Australia: The Land Where Time Began

A biography of the Australian continent 

The Earth – A Planetary System that Has Been Evolving

When the Earth is considered as a planetary system its components are the crust, mantle, core, atmosphere, hydrosphere and biosphere, all of which behave as a system within a system, i.e. a subsystem. Many of these components have been interacting with each other on various time scales as the Earth evolved from its formation 4.6 Ga to the present.

A set of variables characterise the state of the Earth system, and also its subsystems, as the Earth system evolved. Temperature and pressure, as well as various compositional variables, are the most important. If a system is at equilibrium there are no changes as it evolves. But if 1 or more variables perturb the system in this case the Earth responds by reaching a new equilibrium state.

Feedback loop – This is change and response of a system to change that is self-perpetuation.

Positive feedback loop – a response by the system that amplifies the change.

E.g. if more CO₂ is released into the atmosphere, that already has a high concentration of CO_2, by volcanic eruptions, greenhouse warming should be promoted which should cause the temperature to rise.

Negative feedback loop – a response by the system that reverses or diminishes the change.

E.g. volcanism is a positive feedback when it adds more CO₂ to the atmosphere when the atmosphere already has high levels of CO₂, which should promote greenhouse warming that results in the temperature of the Earth rising. If CO₂ is drained from the atmosphere by increased rates of weathering on the continents resulting from the temperature rise it is an example if negative feedback. In the Earth a single subsystem affects many other subsystems, therefore many positive and negative feedbacks occur as the Earth attempts to reach a new state of equilibrium. These feedbacks may act over the short term or long term. The short-term changes can range from hours to 10s of thousands of years, such as short-term climate changes, or if long-term over 10s to hundreds of millions of years, such as climate changes related to the breakup and dispersal of a supercontinent.

The thermal history of a planet is the major driving force of that planet. Many aspects of planetary evolution control the methods and rates at which a planet cools, either directly or indirectly. Thermal history determines timing of core formation, and whether a core forms, in silicate-metal planets such as the Earth. It determines if there is a molten core, which in turn determines if the planet will have a global magnetic field, which is generated by dynamo action in the outer core. This magnetic field, which in turn interacts with the solar wind and cosmic rays, traps high-energy particles in magnetic belts that surround the planet. As life cannot exist in the presence of intense solar wind or cosmic radiation, the evolution of life on a planet is affected by the protection that is provided from high-energy particles from the Sun and cosmic rays.

Tectonic, crustal and magmatic history is also influenced strongly by planetary thermal history. Only planets that recycle lithosphere into the mantle, such as the Earth, appear to be capable of generating continental crust, collisional orogens, and supercontinents. Calc alkaline magmas that are widespread are typically produced at subduction zones. Planets that cool by mantle plumes and delamination of lithosphere in a stagnant lid tectonic regime, possibly as Venus does at the present, have mafic magmas that are widespread, with little component of felsic to intermediate, and they may or may not have continents.

Climate is a reflection of complex interactions of the ocean/atmosphere subsystem with tectonic and magmatic components, as well as interactions with the biosphere. Also, solar energy and impacts of comets or asteroids can severely affect the evolution of the climate. All the subsystems of a planet are affected, directly or indirectly, by the thermal history of the planet. There are 2 types of energy sources, internal and external, involved with the energy supply of the Earth. Generally the effects on the planetary evolution of internal energy sources are over the long term, more than 10 million years, while external sources act over the short term, much less than 10 million years.

Sources & Further reading

1. Condie, Kent C., 2016, Earth as an Evolving Planetary System 3^rd Edition, Elsevier