Earth’s Interior & Plate Tectonics — Chapter Practice, Grade 9

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

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.

Problem 11

At a divergent boundary, plates...
  1. Move apart and new crust forms
  2. Collide and one plate sinks beneath the other
  3. Slide sideways past each other
  4. Stop moving entirely
Answer and explanation

Move apart and new crust forms

Divergent boundaries pull plates apart, letting magma rise up and cool into new crust.

Problem 12

Which plate boundary causes the San Andreas Fault's frequent earthquakes but almost no volcanoes?
  1. Divergent
  2. Convergent
  3. Transform
  4. Subduction zone
Answer and explanation

Transform

The San Andreas Fault is a transform boundary, where plates grind sideways past each other, causing earthquakes but building very little new crust or magma.

Problem 13

Two oceanic plates converge, and one subducts beneath the other. What landform is this most likely to build?
  1. A flat abyssal plain
  2. A volcanic island arc
  3. A mid-ocean ridge
  4. A transform fault
Answer and explanation

A volcanic island arc

When one oceanic plate subducts beneath another, the melting plate feeds a curving chain of volcanoes that rises up as a volcanic island arc, like Japan.

Problem 14

Explain why continental-continental collisions build tall mountains but very few volcanoes.
Answer and explanation

Neither continental plate is dense enough to subduct into the mantle, so instead of melting into magma, the crust on both sides crumples and folds upward into mountains.

Problem 15

Which best explains why oceanic-continental convergent boundaries produce volcanoes, while continental-continental boundaries usually do not?
  1. Oceanic crust is denser and subducts, melting into magma, while continental crust on both sides is too light to subduct
  2. Continental crust always melts at a lower temperature than oceanic crust
  3. Volcanoes can only form near oceans, regardless of plate type
  4. Transform motion at these boundaries generates the magma
Answer and explanation

Oceanic crust is denser and subducts, melting into magma, while continental crust on both sides is too light to subduct

Dense oceanic crust subducts and melts into magma that feeds volcanoes, but when two continental plates meet, neither is dense enough to sink, so the crust just folds instead of melting.

Problem 16

An oceanic plate approaches a continental plate and earthquake depths increase inland. Explain the likely boundary. Explain the reasoning in your own words.
Answer and explanation

An oceanic–continental convergent boundary with a subducting oceanic plate is consistent with the evidence.

The depth pattern indicates a descending slab.

Dense oceanic lithosphere can subduct beneath continental lithosphere.

Response criteria

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

Problem 17

Two oceanic plates move apart. What boundary and crust process are expected? Show the reasoning, not just the conclusion.
Hints

Not every convergent boundary has the same plate types.

Answer and explanation

Divergent boundary; magma can form new oceanic crust near a spreading ridge.

Problem 18

A fault has horizontal slip with little crust creation. Which boundary model fits?
Hints

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

Answer and explanation

A transform boundary, where plates slide past each other, is consistent with the supplied motion.

Problem 19

A sinking slab changes the conditions for melting above itA sinking slab changes the conditions for melting above itOceanic plate →← upper plateCold slab descendsWater helps meltingMelting occurs mainly in overlying mantle; the slab is not a liquid belt.
This original cross-section shows an ideal oceanic subduction setting. Layer thicknesses, angles and distances are exaggerated. The rising red arrow marks a magma pathway after water-assisted mantle melting; the diagram does not locate an actual monitored volcano or predict its eruptions.
Interpret the supplied visual model. Would mountains alone uniquely identify subduction? Name a stronger evidence combination.
Answer and explanation

No. A trench, earthquake depths increasing along the descending plate and an appropriately located volcanic arc together support subduction more strongly. Continental collision can also make high mountains.

Follow the descending blue slab path, then the upward red magma pathway. These are related processes in different regions rather than one arrow tracing molten slab material.

No. A trench, earthquake depths increasing along the descending plate and an appropriately located volcanic arc together support subduction more strongly. Continental collision can also make high mountains.

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 20

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 21

What is the epicenter of an earthquake?
  1. The point underground where the rupture starts
  2. The point on the surface directly above the focus
  3. The instrument that records shaking
  4. The slowest-moving seismic wave
Answer and explanation

The point on the surface directly above the focus

The epicenter is the point on Earth's surface located directly above the focus (hypocenter), where the rupture actually begins underground.

Problem 22

Which type of seismic wave cannot travel through liquid?
  1. P-waves
  2. S-waves
  3. Surface waves
  4. All seismic waves travel through liquid
Answer and explanation

S-waves

S-waves (secondary waves) shake side to side and cannot travel through liquid, unlike P-waves.

Problem 23

A magnitude 4 earthquake and a magnitude 6 earthquake occur. About how many times more ground-shaking does the magnitude 6 quake have?
Answer and explanation

100

Each whole number on the magnitude scale means about 10× more shaking. From 4 to 6 is two steps, so 10 × 10 = 100.

Problem 24

Which volcano type has gentle, broad slopes built from runny, low-viscosity basaltic lava?
  1. Composite (stratovolcano)
  2. Cinder cone
  3. Shield volcano
  4. Caldera
Answer and explanation

Shield volcano

Shield volcanoes, like Hawaii's, are broad and gently sloped because their runny basaltic lava spreads out easily before hardening.

Problem 25

Hawaii's volcanoes formed thousands of kilometers from the nearest plate boundary. What best explains this?
  1. A hot spot — a rising plume of hot mantle rock that can melt through the middle of a plate
  2. A transform boundary running through the open ocean
  3. An unusually thick section of oceanic crust
  4. A meteor impact that melted the crust
Answer and explanation

A hot spot — a rising plume of hot mantle rock that can melt through the middle of a plate

Hot spots are rising plumes of hot mantle rock that can burn through a plate far from any boundary, which is how the Hawaiian volcanoes formed.

Problem 26

Two seismograph stations record the same earthquake. Station A shows a big time gap between the P-wave and S-wave arrivals. Station B shows almost no gap. Which station is farther from the epicenter, and why?
Answer and explanation

Station A is farther away. P-waves always travel faster than S-waves, so the longer the waves travel, the more the P-wave pulls ahead — creating a bigger gap between their arrival times.

Problem 27

Two towns equally far from an earthquake report different damage. Give a plausible explanation and evidence to collect. Explain the reasoning in your own words.
Answer and explanation

Different soils or buildings can produce different damage; site and construction evidence is needed.

Compare local ground, building design and duration of strong shaking.

Damage is a local effect, not a direct measurement of event magnitude alone.

Response criteria

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

Problem 28

Why can an earthquake occur without a nearby volcano? Show the reasoning, not just the conclusion.
Hints

Use the supplied model rather than assuming one speed for all Earth layers.

Answer and explanation

Fault slip can release accumulated stress independently of volcanic activity.

Problem 29

Two stations record different P–S delays. Which is likely farther, using the same calibrated model?
Hints

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

Answer and explanation

The station with the larger delay, subject to path and model assumptions.

Problem 30

There is more than one way to make a partial meltThere is more than one way to make a partial meltDecompressionAdded waterAdded heatPressure decreasesMelting threshold fallsEnergy entersPartial melt can form through different changes in conditions.
Three original condition-change models compare decompression, water-assisted melting and heat input. The figure is qualitative and does not specify a real rock’s melting curve. Blue marks water addition; red arrows do not represent a measured lava flow.
Interpret the supplied visual model. Why is “it got hotter” not the only possible explanation for magma formation?
Answer and explanation

Reduced pressure or added water can permit partial melting without the same required temperature rise. The setting and evidence identify which mechanism is appropriate.

Name which condition changes in each panel. The boxes stand for selected rock regions, while arrow color distinguishes pressure-related rise, water addition and energy transfer.

Reduced pressure or added water can permit partial melting without the same required temperature rise. The setting and evidence identify which mechanism is appropriate.

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 31

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 32

Who proposed the idea of continental drift in 1912?
  1. Isaac Newton
  2. Alfred Wegener
  3. Charles Darwin
  4. Albert Einstein
Answer and explanation

Alfred Wegener

Alfred Wegener proposed continental drift in 1912, based on evidence like matching coastlines and fossils.

Problem 33

What name did Wegener give to the single supercontinent he believed once existed?
  1. Atlantis
  2. Laurasia
  3. Pangaea
  4. Gondwana
Answer and explanation

Pangaea

Wegener called the ancient supercontinent Pangaea, meaning 'all land.'

Problem 34

Which piece of evidence did Wegener use to support continental drift?
  1. Matching fossils of the same species on continents now separated by oceans
  2. Modern GPS measurements of plate motion
  3. Satellite photographs of Earth
  4. Identical languages spoken on separated continents
Answer and explanation

Matching fossils of the same species on continents now separated by oceans

GPS and satellites didn't exist in 1912 — Wegener relied on evidence like matching fossils, rock layers, coastlines, and climate clues.

Problem 35

Why was Wegener's continental drift hypothesis rejected by most scientists for decades?
Answer and explanation

Wegener could not explain a physical mechanism for how huge, solid continents could move or plow through solid ocean floor, so most scientists didn't accept the idea until seafloor spreading was discovered in the 1960s.

Problem 36

What discovery in the 1960s finally gave continental drift a believable mechanism?
  1. The invention of the seismograph
  2. Seafloor spreading and paleomagnetic stripes on the ocean floor
  3. The discovery of Mesosaurus fossils
  4. Wegener's original 1912 publication
Answer and explanation

Seafloor spreading and paleomagnetic stripes on the ocean floor

Seafloor spreading, confirmed by symmetric paleomagnetic stripes recorded in ocean-floor rock, finally explained how continents could move — giving plate tectonics its mechanism.

Problem 37

Ocean-floor rocks are youngest at a ridge and older on both sides. What explanation fits? Explain the reasoning in your own words.
Answer and explanation

Seafloor spreading fits the age pattern; magnetic stripes and plate-motion data provide additional tests.

New crust can form at the ridge.

As plates separate, older crust moves away symmetrically.

Response criteria

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

Problem 38

What would matching ancient mountain belts across an ocean add to fossil evidence? Show the reasoning, not just the conclusion.
Hints

Similar appearance alone is weaker than dated, identified specimens.

Answer and explanation

An independent geological line supporting former connection, if rock ages and structures correspond.

Problem 39

Why does drift evidence not imply the continents move through a fixed unmoving ocean floor?
Hints

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

Answer and explanation

Modern plate tectonics includes motion and creation/recycling of oceanic lithosphere as well as continents.

Problem 40

Magnetic polarity bands record formation and later spreadingMagnetic polarity bands record formation and later spreadingSpreading ridgeOlder ←Youngest→ OlderCompare paired polarity bands and independently dated crust.
Original idealized magnetic-band model shows paired polarity records and an age trend away from a spreading center. Band widths are simplified and do not supply actual reversal durations. The central line is the ridge; stripes symbolize mineral records rather than physical forces.
Interpret the supplied visual model. What two independent measurements would help test the spreading interpretation?
Answer and explanation

Compare dated crust ages with distance from the ridge and measure magnetic polarity patterns. Agreement of age trend and corresponding bands supports the model more than either alone.

Start at the ridge and compare successive bands moving outward on both sides. Color represents polarity record, not the literal visible color of ocean crust.

Compare dated crust ages with distance from the ridge and measure magnetic polarity patterns. Agreement of age trend and corresponding bands supports the model more than either alone.

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 41

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 42

A larger S–P arrival gap usually indicates a station is…
  1. closer to the source
  2. farther from the source
  3. directly above every source
  4. measuring only sound
Answer and explanation

farther from the source

P and S arrivals separate more as their paths lengthen.

Problem 43

Why can one station’s distance estimate not give a unique epicenter?
Answer and explanation

Many locations can lie at the same distance from one station.

Problem 44

Name a way to reduce earthquake risk without preventing earthquakes.
Answer and explanation

Strengthen buildings or infrastructure and improve preparedness; these reduce vulnerability.

Problem 45

Two towns have similar hazard; one has weak crowded buildings and the other reinforced sparse buildings. Compare risk. Explain the reasoning in your own words.
Answer and explanation

The first town plausibly has higher risk; a numerical estimate needs more detailed evidence.

Hazard is comparable, but exposure and vulnerability differ.

The crowded weak buildings increase the potential consequences.

Response criteria

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

Problem 46

How could a town reduce earthquake risk without changing plate motion? Show the reasoning, not just the conclusion.
Hints

Separate event likelihood from likely consequences.

Answer and explanation

Improve building resilience, emergency planning and infrastructure vulnerability; these reduce consequences.

Problem 47

Three stations give distances to a quake. Why intersect distance circles?
Hints

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

Answer and explanation

Each distance constrains possible locations; their intersection estimates a common epicenter under the model.

Problem 48

A probability forecast differs from warning about an event already detectedA probability forecast differs from warning about an event already detectedDefined event • region • time interval • uncertaintyA 30-year probability is not a date on a calendar.A detected event can trigger warning before stronger shaking arrives.Rupture detectedLater shaking arrival
Original conceptual distinction between defined-interval probabilities and detected-event warning. The diagram supplies no actual hazard probability, geographic warning range or guaranteed seconds of lead time. Use official local information for operational safety decisions.
Interpret the supplied visual model. Why does early warning not demonstrate an ability to name the exact earthquake date before rupture?
Answer and explanation

Warning follows detection of an event already underway. Exact-date prediction before rupture is a different and unsupported claim in this model.

Read the chronological arrows as detection followed by later shaking at a selected distant place. This is not a guaranteed advance prediction.

Warning follows detection of an event already underway. Exact-date prediction before rupture is a different and unsupported claim in this model.

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 49

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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