The rifting happened in two stages with a long intermission. The first serious motion came in the lower or middle Eocene, followed by seafloor spreading in the late Eocene and early Oligocene. Then, for roughly 30 million years, the system went quiet. During that quiet, evaporating seawater left behind massive salt deposits across the basin, thick layers of halite, anhydrite, and gypsum that would later cause geological trouble. About five million years ago, spreading resumed, and the new rifting had to tear through sediments that had already settled on the older crust. The axial trough, the actively spreading center of the sea floor today, is the result. Earthquakes with normal faulting along the rift valley still show that the motion continues.

Geologists describe the mechanism in three steps. First, a thermal anomaly rises in the mantle and thins the continental lithosphere above it, lifting the asthenosphere toward the surface. Second, the crust stretches and thins, and basaltic dikes intrude along the weak spots. Third, the dike injections concentrate into a narrow axial zone and true seafloor spreading begins, leaving the characteristic magnetic stripes that Fred Vine and Drummond Matthews predicted in 1963. From that point the rift propagates along its axis, though it can be locked temporarily by shear or fracture zones. The Red Sea is doing all of this now, in real time. The African Plate is rotating at 0.9270 degrees per million years. The Arabian Plate is rotating at 1.1616 degrees per million years. The difference between those two numbers is the sea.

What lies beneath the central trough is oceanic crust, on that point geologists agree. What lies beneath the flanks and coastal plains is the subject of ongoing argument. One school holds that the entire Red Sea basin is underlain by oceanic crust. Another argues that only part of the main trough is, and that elsewhere the crust remains continental but injected with basaltic dikes. A third suggests that the lower crust in the rift consists of meta-sedimentary rift material sitting directly on the upper mantle. The seismic data from the axial trough is clean, straightforwardly oceanic. The main trough is confusing, with sharp lateral changes in basement velocity jumping between continental and oceanic signatures over short distances. The sea is a geological adolescent, with the body parts of two different adults.

In the southern Red Sea, the basaltic stratovolcano Jabal al-Tair rises as an island at the mouth of the sea, roughly halfway between Yemen and Eritrea, just northwest of the Bab al-Mandab strait. On September 30, 2007, it erupted after 124 years of dormancy, reminding everyone that this rift is hot. Farther south, across the Afar Depression where the Red Sea Rift meets the East African Rift and the Aden Ridge, the land itself is below sea level and the triple junction is an open-air laboratory for continental breakup. The three plate boundaries meet in the Horn of Africa, and the triangular plate between them is slowly separating from the rest of the continent along the East African Rift. In ten or twenty million years, that plate may be an island.

In 1949, an oceanographic survey of the central Red Sea reported something that sounded impossible: the water at the bottom of the axial trough was hot. Subsequent work through the 1960s confirmed it. Saline brines at roughly 60 degrees Celsius were sitting in pools on the sea floor, denser than the water above them, keeping their heat. They were accompanied by metalliferous muds rich in iron, manganese, zinc, and copper. The brines were rising from the active rift, chemistry of the mantle leaking into the sea. The Red Sea was the first place on Earth where anyone had ever seen hydrothermal brines of that kind. Decades later, similar systems would be found at spreading centers worldwide, but the Red Sea gave us the first look, and the muds it produces are still being evaluated as a potential metal resource.

Fly north up the Red Sea and you are flying above a rift that is changing the map of the Earth. The coastlines of Egypt and Saudi Arabia, of Sudan and Eritrea, are the edges of continents pulling apart. The reefs along those coasts grow in waters heated from below as well as from above. The salt layers beneath the sea floor are the frozen evidence of a desiccation event tens of millions of years old. Somewhere far below the keel of any ship crossing that sea, basalt is being injected into new cracks, cooling, recording the direction of the magnetic field as it hardens. A centimeter a year is slow on human time. On geologic time, it is enough to open a new ocean from scratch.