Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart. As the plates move apart, the rocks break and form a crack between the plates. Earthquakes occur along the plate boundary. Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano.
(Fig 1.11) Sea floor Spreading.
As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor. This chain is called an oceanic ridge. The boundaries where the plates move apart are
'constructive' because new crust is being formed and added to the ocean floor. The ocean floor
gradually extends and thus the size of these plates increases. As these plates get bigger, others
become smaller as they melt back into the Earth in the process called subduction.
The new rock at the edge has no sediments like the sand or mud, since it is formed only recently.
Farther away from the ridge, sand and mud gradually settle on it, in an ever-thickening blanket. The oldest rocks may have 14,000 feet of sand and other sediments resting on top of it.
An example of an oceanic ridge is the Mid-Atlantic Ridge. It is one part of a system of mid-oceanic ridges that stretches for 50,000 miles through the world's oceans. The underwater mountains of the ridge may not be more than two miles higher than the surrounding sea floor.
On the whole, sea-floor spreading is basically volcanic, but it is a slow and regular process, without the explosive outbursts of the volcanoes on land.