Rift

A rift is a linear zone where the Earth's lithosphere is being pulled apart. The lithosphere is the solid outer layer of our planet. This process is a key part of extensional tectonics, which describes how the Earth's crust stretches. Rifts create long depressions in the ground. Scientists often call these central downfaulted areas a graben. If the rift is a half-graben, it is controlled by a single main fault. When these rifts remain above sea level, they form rift valleys. These valleys can fill with water to create large rift lakes.

The way a rift forms follows a specific sequence of stages. At the beginning, the upper lithosphere starts to extend. This happens on a series of small, unconnected normal faults. These faults create isolated basins. This is often called passive rifting because it is caused by horizontal stresses. During this early stage, there is usually no magmatic activity. As the rift reaches a mature stage, these individual fault segments grow. They eventually link together to form much larger bounding faults. This concentration of extension causes the ground to sink more continuously along the rift axis.

As rifting reaches its climax, the crust thins significantly. This causes the Earth's surface to subside, or sink. At the same time, the Moho, which is the boundary between the crust and the mantle, rises. The mantle lithosphere also thins. This allows the asthenosphere, a hot layer of the mantle, to rise closer to the surface. This brings intense heat into the thinning lithosphere. The high heat can cause dehydration melting. This process can create metamorphic rocks like granulites or granites. This specific type of mountain-building in an extensional setting is called rifting orogeny.

Rifts are not always one long, continuous line. Instead, they usually consist of separate segments. The length of these segments depends on the thickness of the lithosphere. In cold, thick areas like the Baikal Rift, segments can be longer than 80 km. In warmer, thinner areas, segments may be less than 30 km. Where these segments meet, they form accommodation zones. These zones handle the differences in how the faults move. They can be simple relay ramps or very complex structures.

Some rifting processes eventually lead to a total continental breakup. This transition often happens at a triple junction. A triple junction is a place where three rifts meet over a hotspot. In many cases, two of these rifts evolve into seafloor spreading. This creates new oceanic crust at a divergent boundary. The third rift may fail to continue. These failed rifts are known as aulacogens. Successful rifting can create magma-rich margins with many volcanic features. Other margins are magma-starved and rely on large-scale faulting.

Rifts can also show a history of multiple phases over millions of years. The North Sea rift is a great example of this. It showed evidence of several separate rifting phases. This process lasted from the Permian period through the Earliest Cretaceous. That is a total period of over 100 million years. After rifting stops, a process called post-rift subsidence occurs. The mantle beneath the rift cools down. This causes a broad area of the ground to sink. Scientists use models like the McKenzie model or the flexural cantilever model to study this sinking.

These geological structures are very important for the global economy. The sedimentary rocks found in continental rifts hold many minerals and energy sources. For example, SedEx mineral deposits form when hydrothermal fluids are expelled at the seabed. Rifts are also major sites for oil and gas. About 30 percent of the world's giant oil and gas fields are found in rifts. In 1999, it was estimated that rifts held 200 billion barrels of recoverable oil. The Viking Graben and the Gulf of Suez Rift are notable examples of these energy-rich areas.