Evidence for sea-floor spreading
Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 11, Ch 4 "Distribution of Oceans and Continents"
Meaning
NCERT gives five facts that showed the ocean floor is slowly widening:
- Volcanic eruptions are common all along mid-oceanic ridges, which are underwater mountain chains, and they bring up huge amounts of lava.
- Rocks at equal distances on either side of a ridge crest match in age, chemistry and magnetism. The youngest rocks lie at the crest and have normal polarity, meaning they are magnetised in the same direction as today's field.
- Ocean crust is nowhere older than 200 million years, while some continental rocks are 3,200 million years old.
- Ocean-floor sediments are surprisingly thin and nowhere older than 200 million years.
- Earthquakes are deep at trenches and shallow at ridges.
Together these facts led Harry Hess to propose sea-floor spreading in 1961.
Example
At the Mid-Atlantic Ridge, rocks get older the farther they are from the crest, on both sides alike. Iceland is a part of this ridge that rises above sea level, and its frequent eruptions show the crest is active.
Don't confuse with
- Fossil evidence for continental drift: Fossils support continental drift. "Fossil distribution was used as evidence for sea-floor spreading" is false.