Extensional tectonics

Extensional tectonics

Extensional tectonics is the study of how Earth's outer layers stretch and change shape. It looks at the forces that pull and lengthen the planet's crust or lithosphere. These forces create special patterns in the rocks. This helps scientists learn how the Earth moves and changes over time. By studying extensional tectonics, we can discover more about the planet's history and how landscapes form.

Deformation styles

The shapes and structures in Earth's crust depend on how much it stretches. We measure this stretching using the beta factor. This factor compares the thickness of the crust before and after stretching.

When the crust stretches only a little, we see faults — cracks where the ground moves. These faults can form valleys and tilted blocks of land.

When the crust stretches a lot, the faults can become too flat to move. New faults form, and sometimes they bring up deep, hot rocks to the surface. These special areas are called metamorphic core complexes. Near the oceans, stretching can create big curved faults and folded landforms.

Geological environments associated with extensional tectonics

Areas where the Earth's crust stretches are linked to several special places.

Continental rifts

Main article: Rift

Rifts are long areas where the Earth's crust stretches. They can be from less than 100 km to several hundred km wide. These areas have faults and blocks of land that move. One side often moves more than the other, creating a shape called a half-graben. Other shapes include metamorphic core complexes and tilted blocks. Examples of active rifts are the Baikal Rift Zone and the East African Rift.

Divergent plate boundaries

Main article: Divergent plate boundary

Divergent plate boundaries are places where the Earth's crust stretches as new crust forms at the mid-ocean ridge and the land opens up.

Gravitational spreading of zones of thickened crust

Main article: Orogenic collapse

When parts of the Earth's crust become very thick, such as after two landmasses crash together, they can spread out sideways. This spreading can happen even while the crash is still occurring. After the crash ends, the thick crust often collapses under its own weight, forming large stretching faults. For example, big stretching happened during the Devonian time right after the end of the Caledonian orogeny, especially in East Greenland and western Norway.

Releasing bends along strike-slip faults

When a strike-slip fault bends and creates a gap, such as a left-stepping bend on a fault that moves left, it can cause the land to stretch or transtension. These bends are called releasing bends or extensional stepovers. They often form pull-apart basins or rhombochasms. Examples include the Dead Sea, formed at a left-stepping offset of the Dead Sea Transform, and the Sea of Marmara, formed at a right-stepping offset on the North Anatolian Fault.

Back-arc basins

Main article: Back-arc basin

Back-arc basins form behind many places where one part of the Earth's crust dives under another, due to the movement of the oceanic trench rolling back, which creates a zone of stretching parallel to the island arc.

Passive margins

Main article: Passive margin

A passive margin built over a weaker layer, like a squished mudstone or salt, tends to spread out sideways because of its own weight. The inner part of the land moves with stretching faults, while the outer part gets squished.