The Earth is unique in the solar system for a number of features.
It has an iron core and an oxygen-rich atmosphere. Its surface has been formed and reformed continuously by tectonic forces and the presence of life, which is responsible for the composition of the crust and the atmosphere.
The core is one-third of the Earth's mass, and formed in the first 100 million years. It consists of iron, which, being heavier than silicates, sank while the planet was still molten.
There has been little mixing between the core and the mantle.
The mantle is stratified and fluid, supporting the continental crust, while plate tectonics create the pattern of formation and reformation of the oceanic crust.
Studies of meteorites provide some evidence of the average composition of the solar system. This knowledge allows a calculation about how and when the mantle and crustal layers came into being.
The Earth's surface is not a static, solid rock. It is in constant movement: tectonic plates sliding over and under each other, material falling beneath the surface and re-emerging millions of years later, and geophysical cycles, rapid or lethargic cycles of biochemical elements and substances, which convey material and nutrients essential for the life and stability of the atmosphere and oceans.
Whereas core parts of continents formed more than 2.6 billion years ago, oceanic plates are in constant motion, being created and destroyed on a cycle of the order of 200 million years. It is the relative differences in movements of the plates which causes the changing shape of oceans and continents, the formation of mountains, and the occurrence of volcanoes and earthquakes.
The Earth is layered, with an iron-rich core, a mantle rich consisting of silicate minerals, and a rather thin but rigid crust. In its early phases, the planet was very hot and dynamic, with volcanoes and earthquakes shaping the landmasses, and filling the atmosphere with gas and water from the deeper rocks.
The first crust was basalt, a hard, black rock with microscopic crystals. Basalt makes the rock of volcanic islands, and the sea floor beneath the sedimentary layer.
Since granite and related rock is less dense than basalt, the continental crust it forms naturally floats to the top. It is very resilient compared to oceanic crust, and original rock from 4 billion years ago can still be found today. The majority of the cores of continents formed more than 2.6 billion years ago.
As molten rock, or magma, cools, crystals form with characteristics and composition in accordance with the prevailing conditions, mainly temperature and pressure. Feldspars are early formers, and the Earth's crust is 60% feldspar.
[Indicatively, the Moon has rocks which contain plagioclase, a form of feldspar with sodium and calcium.]
The process by which the density differences between surface and deep magma leads to different mineral formation is referred to as fractional crystallization. The families of rocks on Earth are due to fractional crystallization.
A difference in the way fractional crystallization occurs on the Earth is due to the presence of water, leading to plagioclase sinking in wet magma, rather than floating, as it did on the dry Moon.
Granite is a mix of feldspar and minerals like quartz, and is formed in subduction zones, where the ocean floor slides beneath continents.
The Earth is unique in the solar system for its type and composition of atmosphere. Whatever original atmosphere left from the Solar Nebula has largely been lost. For example, there is far less neon in the Earth's atmosphere than its average distribution through the solar system.
The original atmosphere has been replaced by gases which have originated from within the planet, mainly due to volcanic activity, and life mechanisms. Volcanoes released sulphur dioxide, carbon monoxide and hydrogen.
The surface of the early Earth would have been a hot and steamy place, made up of growing oceans with archipelagos of volcanoes spewing hydrogen compounds, such as hydrogen sulphide ($H_2S$), hydrogen chloride(HCl), and hydrogen gas, as well as $CO_2$ and $SO_2$. This process continues, but most of the volcanoes are located underwater today.
Oxygen is formed when solar energy breaks down carbon dioxide and water. Life, through photosynthesis has been largely responsible for the considerable accumulation of oxygen in the atmosphere, leading to the current 21% by volume.
Life began on Earth quite soon after the temperature had reduced enough. Around 3.5 billion years ago, prokaryotes were creating its energy from chemosynthesis, but soon very simple, monocellular algae began to use the Sun's energy via photosynthesis.
Although life has been on the Earth since at least 3.5 billion ago, it took 3 billion more years before complex life, such as plants, using photosynthesis, could begin to dominate the planet, and change the biosphere to its own liking.
Around 350 million years ago, there were forests in sufficiency to be laying down coal deposits. Despite 6 major extinction events, in which up 95% of species ceased to be, life soldiered on to arrive at our world, the Holocene, and the greatest challenge to survival of the species yet: homo sapiens and its planet-shaping skills and unchecked power.
Liquid water on the surface of the Earth came from condensing volcanic gases from the mantle. As the continental crust formed and rose above the heavier basalt, shallow seas began to form.
The algal life that developed in this aquatic environment boosted the atmospheric levels of oxygen. Carbon dioxide was absorbed by photosynthetic processes. The carbon part was retained, gradually accumulating into deposits of calcite (calcium carbonate), trapping the carbon in the hydrosphere. There was therefore a net return of oxygen to the atmosphere.
Content © Andrew Bone. All rights reserved. Created : August 19, 2015 Last updated :February 21, 2016
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