Seafloor spreading

Seafloor spreading happens at the bottom of an ocean as tectonic plates move apart. The seafloor moves and carries continents with it. At ridges in the middle of oceans, new oceanic crust is created. The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges.
At the Mid-Atlantic Ridge (and other places), material from the upper mantle rises through the faults between oceanic plates. It forms new crust as the plates move away from each other. The new crust then slowly moves away from the ridge. It is a place of earthquakes and volcanos. Seafloor spreading helps explain continental drift in plate tectonics. At oceanic trenches, seafloor crust slides down and under continental crust.
Earlier theories (e.g. by Alfred Wegener) of continental drift were that continents 'plowed' through the ocean. The modern idea is that the ocean floor itself moves and carries the continents with it as it expands from a mid-ocean ridge. Today, it is accepted. The phenomenon is caused by convection in the weak upper mantle, or asthenosphere.
Additionally spreading rates determine if the ridge is a fast, intermediate, or slow. As a general rule, fast ridges see spreading rate of more than 9 cm/year. Intermediate ridges have a spreading rate of 4-9 cm/year while slow spreading ridges have a rate less than 4 cm/year.
Mid-ocean ridge
A mid-ocean ridge is an underwater mountain system. This consists of mountain chains, with a rift valley running along its spine, formed by plate tectonics. A mid-ocean ridge marks the boundary between two tectonic plates which are moving apart. A mid ocean ridge is made by a divergent boundary.
The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean. The mid-oceanic ridge system is the longest mountain range in the world. The continuous mountain range is long. It is several times longer than the Andes, the longest continental mountain range. The total length of the oceanic ridge system is long.
How it works
Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor and into the crust at and near rifts along the ridge axes. The crystallized magma forms new crust of basalt and gabbro.
The rocks making up the crust below the sea floor are youngest at the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of decompression melting in the underlying Earth's mantle.
The oceanic crust is made up of rocks much younger than the Earth itself: oceanic crust in the ocean basins is everywhere less than 200 million years old. The crust is in a constant state of 'renewal' at the ocean ridges. Moving away from the mid-ocean ridge, ocean depth progressively increases; the greatest depths are in ocean trenches. As the oceanic crust moves away from the ridge axis, the peridotite in the underlying mantle cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere.
Slow spreading ridges like the Mid-Atlantic Ridge have large, wide rift valleys, sometimes as big as 10-20 km wide and very rugged terrain at the ridge crest. By contrast, fast spreading ridges like the East Pacific Rise are narrow, sharp incisions surrounded by generally flat topography that slopes away from the ridge over many hundreds of miles.
References
Oceanography
Plate tectonics