Plate Tectonics

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

The Great Rift Valley looks, on a map, like a scar. It is one, in the most literal geological sense: a divergent plate boundary, the line along which the African Plate is being pulled apart into two smaller plates, the Nubian to the west and the Somali to the east. At a divergent boundary, the British Geological Survey explains, the crust is pulled apart to create a new pathway for rising hot magma to flow onto the surface, the same process that built the Mid-Atlantic Ridge, except that the East African Rift is doing it on land, in slow motion, where it can be watched rather than only mapped by sonar under two miles of ocean.

The rifting is not uniform. In the north, along the Red Sea and the Gulf of Aden, the plates have already pulled far enough apart that seawater has flooded in, and the Red Sea is, geologically speaking, a young ocean still widening. Follow the rift south into East Africa and the process is younger still: the valley floor has dropped and cracked, volcanic highlands have risen along its shoulders, and a chain of deep lakes, Lake Tanganyika and Lake Malawi among them, has filled the lowest stretches, but the plates have not yet pulled far enough apart for the sea to reach in. Given enough time, tens of millions of years on the timescale plate motions actually run on, geologists expect the same story to repeat: continued spreading, an eventual connection to the sea, and a new ocean where a rift valley used to be, the way the Red Sea and the Atlantic before it both began.

That the Great Rift Valley is a rift becoming an ocean, and not merely a scenic trench, is also why it matters to more than geology. The volcanic highlands the rifting raised, and the lakes it filled, sit at the center of the fossil record used to study early human evolution, a record this atlas's own Great Rift Valley entry already carries. The valley is the plate boundary and the exhibit at the same time, which is part of what makes it worth a page of its own on an atlas built around this atlas's own physical shape.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Earth's lowest and highest points are both plate-tectonic monuments, and both are convergent boundaries, plates colliding rather than separating, yet they look nothing alike. The difference is what happens when two plates meet: whether one is dense enough to be shoved beneath the other, or whether neither one is.

At the Mariana Trench, the answer is subduction. The Pacific Plate, made of dense basaltic ocean floor, meets the smaller Mariana Plate and simply goes under it, bending downward into the mantle. The trench itself is the crease where the bending begins, a crescent-shaped scar nearly seven miles deep at its lowest point, the Challenger Deep, first sounded in 1875 and visited by only a handful of crewed and robotic descents since. Subduction here does not build anything upward; it consumes ocean floor and, as the plunging plate melts at depth, feeds the volcanic arc of the Mariana Islands strung out beside the trench.

At the Himalayas, the answer is the opposite, because neither the Indian Plate nor the Eurasian Plate is oceanic crust dense enough to subduct beneath the other. When continental crust meets continental crust head-on, the collision has nowhere to go but up: both plates crumple, fold and thicken, and the crust piles into the highest mountain range on Earth, still rising today as India continues to push north into Asia. The Himalayas hold Earth's highest point, the summit of Mount Everest, for the same structural reason the Mariana Trench holds its lowest: both are convergent boundaries, and the only variable that decides whether a convergence digs a trench or raises a roof is what kind of crust is doing the converging.

Set side by side, the Mariana Trench and the Himalayas are not two unrelated superlatives that happen to share a mechanism in passing. They are the same process, plate convergence, run to its two opposite structural conclusions, subduction where one plate could go under and collision where neither could.