Moving Plates
Geology & Plate Tectonics Fundamentals
Course 1 · Chapter 3 · Plate Tectonics: The Theory and the Evidence
Look at a map of the Atlantic and the coast of South America seems to fit against the coast of Africa. Many people noticed this long before it became science. In 1912 the German meteorologist Alfred Wegener argued that the continents had once been joined and had drifted apart, and most geologists of his day rejected the idea, partly because he could not explain what moved them. It took until the 1960s for the evidence to turn the argument, and the resulting theory, plate tectonics, is now the framework for almost everything else in geology. This chapter follows that change of mind: the early evidence, the discoveries on the ocean floor, the plates and their boundaries, and the causes of movement. Dates and names are from general knowledge and flagged "check" where they matter.
Wegener and the Case for Drift
The fit
The edges of the continents, especially when matched at the edge of the continental shelf and not at the coast, line up well (check). On its own, a good fit is suggestive, not proof.
Matching fossils
The same fossil plant Glossopteris and the freshwater reptile Mesosaurus are found on continents now separated by oceans (check). Wegener argued that they could not have crossed an ocean.
Matching rocks and climate
Mountain belts and rock types continue across the Atlantic, and glacial deposits of the same age appear in places that are now warm, such as India and southern Africa (check).
The Ocean Floor Changes the Argument
| Discovery (check each date) | What it showed |
|---|---|
| Mid-ocean ridges (surveys after the Second World War) | A continuous mountain chain winds through the world's oceans, with a rift valley along its crest |
| Young ocean floor (1950s and 60s) | Ocean-floor rocks are far younger than the continents, with the youngest at the ridges; no known ocean floor is older than about 180 million years |
| Seafloor spreading (Harry Hess, early 1960s) | New crust forms at the ridges and moves outward; old crust sinks back at trenches |
| Magnetic stripes (Vine, Matthews and Morley, 1963) | Rocks on either side of a ridge record the Earth's magnetic field, which has reversed many times, in a mirror-image pattern of stripes |
| Earthquake locations (1960s) | Quakes trace narrow belts, outlining plates and dipping zones where one plate descends beneath another |
The magnetic stripes were the turning point. When lava cools, magnetic minerals in it lock in the direction of the field at that time. A symmetrical pattern on either side of a ridge is just what you would expect if crust is made at the centre and carried away in both directions (check). Satellite measurements now track plate movement directly, at a few centimetres a year, about the speed at which fingernails grow (check).
The Plates and Their Boundaries
The outer shell of the Earth, the lithosphere, is broken into about a dozen large plates and several smaller ones (check). Each plate is a rigid slab of crust and the uppermost mantle, riding on the softer, weaker layer beneath, the asthenosphere.
| Boundary | Movement | Typical features (check examples) |
|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridges, rift valleys; Iceland sits on the Mid-Atlantic Ridge; East African Rift |
| Convergent: ocean meets continent | Denser ocean plate sinks beneath (subduction) | Deep trenches, volcanic mountain chains such as the Andes |
| Convergent: continent meets continent | Neither sinks easily, so crust crumples | Himalaya, Alps |
| Transform | Plates slide past each other | San Andreas Fault in California |
Most earthquakes and volcanoes occur at plate boundaries, which is why they cluster in belts such as the "Ring of Fire" around the Pacific (Chapter 4). Intraplate volcanoes such as Hawaii are explained by hot spots, plumes of hot rock rising from deep in the mantle (check).
What Drives the Plates?
- Mantle convection. Heat from the interior makes the mantle flow very slowly, as Holmes proposed. The exact pattern is still debated (check).
- Slab pull. The cold, dense edge of a plate sinking at a trench pulls the rest of the plate along. Many geologists regard this as the largest force (check).
- Ridge push. The elevated ridge slides down, nudging the plate away. This is generally considered smaller (check).
- Why it matters. The cause is understood in outline but not in every detail. That is normal in science: the theory rests on evidence of movement, which is firm, more than on a complete account of the engine, which is still being refined.
Exercises
Explain why most geologists rejected Wegener in the 1910s and 1920s, and what changed by the 1960s. Name two pieces of evidence that carried most weight.
A plate moves 2.5 cm a year. How far does it travel in 100 million years, and what does your answer tell you about the width of the Atlantic?
Choose one of the four boundary types and describe the landscape you would expect, the likely hazards, and one real example you could look up.
Quick Reference
- Drift (1912): Wegener's fit, fossil and rock evidence, without a good mechanism
- Seafloor spreading: new crust at ridges; magnetic stripes in mirror image
- Plates: about a dozen large lithosphere slabs moving a few cm a year
- Boundaries: divergent, convergent (ocean or continent), transform
- Drivers: mantle convection, slab pull and ridge push