Plate Boundaries — Practice · Grade 9 Science

Chapter: Earth’s Interior & Plate Tectonics

What this practice covers

By the end of Plate Boundaries you should be able to:

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

Vocabulary used: divergent boundary, convergent boundary, transform boundary, subduction.

Problem 1

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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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.

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