Depending on whether the context is plate tectonics or volcanology, the term "rift zone" can mean two different but related things; in general terms, it can be regarded as an area where rock in the Earth’s crust has been stretched, resulting in fissures and fractures through which magma can rise, as lava, to the surface. Lava can vary in composition according to its origin, and different types of lava have different physical characteristics. Rift zones form in areas where the magma below the surface is basaltic in nature — this type of lava is relatively runny and has a low gas content. This means that eruptions are not explosive: instead, the lava tends to flow relatively quietly out of fractures in the crust. Basaltic, or mafic, lava comes from deep in the mantle and is associated with spreading centers, or areas where continental plates are moving apart.
Rift zones can form at these spreading centers, where the crust is stretched by convection within the mantle. There are a number of rift zones of this type in the form of mid-oceanic ridges, such as the North Atlantic Ridge, where the North American and Eurasian plates are moving apart. Here, the oceanic crust is fracturing, and ridges are formed by the solidification of magma rising to the surface. The gradual spreading apart of the oceanic crust in these areas limits the extent to which ridges can build up, but in some particularly active areas, sometimes called “hot spots,” the new rock that is being formed relatively rapidly can reach the surface, resulting in volcanic islands such as Iceland and the Hawaiian islands.
Tectonic rift zones can also appear on land where a new split is forming, creating plates that move apart. A good example is the Great Rift Valley in East Africa. The land sinks along the line of the split, forming a widening valley between two volcanic ridges. Eventually, when the land sinks below sea level, ocean water will flow in, forming a new ocean.
Over time, repeated outpourings of basaltic lava that occur at hot spots can form a type of large volcano known as a shield volcano. These have long, gentle slopes formed from many layers of solidified basaltic lava, and localized rift zones can form on their slopes, due to stretching of the rock due to gravity. Large fractures can form at points of weakness in the rock, allowing lava to flow to the surface and resulting in lateral eruptions, as opposed to eruptions from the main vent. As with ocean floor eruptions, these are non-explosive in nature. Mauna Loa on Hawaii provides a textbook example of this kind of rift zone.
There are a number of distinctive geological features that are associated with rift zones on shield volcanoes. Dikes are wall-like structures formed by lava that has risen into fractures in the rock and solidified. Often, the solidified lava is harder than the surrounding rock, which erodes more quickly, leaving the dike exposed. Fissure eruptions may eject blobs of molten lava, known as “spatter,” a few meters into the air. These can accumulate around eruption sites, forming spatter cones and more linear structures called ramparts.
Rift zones are not confined to the Earth. On Mars, the enormous canyon known as Valles Marineris is a huge rift zone that, at 2,000 miles (3,000 kilometers) long and up to 12,500 feet (3,800 meters) deep, dwarfs any similar features on our planet. It is thought to have formed over a period from 3.5 billion to 2 billion years ago and seems to have resulted from the stresses associated with major volcanic activity nearby.