Plate Tectonics
In 1912, Alfred Wegener noticed South America's coast fit Africa's like a jigsaw puzzle piece. He proposed the continents had once been joined, and scientists laughed at him. Forty years later, seafloor spreading proved him right. In this lesson you'll learn the theory of plate tectonics, the evidence behind it, and the three types of plate boundaries shaping our world right now.
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Q1 · Look at a world map. Does South America's east coast look like it could fit into Africa's west coast? Why might that be?
Q2 · If the continents have been moving for hundreds of millions of years, could they eventually move again into a new arrangement?
● Know
- Alfred Wegener's continental drift hypothesis and key evidence
- The mechanism of plate movement (mantle convection)
- The three types of plate boundaries and the features they form
● Understand
- Why scientists initially rejected Wegener's idea
- How seafloor spreading at mid-ocean ridges confirms plate tectonics
- Why Australia experiences earthquakes despite being far from most boundaries
● Can do
- List at least three pieces of evidence for continental drift
- Describe what happens at each type of plate boundary
- Explain Australia's movement using plate tectonic theory
Alfred proposed in 1912 that the continents had once been joined as the supercontinent . The movement of tectonic plates is driven by currents in the .
In 1912, Alfred Wegener noticed something strange: the coastlines of South America and Africa matched like puzzle pieces. He proposed that all continents were once joined as one supercontinent, Pangaea. Scientists laughed at him. He had no mechanism. Forty years later, seafloor spreading proved him right.
Wegener's four lines of evidence:
- Jigsaw coastlines, the east coast of South America fits into the west coast of Africa almost perfectly.
- Matching fossilsGlossopteris (a plant) and Mesosaurus (a freshwater reptile) fossils are found on continents now separated by thousands of kilometres of deep ocean. They couldn't have crossed the open sea.
- Matching rock formations, identical rock sequences and mountain chains appear across South America and Africa, as if a puzzle piece was pulled apart.
- Ancient climate evidence, coal deposits (from tropical swamp plants) are found in Antarctica. Glacial scratches are found in tropical Africa. This only makes sense if the continents were in different positions.
Pangaea began breaking up about 200 million years ago. The southern supercontinent, Gondwana, split into South America, Africa, Australia, Antarctica and India.
- Wegener 1912: proposed Pangaea, all continents once joined.
- Evidence: (1) jigsaw coastlines, (2) matching fossils, (3) matching rocks, (4) ancient climate clues.
- Gondwana = southern supercontinent that split into South America, Africa, Australia, Antarctica, India.
Wegener's big problem: what drives the continents? In the 1960s, Harry Hess proposed seafloor spreading:
- Hot mantle material rises at mid-ocean ridges, creating new ocean floor.
- The new floor spreads outward. Old seafloor sinks back at subduction zones.
- This conveyor belt is driven by convection in the mantle, hot material rises, spreads, cools, and sinks.
There are three types of plate boundaries:
| Boundary type | Movement | Features formed | Example |
|---|---|---|---|
| Divergent | Plates moving apart | Mid-ocean ridges, rift valleys | Mid-Atlantic Ridge; East African Rift |
| Convergent | Plates moving together | Mountain ranges, ocean trenches, volcanoes (subduction) | Himalayas (India + Eurasia); Andes (Pacific + South America) |
| Transform | Plates sliding past each other | Fault lines, earthquakes | San Andreas Fault (California) |
Modern evidence has eliminated any doubt about plate tectonics:
- GPS measurements directly show plates moving in real time. Australia moves ~7 cm north per year, one of the fastest-moving plates. In your 70-year lifetime, Australia will shift about 4.9 metres closer to Asia.
- Paleomagnetic stripes: when new seafloor forms, iron minerals lock in the direction of Earth's magnetic field. Symmetric striped patterns on either side of mid-ocean ridges mirror each other perfectly, direct evidence of seafloor spreading.
- Ages of ocean floor: the oldest ocean floor is found furthest from mid-ocean ridges, exactly as seafloor spreading predicts.
Australian context: Australia is on the Indo-Australian Plate. Our earthquakes occur mainly in the interior, we're far from most plate boundaries, but stress builds up within the plate itself. The collision between the Indo-Australian Plate and the Eurasian Plate is building the Himalayas right now.
Glossopteris is a fossil plant found on every Southern Hemisphere continent, including Antarctica. These continents are thousands of kilometres apart today, separated by deep oceans. Predict how Glossopteris seeds could have spread so widely, and what this evidence tells us about Earth's past.
How close was your prediction?
Good reasoning, you connected the fossil distribution to a former land connection.
Key idea: heavy seeds can't cross oceans. If the same plant is on every continent, those continents must have once been connected.
In your workbook, draw a table with three columns: Evidence type | Observation | What it supports. Complete it for all four of Wegener's pieces of evidence (jigsaw coastlines, matching fossils, matching rocks, ancient climate).
For each scenario below, write whether it describes a divergent, convergent, or transform boundary, and explain why:
- The Himalayas are still growing taller as India pushes into Eurasia.
- A rift valley is forming in East Africa as the African continent splits apart.
- Earthquakes occur along the San Andreas Fault in California as two plates grind past each other.
- New ocean floor is being created at the Mid-Atlantic Ridge.
At the start of the lesson, you learned that Australia is moving north at about 7 cm per year, roughly as fast as your fingernails grow!
Now that you've studied plate tectonics, explain why Australia is moving. What force is driving it, and what does the theory of Pangaea tell us about how continents have always been on the move?
Q1. Describe three pieces of evidence that Alfred Wegener used to support his theory of continental drift. (3 marks)
Q2. Explain the difference between a divergent and a convergent plate boundary. Give one example of a feature formed at each. (4 marks)
Q3. Explain why Australia experiences earthquakes even though it is far from most plate boundaries. (3 marks)
Answers
▾MCQ 1
C GPS didn't exist in 1912 when Wegener proposed his theory. His four lines of evidence were coastline fit, matching fossils, matching rocks, and ancient climate clues.
MCQ 2
C Mid-ocean ridges are divergent boundaries where plates pull apart and new seafloor is created by upwelling magma.
MCQ 3
B Australia moves approximately 7 cm north per year, making it one of the fastest-moving tectonic plates on Earth.
MCQ 4
B Oceanic crust is denser than continental crust. Where they collide, the denser oceanic plate sinks (subducts) beneath the continental plate, often creating a chain of volcanoes and a deep ocean trench.
MCQ 5
B Convection currents in the mantle drive plate movement. Hot material rises at ridges, spreads, cools, then sinks at subduction zones, a giant conveyor belt.
Short Answer 1
Model answer: (1) Jigsaw coastlines, South America's east coast fits against Africa's west coast like puzzle pieces. (2) Matching fossils, identical plant (Glossopteris) and animal (Mesosaurus) fossils found on continents now separated by oceans; they couldn't have crossed the sea. (3) Matching rock formations, identical rock sequences and ancient mountain chains appear across South America and Africa, suggesting they were once joined.
Short Answer 2
Model answer: A divergent boundary is where two plates move apart. New crust forms as magma fills the gap. Example: the Mid-Atlantic Ridge, where new ocean floor is created and Iceland sits on it. A convergent boundary is where two plates move toward each other. If oceanic meets continental, the oceanic plate subducts; if two continental plates collide, mountains form. Example: the Himalayas formed where the Indo-Australian Plate collides with the Eurasian Plate.
Short Answer 3
Model answer: Australia is on the Indo-Australian Plate and is far from most active plate boundaries. However, tectonic forces push and pull at the entire plate continuously. Stress accumulates within the plate itself, not just at its edges, and is periodically released as intraplate earthquakes. The 1989 Newcastle earthquake (magnitude 5.6) is an example of this intraplate seismicity. Building codes across Australia now account for this risk.