Ocean floor mapping
Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 11, Ch 4 "Distribution of Oceans and Continents"
Meaning
Ocean floor mapping means the detailed surveys of the sea bed carried out after World War II, using echo-sounding (sonar). Sonar sends sound waves down and times their echoes to measure depth. The surveys showed that the ocean floor is not a flat plain. It has long underwater mountain ranges and deep trenches, mostly close to continental margins. They also found three key facts:
- mid-oceanic ridges are the most volcanically active places;
- ocean-crust rocks are much younger than continental rocks;
- rocks at equal distances on either side of a ridge match in age and make-up.
These findings led directly to the theory of sea-floor spreading.
Example
Mapping revealed the mid-oceanic ridge system, a chain of underwater mountains running through all the oceans. It includes the Central Indian Ridge in the Indian Ocean, south-west of India. The ridge is the longest mountain system on earth (NCERT: over 70,000 km).
Don't confuse with
- Continental evidence for drift: Wegener's evidence (coastline fit, fossils, tillite) came mostly from the land. Ocean floor mapping gave new evidence from the sea bed.