Divergent Plate Boundaries

Mid-Ocean Ridges

At oceanic divergent boundaries, new lithosphere is born hot and cools over millions of years. As it cools it shrinks, thus the fresh seafloor 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 seafloor 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 study of mid-ocean ridges helped establish the theory of plate tectonics in the 1960s. Geomagnetic mapping showed large, alternating "magnetic stripes" in the seafloor, a result of Earth's ever-changing paleomagnetism. These stripes mirrored each other on both sides of divergent boundaries, giving geologists irrefutable evidence of seafloor spreading. 

Iceland

At over 10,000 miles, the Mid-Atlantic Ridge is the longest mountain chain in the world, stretching from the Arctic to just above Antarctica. Ninety percent of it, however, is in the deep ocean. Iceland is the only place that this ridge manifests itself above sea level, but this is not due to magma buildup along the ridge alone.

Iceland also sits on a volcanic hotspot, the Iceland plume, which uplifted the ocean floor to higher elevations as the divergent boundary split it apart. Because of its unique tectonic setting, the island experiences multiple types of volcanism and geothermal activity. Over the past 500 years, Iceland has been responsible for roughly a third of the total lava output on Earth. 

Continental Spreading

Divergence happens in the continental setting too—that's how new oceans form. The exact reasons as to why 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 Nubian plate. Because Arabia has run into southern Asia while Africa remains stable, the Red Sea won't widen into a Red Ocean soon. 

Divergence is also going on in the Great Rift Valley of East Africa, forming the boundary between the Somalian and Nubian plates. But these rift zones, like the Red Sea, have not opened much even though they are millions of years old. Apparently, the tectonic forces around Africa are pushing on the continent's edges.

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 were once integrated with a larger continent.

Early in the 1900s, that ancient continent was given the name Gondwanaland. Since then, we have used the spreading of the mid-ocean ridges to track all of today's continents to their ancient combinations in earlier geologic times.

String Cheese and Moving Rifts

One fact not widely appreciated is that divergent margins move sideways just like the plates themselves. 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 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.

Edited by Brooks Mitchell