Orogenic collapse

In geology, orogenic collapse is when parts of the Earth's crust become thinner and spread out. This happens because the thick crust has a lot of energy and moves to areas with less energy. Orogenic collapse can start while mountains are still being built up by the movement of tectonic forces.

After the main mountain-building stops, this process is called post-orogenic collapse or post-orogenic extension. These processes are important because they are part of the Wilson Cycle, the cycle of how continents move, collide, and then break apart again. Learning about orogenic collapse helps scientists understand how Earth's surface changes over millions of years.

Description

Orogens, also called mountain ranges, form when tectonic plates crash into each other. This pushes the Earth's crust up and makes it thicker, creating mountains.

But as mountains grow taller, the thick crust can become unstable. It may spread out and become thinner — this is called orogenic collapse.

There are two main reasons for this collapse. First, the heavy crust is pulled down by gravity, causing it to break and spread out. Second, the thick crust sinks into the Earth’s mantle. The heat there makes rocks softer and easier to move. This heat allows material from deeper down to rise, helping to make the crust thinner. While erosion and eduction also wear down mountains, orogenic collapse is an important way mountains can become lower.

Models

A fixed-boundary collapse happens when the Earth's outer layer grows too thick while forces from plate movements keep pushing. This can bring up parts of the Earth that were buried deep down.

Free-boundary collapse occurs when the forces pushing the Earth's layers stop. This lets the thick crust move freely. The surface stretches and deeper layers flow toward thinner areas. The stretching can make cracks on the surface, like how Camembert cheese might sag and split if left out overnight. This process involves extension of the surface and normal faulting.

Examples

Caledonian orogeny

Main article: Caledonian orogeny

The Scandinavian Caledonides is a mountain chain that was once very tall. It was up to 8–9 km high before it got thinner and spread out during the Devonian. This process made big changes in the land, like the Nordfjord-Sogn Detachment. Today, the height of the Scandinavian Mountains comes from things that happened much later, during the Cenozoic.

Basin and Range Province

The Basin and Range Province in the western United States used to be a high flat area. Over time, it stretched and got thinner, breaking into pieces called fault blocks. Scientists are not sure why this happened, but it might be because of changes in how the Earth's plates move. This includes the change from a subduction zone to a transform boundary between the North American and Pacific plates.

Aegean Sea Plate

The Aegean Sea Plate is part of the Earth's crust that has gotten thinner over time. It is located between the Mediterranean and the Black Sea. After some time making mountains, the crust got even thinner because of movements deep inside the Earth involving the African Plate.

Variscan orogeny

See also: Variscan chain and Variscan orogeny

The Variscan orogeny happened when two big parts of Earth's crust, Laurussia and Gondwana, bumped into each other to form Pangaea. This made a thick, high flat area. Later, the thick crust got thinner because of natural processes, including movements deep inside the Earth.

Tibetan Plateau

See also: Geology of the Himalayas

The Tibetan Plateau is being pushed together by the crash of the Indian and Eurasian plates. But it is also stretching from side to side. This stretching probably happens because the thick plateau is spreading out under its own weight, along with movements deep inside the Earth.