Rifts break through the weak spots.

Rifts break through the weak spots

The water-etched lower boundaries of Saudi Arabia -- the Gulf of Suez on the west and the Gulf of Aqaba on the east -- illustrate how rifts in the upper earth tend to follow the path of least resistance, traveling through the weakest areas, according to Michael S. Steckler and Uri S. ten Brink of the Lamont-Doherty Geological Observatory in Palisades, N.Y.

In a paper to be published in EARTH AND PLANETARY SCIENCE LETTERS, the two geophysicists state that rifts underlying both the Suez and Aqaba were formed by the same movement between the African and Arabian plates. The Suez rift formed first, propagating northwestward from the Red Sea until about 17 million years ago, when it reached the Mediterranean Ocean, where the lithosphere (the upper 100 kilometers of crust and upper mantle) is much stronger than under the African continent. At this Mediterranean barrier, the Suez rift slowed almost to a half, and a new rift opened from the Red Sea northeastward, Steckler says. This one, running through the Gulf of Aqaba and farther north through the Dead Sea, continues today, Steckler says, and is likely someday to split the African and Arabian continents apart.

The lithosphere under continents is mostly weaker than that under oceans because it is made up of a thicker portion of crust and a correspondingly thinner portion of mantle, Steckler explains. Continental lithosphere is strongest in midcontinent areas, where it is oldest and therefore coldest. It is warmest, and weakest, at the edges, Steckler says.

This explains why so many of the world's fault zones skirt the edges of continents, Steckler says. The Aqaba rift, for example, runs along the weakest zone on the eastern edge of the African continent. Similarly, the San Andreas fault slices along the western edge of CAlifornia, and the Fairweather and Queen Charlotte faults trim the edges of Canada and Alaska.