About Divergent ZonesWhere lithospheric plates move apart
Where the lithospheric plates move apart from each other, the boundary between them becomes a divergent margin. Unlike convergent margins, divergent margins involve only oceanic or only continental lithosphere, not one of each. Today the vast majority of divergent margins are in the ocean, where they were not mapped or understood until late in the 20th century. Of all these oceanic divergent margins, only Iceland stands above the waves. Two examples on the continents are the Afar region of east Africa and the Imperial Valley of California/Mexico.
New lithosphere is born hot and cools over millions of years. As it cools it shrinks, thus the fresh sea floor stands higher than the older lithosphere on either side. This is why divergent zones take the form of long, wide swells running along the ocean floor: mid-ocean ridges. The ridges are only a few kilometers high but hundreds wide. The slope on the flanks of a ridge means that diverging plates get an assist from gravity, a force called "ridge push" that together with slab pull accounts for most of the energy driving the plates. On the crest of each ridge is a line of volcanic activity. This is where the famous black smokers of the deep sea floor are found.
Plates diverge at a wide range of speeds, giving rise to differences in spreading ridges. Slow-spreading ridges like the Mid-Atlantic Ridge have steeper-sloping sides because it takes less distance for their new lithosphere to cool. They have relatively little magma production so that the ridge crest can develop a deep dropped-down block, a rift valley, at its center. Fast-spreading ridges like the East Pacific Rise make more magma and lack rift valleys.
The best example on Earth today is the narrow Red Sea, where the Arabia plate has pulled away from the Africa plate. Because Arabia has run into southern Asia while Africa isn't going anywhere, the Red Sea won't widen into a Red Ocean soon. It probably isn't a typical case.
Divergence is also going on in the great rift valleys of East Africa, forming the boundary between the Africa and Nubia plates. But these rift zones, like the Red Sea, have not opened much though they are millions of years old. Apparently the tectonic forces around Africa are pushing on the continent's edges.
One fact not widely appreciated is that divergent margins move sideways just like the plates themselves do. To see this for yourself, take a bit of string cheese and pull it apart in your two hands. If you move your hands apart, both at the same speed, the "rift" in the cheese stays put. If you move your hands at different speedswhich is what the plates generally dothe rift moves too. This is how a spreading ridge can migrate right into a continent and vanish, as is happening in western North America today.
This exercise should demonstrate that divergent margins are passive windows into the asthenosphere, releasing magmas from below wherever they happen to wander. While textbooks often say that plate tectonics is part of a convection cycle in the mantle, that notion cannot be true in the ordinary sense. Mantle rock is lifted to the crust, carried around, and subducted somewhere else, but not in the closed circles called convection cells.