and which risk component the retrofit changes.
Answer and explanation

The distance defines an approximate set of possible source locations around the station, not a unique direction. The retrofit changes vulnerability, while local physical hazard is not necessarily changed. Additional stations and verified building-performance evidence are needed.

The distance defines an approximate set of possible source locations around the station, not a unique direction. The retrofit changes vulnerability, while local physical hazard is not necessarily changed. Additional stations and verified building-performance evidence are needed.

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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