![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
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 CO2, 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.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |