Nance_1988_Supercontinent+Cycle

The Earth's continents have repeatedly joined to form supercontinents and then split apart.
This process of formation and destruction is believed to be cyclic, influencing geology, climate, and biological evolution.

Plate tectonics is not random; it is characterized by orderly processes.
The lithosphere (the rigid outer layer of the Earth) is made up of a mosaic of tectonic plates that float on the asthenosphere (a hot, plastic layer of the mantle).
Plate movements occur at an average speed of a few centimeters per year, primarily driven by sea-floor spreading.
Sea-floor spreading involves molten material from the asthenosphere rising at ocean ridges and forming new oceanic crust.
Oceanic crust can either carry continents with it or sink beneath them in a process called subduction.
Continents can collide and merge to form larger landmasses or move apart, creating new oceans.

Authors: R. Damian Nance, Thomas R. Worsley, and Judith B. Moody, specialists in tectonics, oceanography, and geochemistry.
Past influences from Don L. Anderson (California Institute of Technology) and J. Umgrove (1947) guided the development of the supercontinent cycle theory.

Primary driver of tectonic plates is heat from radioactive decay in the mantle; however, heat conduction and loss through the crust are crucial.
Continental crust is less efficient than oceanic crust in conducting heat, leading to accumulation of heat under supercontinents, causing them to dome and eventually break apart.
This resembles a coffee percolator where heat is continuously input but released in bursts.

The model builds on the Wilson cycle, which involves the opening and closing of ocean basins.
Features of the Wilson cycle:
- Formation of volcanic hot spots connected by rift valleys leads to the splitting of continents.
- New oceans develop from rift valleys as continental fragments move apart.
- The ocean floor ages, cools, and becomes dense before subduction occurs, reversing the cycle.

Supercontinent cycles shape the geology and climate of Earth and influence biological evolution.
An observed cyclicity in geological records suggests a period of about 500 million years for one complete supercontinent cycle.
Significant geological events, such as mountain building and rifting, follow a regular pattern every few hundred million years.
- Intense mountain building episodes have occurred at approximate intervals of 400-500 million years.

The supercontinent cycle is predicted to have significant effects on global sea level due to the interplay of ocean basin volume and continental elevation.
Sea level is affected by:
- Age of sea floor, determined by spreading rates and subduction.
- Accumulation of heat under continents causing uplift.
The processes of breakup and assembly of supercontinents lead to fluctuations between low and high sea levels, influencing sedimentation patterns, biological productivity, and nutrient availability.

The supercontinent cycle influences global climate patterns due to factors such as sea level changes, continental weathering, and ocean circulation.
Lower sea levels lead to:
- Enhanced weathering of continental rocks, drawing down atmospheric CO2.
- Glaciation and its associated effects on ocean circulation and marine productivity.

The biological record supports the theory with indications of low species diversity during supercontinent existence and increased diversity thereafter.
Historical evidence suggests links between sea level changes and major biological innovations, such as the appearance of shelled organisms and multicellular life.
Isotopic analysis of marine sediments provides additional support for correlation between geological events and biological changes.

The supercontinent cycle provides a framework for understanding Earth's tectonic history and the evolution of life, driven by cyclic geological processes.