Plate Tectonics & Earth's Structure — Science

  1. Grade 9 — Earth's Internal Structure
  2. Grade 9 — Plate Boundaries
  3. Grade 9 — Earthquakes & Volcanoes
  4. Grade 9 — Evidence for Continental Drift

Apply it in a new setting

Problem 1

Off a continent's coast, a deep ocean trench runs parallel to the shore, a chain of explosive volcanoes rises just inland, and earthquakes start shallow near the trench and get deeper moving inland. What is happening here?
  1. A dense oceanic plate is subducting beneath the continental plate at a convergent boundary
  2. Two plates are pulling apart at a divergent boundary, creating new crust
  3. Two plates are sliding side by side at a transform boundary
  4. A hot spot is erupting in the middle of a plate, far from any boundary
Hints

A deep offshore trench plus a parallel volcanic chain is a classic signature — which boundary makes both?

Earthquakes that get deeper as you move inland trace a plate sinking at an angle.

Divergent and transform boundaries don't produce deep trenches with explosive inland volcanoes.

Answer and explanation

A dense oceanic plate is subducting beneath the continental plate at a convergent boundary

A deep trench, a parallel arc of explosive volcanoes, and earthquakes deepening inland are the hallmarks of subduction.

Dense oceanic crust bends down and sinks beneath the lighter continental crust, melting to feed the volcanoes (like the Andes).

That is an oceanic-continental convergent boundary — A.

Problem 2

Along a certain boundary, rock formations that were once continuous are now offset sideways, frequent strong earthquakes occur, but there are almost no volcanoes and no new crust is being made or destroyed. What type of boundary is this?
  1. Divergent boundary
  2. Transform boundary
  3. Oceanic-continental convergent boundary
  4. Hot spot
Hints

Which boundary has plates grinding sideways past each other?

The key clues: sideways offset, lots of earthquakes, very few volcanoes, no crust created or destroyed.

Volcanoes need magma from spreading or subduction — is either happening here?

Answer and explanation

Transform boundary

Sideways-offset rock and frequent earthquakes with almost no volcanoes point to plates sliding horizontally past each other.

Because no crust is created (divergent) or destroyed (convergent), there's little magma, so few volcanoes — matching a transform boundary like the San Andreas Fault.

The answer is B.

Problem 3

Two seismograph stations record the same earthquake. At Station X, the P-waves arrive a full 4 minutes before the S-waves. At Station Y, the P-waves and S-waves arrive almost together. Which station is closer to the epicentre?
  1. Station X, because a bigger P-to-S time gap means it's closer
  2. Station Y, because a small P-to-S time gap means it's closer
  3. They are exactly the same distance away
  4. It's impossible to tell from arrival times
Hints

P-waves travel faster than S-waves, so the faster wave pulls ahead as they travel.

The farther the waves travel, the more the gap between P and S arrivals grows.

A tiny gap means the waves haven't had much distance to separate.

Answer and explanation

Station Y, because a small P-to-S time gap means it's closer

P-waves are faster than S-waves, so the longer they travel, the larger the gap between their arrival times becomes.

A big P-to-S gap (Station X) means a long distance; a tiny gap (Station Y) means a short distance.

So Station Y, with almost no gap, is closer to the epicentre — B.

Problem 4

The earthquake magnitude scale is logarithmic: each whole-number step means about 10× more ground-shaking. About how many times more ground-shaking does a magnitude 7 earthquake produce than a magnitude 4 earthquake?
Hints

Count the whole-number steps from 4 up to 7.

Each step multiplies the shaking by about 10.

Multiply 10 by itself once for each step.

Answer and explanation

1000 times

From magnitude 4 to 7 is 3 whole-number steps (4→5→6→7).

Each step is about 10× more shaking, so multiply: 10 × 10 × 10 = 10^3.

That is 1000 times more ground-shaking.

Problem 5

Two continents that were once joined are now about 5,000 km apart. If the plates carrying them have drifted apart at an average of about 2.5 cm per year, roughly how many MILLION years ago were they joined? (1 km = 100,000 cm.)
Hints

First convert the separation distance from kilometres to centimetres so the units match the rate.

Time = distance ÷ rate.

After dividing to get years, convert to millions of years.

Answer and explanation

200 million years

Convert the distance: 5,000 km × 100,000 cm/km = 5×10^8 cm.

Time = distance ÷ rate = 5×10^8 cm ÷ 2.5 cm/year = 2×10^8 years.

2×10^8 years = 200 million years ago (about when Pangaea began breaking up).

Problem 6

Surveys of the ocean floor reveal a pattern of magnetic 'stripes' in the rock that is a near-perfect mirror image on the two sides of a mid-ocean ridge. What does this symmetry best support?
  1. Seafloor spreading — new crust forms at the ridge and moves outward in both directions, recording Earth's magnetic reversals as it cools
  2. That the ocean floor has never moved since Earth formed
  3. That continents plow directly through solid ocean floor
  4. That the stripes were painted on by ocean currents
Hints

What could make the same pattern appear symmetrically on both sides of a ridge?

As new crust cools at the ridge, iron minerals lock in the direction of Earth's magnetic field at that moment.

Earth's magnetic field reverses over geologic time, and the crust records each reversal.

Answer and explanation

Seafloor spreading — new crust forms at the ridge and moves outward in both directions, recording Earth's magnetic reversals as it cools

New ocean crust forms at the ridge, and as it cools, iron-bearing minerals freeze in the current direction of Earth's magnetic field.

Because the field reverses over time and the crust spreads outward evenly, matching stripes are laid down on both sides — a mirror image.

This symmetry is direct evidence of seafloor spreading, the mechanism for drift — A.

Problem 7

A student says, 'Tectonic plates float and slide around on a hidden ocean of liquid magma.' Which correction is scientifically accurate?
  1. The student is right; the whole mantle is liquid magma
  2. Plates ride on the asthenosphere, which is solid rock that flows very slowly (like a glacier of ice), not a layer of liquid magma
  3. Plates rest directly on the liquid outer core
  4. Plates don't move at all, so nothing is beneath them
Hints

Recall the difference between the lithosphere and the asthenosphere.

Is the asthenosphere liquid, or solid rock that can slowly flow?

Compare it to a glacier: solid, yet still creeps.

Answer and explanation

Plates ride on the asthenosphere, which is solid rock that flows very slowly (like a glacier of ice), not a layer of liquid magma

The rigid plates (lithosphere) ride on the asthenosphere, the upper-mantle layer just beneath them.

The asthenosphere is solid rock that flows extremely slowly over long times — like solid glacier ice creeping downhill — not a sea of liquid magma.

So the accurate correction is B.

Browse all courses