Evidence for Continental Drift — Practice · Grade 9 Science

Chapter: Earth’s Interior & Plate Tectonics

What this practice covers

By the end of Evidence for Continental Drift you should be able to:

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

Vocabulary used: continental drift, Pangaea, seafloor spreading, paleomagnetism.

Problem 1

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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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.

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