Earthquakes & Volcanoes — Practice · Grade 9 Science

Chapter: Earth’s Interior & Plate Tectonics

What this practice covers

By the end of Earthquakes & Volcanoes you should be able to:

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

Vocabulary used: focus (hypocenter), epicenter, seismic wave, hot spot.

Problem 1

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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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 11

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.

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