Worksheets › Class 11 › Distribution of Oceans and Continents › Cheat sheet
Class 11 Distribution of Oceans and Continents: cheat sheet
CBSE Social Science. Continental drift and its evidence, sea floor spreading, plate tectonics, the three kinds of plate boundary and the journey of the Indian plate.
Key ideas
- Wegener said all continents were one super continent, Pangaea, ringed by the ocean Panthalassa; it began to split about 200 million years ago.
- Evidence: the jig-saw fit of the Atlantic coasts, rocks of the same age across oceans, matching tillite, placer gold in Ghana from Brazil, and fossils like Mesosaurus.
- Wegener’s forces were too weak; Arthur Holmes suggested convection currents in the mantle as the driving force.
- Hess’s sea floor spreading: lava at mid-ocean ridges pushes the floor apart, and old floor sinks at trenches, so ocean crust is young.
- A plate is a rigid slab of lithosphere moving over the asthenosphere; there are seven major plates and several minor ones.
- Divergent boundaries make new crust, convergent boundaries destroy it in subduction zones, and transform boundaries neither make nor destroy it.
- India moved north from near Australia, poured out the Deccan Traps and hit Asia 40–50 million years ago, raising the Himalayas.
Remember
| Key years | Wegener 1912; Hess 1961; Bullard’s computer fit 1964; plate tectonics 1967 |
| Continents | about 29% of the earth’s surface |
| Oldest ocean floor | no older than 200 million years |
| Plate movement | 1 cm a year for 1 million years = 10 km (1 km = 100,000 cm) |
| Time to open a gap | million years = width in km ÷ (rate in cm a year × 10) |
| Fastest spreading | East Pacific Rise, over 15 cm a year; slowest the Arctic Ridge |
| Divergent example | Mid-Atlantic Ridge |
Watch out
- Swapping Pangaea (the land) and Panthalassa (the ocean).
- Getting the unit change wrong: cm a year times a million years is cm, so divide by 100,000 for km.
- Saying crust is made at convergent boundaries (it is destroyed there).
Worked example
Suppose two plates move apart at 15 cm a year. (a) How many km apart do they move in 17 million years? (b) The two Mesosaurus localities are 4,800 km apart: how many million years would it take to open that gap at this rate? (c) What kind of plate boundary forms where plates move apart?
Working: 15 cm × 1 million = 150 km per million years
Answer: (a) 2,550 km (b) 32 million years (c) a divergent boundary (a spreading site), such as the Mid-Atlantic Ridge
Practise Distribution of Oceans and Continents
Revise with this page, then practise: every Distribution of Oceans and Continents sheet in the app is new, with an answer key and working.
Coming soon for Android and iPhone.