Ocean floor and sea floor spreading

Post-drift studies. Most of Wegener's evidence came from the continents. After the Second World War, ocean-floor mapping added new evidence.

• Convection currents (Arthur Holmes, 1930s): heat from radioactive elements creates temperature differences in the mantle, driving a system of currents. This offered the force that drift theory had lacked.

The ocean floor is not a flat plain but full of relief. Its three main divisions:

• Continental margins: the shift from shore to deep sea: continental shelf, slope, rise and deep-ocean trenches (mostly near continents).
• Abyssal plains (deep-sea basins): wide plains between the margins and the ridges, where sediments from the continents settle.
• Mid-oceanic ridges: an interconnected underwater mountain system, the longest mountain chain on earth, with a central rift of intense volcanic activity.

Earthquakes and volcanoes. On a world map, a line of quakes runs down the middle of the Atlantic into the Indian Ocean, along the mid-oceanic ridges. Another belt follows the Alpine-Himalayan system and the rim of the Pacific. Quakes at ridges are shallow; along the Alpine-Himalayan belt and the Pacific rim they are deep-seated. The Pacific rim, full of active volcanoes, is the 'rim of fire'.

Facts from the ocean floor:

• Eruptions are common along the ridges.
• Rocks equidistant on either side of a ridge's crest match in age, chemistry and magnetism. Rocks nearest the crest have normal polarity and are the youngest; age increases away from it.
• Ocean crust is much younger than continental rock: nowhere older than 200 million years, while some continental rocks are 3,200 million years old.
• Ocean sediments are unexpectedly thin.
• Trenches have deep quakes; ridges, shallow ones.

Sea floor spreading (Harry Hess, 1961): constant eruptions at the ridge crest crack the ocean crust, and new lava wedges in, pushing the floor apart on both sides. Since one ocean's spreading does not shrink another, the old floor must be consumed: it sinks at the oceanic trenches.

Example (new): picture two conveyor belts coming up out of a slot in the floor and moving away from each other: a box placed near the slot a minute ago is closer to it than one placed an hour ago. Rocks beside a mid-oceanic ridge age in the same way with distance from the crest.