Earth's surface is covered by more than a dozen plates (rock plates), which continue to move relative to one another. Plate movements are the driving force behind earthquakes, volcanoes, mountain ranges, and other features.
Basic Knowledge
Plates are rigid rock slabs made of the lithosphere(the crust+the uppermost mantle. Approximately 100 km thick). They move over the asthenosphere(soft mantle) below at a rate of several cm per year (about as fast as fingernails grow).
Types of plates:
・Continental plates: Include continental crust. Because they have a low density (approximately 2.7 g/cm³), they lie above oceanic plates. They do not subduct.
・Oceanic plates: Include oceanic crust. They have a high density (approximately 3.0 g/cm³). They are created at mid-ocean ridges and subduct at ocean trenches.
Four plates near Japan:
①Pacific Plate(oceanic・from east to west)
②Philippine Sea Plate(oceanic・from south to north)
③North American Plate(continental・located in eastern Japan)
④Eurasian Plate(continental・located in western Japan)
The driving forces of plate movement are mantle convection and the force exerted by a sinking, heavy plate as it pulls the rest of the plate (slab pull).
Plate（A rigid rock slab made of the lithosphere. Approximately 100 km thick）
Continental plate（Low density・does not subduct）
Oceanic plate（High density・created at mid-ocean ridges and sinks at trenches）
Mantle convection（The main driving force of plate movement）
Slab pull（The force exerted by a sinking, heavy plate as it pulls the rest）
Mid-ocean ridge（A place where plates are created. A mountain range on the seafloor）
In Depth
The predecessor of plate tectonic theory was Wegener's continental drift hypothesis of 1912. He based it on evidence such as the matching shapes of the west coast of Africa and the east coast of South America, and the existence of the same fossils and strata on both coasts. However, it was initially viewed skeptically because the driving force was unknown. In the 1960s, the seafloor spreading hypothesis (Hess) and studies of paleomagnetism were added, completing the theory of plate tectonics.
Today's continental arrangement was created when the supercontinent Pangaea, which existed about 200 million years ago, split apart. India was once near Antarctica and collided rapidly with Eurasia, forming the Himalayas (India is currently moving northward by approximately 5 cm per year).
- Plate=lithosphere (approximately 100 km thick)
- Continental plate=low density・does not subduct
- Oceanic plate=high density・created at mid-ocean ridges and sinks at trenches
- Near Japan=4 plates (Pacific・Philippine Sea・North American・Eurasian)
- Continental drift hypothesis=Wegener (1912)
- The supercontinent Pangaea split apart about 200 million years ago
Points to Note
① A plate is the lithosphere (not just the crust). ② Oceanic plates subduct, whereas continental plates do not. ③ A slab is a subducted oceanic plate. Because it is colder and denser than its surroundings, it is dragged downward by its own weight.
Practice
- Name all four plates located near Japan.
- Using density, explain why a continental plate does not subduct beneath an oceanic plate.
- State the name of the scientist who proposed the continental drift hypothesis and the year.