Divergent Zones in a Nutshell

Where 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. Two examples on land are the Afar region of east Africa and the Imperial Valley of California/Mexico.

_{Schematic diagram of plate divergence. U.S. Geological Survey image.}

In divergent zones the plates are pulled apart, not pushed apart. The main force driving plate motions is the "slab pull" that arises where plates sink into the mantle under their own weight—the process called subduction. In divergent zones this pulling motion uncovers the hot deep mantle rock of the asthenosphere. As the pressure eases on the deep rocks, they begin to partially melt even though their temperature may fall. (This is called adiabatic melting.) The melted portion expands (as melted solids generally do) and rises, having nowhere else it can go. This magma then freezes onto the trailing edges of the diverging plates, and so the plates grow.

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 very gentle slope 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.

_{Continental divergence forms the Red Sea. More rifting affects East Africa.}

Divergence happens in the continental setting too—that's how new oceans form. The exact reasons it happens where it does, and how it happens, are still being studied.

The best example on Earth today is the narrow Red Sea, where the Arabian plate has pulled away from the African 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.

More divergence is going on within the African plate, in the great rift valleys of East Africa. 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.

_{The Mid-Atlantic Ridge marks where two continents once were one. Most tectonicists assume Africa is fixed while South America and the ridge moved west. Images from National Geophysical Data Center.}
 

A much better example of how continental divergence creates oceans is easy to see in the South Atlantic Ocean. There the precise fit between South America and Africa testifies to the fact that they once were integrated in a larger continent. Early in the 1900s, that ancient continent was given the name Gondwanaland. Since then we have used the mid-ocean ridges to track all of today's continents to their ancient combinations in earlier geologic times.

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 speeds—which is what the plates generally do—the rift moves too. This is how a spreading ridge can migrate right into a continent and vanish, as is happening in the western USA 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 a closed circle.

Back to Plate Tectonics in a Nutshell

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