Geography · Ch 10 — Distribution of Oceans and Continents
Plate Tectonics
Plate Tectonics
Map 4.5 is the visual heart of the plate tectonics section. It takes the abstract idea that the lithosphere is cracked into rigid slabs and makes it concrete: you see the entire globe divided into seven large, labelled plates and a handful of smaller ones, each drawn as a distinct patch of the Earth's surface. The map uses a rectangular graticule, so the whole world fits on one flat frame, and every plate is outlined by a boundary line whose style tells you what kind of edge it is.
The seven major plates are each marked with a roman numeral, I through VII. I is Antarctica and the surrounding oceanic plate; II is the North American plate, which includes the western Atlantic floor and is separated from the South American plate along the Caribbean islands; III is the South American plate, likewise carrying the western Atlantic floor and separated from its northern neighbour along the same Caribbean island chain. IV is the Pacific plate, V is the India–Australia–New Zealand plate, VI is the African plate with the eastern Atlantic floor, and VII is the Eurasian plate with its adjacent oceanic plate. The map also shows the minor plates: the Cocos plate between Central America and the Pacific, the Nazca plate between South America and the Pacific, the Arabian plate (mostly the Saudi Arabian landmass), the Philippine plate between the Asiatic and Pacific plates, the Caroline plate between the Philippine and Indian plates north of New Guinea, and the Scotia plate.
What makes the map instructive is not just the plates themselves but the boundaries drawn between them. A legend distinguishes three kinds of boundary. A double line marks a divergent boundary, where plates pull apart and new crust is generated — the classic example shown is the Mid-Atlantic Ridge, running down the Atlantic and separating the American plates from the Eurasian and African plates. A triangled line marks a convergent boundary, where one plate dives beneath another at a subduction zone; the map labels the Aleutian Trench and the Java Trench as such sites. A single line marks a transform boundary, where plates slide horizontally past each other without creating or destroying crust — the San Andreas Fault is the named example. The East Pacific Rise also appears, a fast-spreading ridge in the Pacific. …
The idea in one line
The continents do not drift on their own — they ride on large, rigid slabs of the Earth's outer shell called tectonic (lithospheric) plates, and it is the movement of these plates that reshapes oceans and continents.
What is a tectonic plate?
A tectonic plate, also called a lithospheric plate, is a massive, irregularly shaped slab of solid rock. Each plate is generally made up of both continental and oceanic lithosphere. These plates move horizontally over the asthenosphere as rigid units — the plates themselves do not deform; they slide as whole pieces.
The lithosphere includes the crust and the uppermost part of the mantle. Its thickness varies:
- 5 to 100 km in oceanic parts
- about 200 km in continental areas
A plate is called a continental plate or an oceanic plate depending on which of the two occupies the larger portion of the plate. For example, the Pacific plate is largely an oceanic plate, while the Eurasian plate may be called a continental plate.
How many plates are there?
The theory of plate tectonics proposes that the Earth's lithosphere is divided into seven major plates and some minor plates. These major plates are surrounded by young fold mountain ridges, trenches, and/or faults.
The seven major plates
| Plate | Description |
|---|---|
| I | Antarctica and the surrounding oceanic plate |
| II | North American plate (with the western Atlantic floor, separated from the South American plate along the Caribbean islands) |
| III | South American plate (with the western Atlantic floor, separated from the North American plate along the Caribbean islands) |
| IV | Pacific plate |
| V | India-Australia-New Zealand plate |
| VI | African plate (with the eastern Atlantic floor) |
| VII | Eurasian plate and the adjacent oceanic plate |
Some important minor plates
- Cocos plate — between Central America and the Pacific plate
- Nazca plate — between South America and the Pacific plate
- Arabian plate — mostly the Saudi Arabian landmass
- Philippine plate — between the Asiatic and Pacific plates
- Caroline plate — between the Philippine and Indian plates, north of New Guinea
What actually moves — the plate, not the continent
These plates have been constantly moving over the globe throughout the history of the Earth. A crucial correction to Wegener's idea: it is not the continent that moves — continents are simply parts of a plate, and what moves is the plate itself.
Moreover, all plates, without exception, have moved in the geological past and shall continue to move in the future.
Wegener had imagined that all continents initially existed as a single supercontinent, Pangaea. But later discoveries show that continental masses, resting on plates, have been wandering throughout the geological period. Pangaea itself was the result of converging different continental masses that were parts of one or the other plates — not the starting point of all continental motion.
Scientists have used palaeomagnetic data to determine the positions held by each present-day continental landmass in different geological periods. The position of the Indian subcontinent (mostly Peninsular India) has been traced using rocks analysed from the Nagpur area.
Types of plate boundaries
There are three types of plate boundaries.
1. Divergent boundaries
Here, new crust is generated as plates pull away from each other. The sites where plates move away from each other are called spreading sites. The best-known example is the Mid-Atlantic Ridge, where the American plates are separated from the Eurasian and African plates.
2. Convergent boundaries
Here, crust is destroyed as one plate dives under another. The location where sinking of a plate occurs is called a subduction zone. Convergence can occur in three ways:
- between an oceanic and a continental plate
- between two oceanic plates
- between two continental plates
3. Transform boundaries
Here, the crust is neither produced nor destroyed as plates slide horizontally past each other. Transform faults are the planes of separation generally perpendicular to the mid-oceanic ridges. Because eruptions do not take place all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the Earth's axis. The rotation of the Earth also has its effect on the separated blocks of the plate portions.
Rates of plate movement
The strips of normal and reverse magnetic field that run parallel to the mid-oceanic ridges help scientists determine the rates of plate movement. These rates vary considerably:
- The Arctic Ridge has the slowest rate — less than 2.5 cm per year …