The Formation of The Earth - Part 2.

Core, Mantle and Crust.

The Earth has three distinct layers: a core, consisting of a solid inner core and a fluid outer core, which together make up more than half the planet's diameter; the mantle, a layer of dense minerals making up most of the rest of the Earth's diameter; and a thin crust composed of rocks and minerals chemically distinct from those of the core and mantle.

Mantle and Crust Minerals.

The intensity of the heat in the interior of the forming Earth encouraged chemical reactions between the iron-rich core and the surrounding material. Most of the elements that combine readily with iron, formed denser minerals which accumulated in a thick shell around the core. This created what is known as the lower mantle.

Lighter materials were formed from elements with a greater affinity for oxygen than for iron. These combined as oxide compounds, predominately silicates. Being lighter, these compounds rose upwards to form the upper mantle and the crust, in effect, floating on the lower mantle.

The Oxygen factor.

Although the core of the Earth is denser than the mantle and crust, the distribution of the various chemical elements between the layers of the Earth's interior was not necessarily determined by the density of the actual elements themselves. The tendency of various elements to interact either with iron or with oxygen could have been the principle determining factor. For example, uranium and thorium are very dense heavy elements, yet they are concentrated in the crust, possibly because they both have a high affinity to oxygen. The importance of the role played by oxygen in the Earth's crust is clearly illustrated by the fact that it makes up 50 percent of the crust by weight.

The Atmosphere and The Oceans.

The formation of the atmosphere and the oceans was directly connected to the formation of the crust. During the period of intense accretion, as the Earth melted and recrystallized, water vapour and other gases were released to add to the gases from the Solar Nebula accumulated during the accretion process. This constituted the Earth's first atmosphere. Then, 5.8 billion years ago, the Earth and moon were subjected to intense meteorite bombardment which is thought to have stripped away the first atmosphere.

Subsequent volcanic activity released nitrogen, carbon dioxide and water vapour, which in turn were broken down by ultraviolet light into hydrogen, oxygen and ozone. The water vapour collected to form oceans, where salts dissolved, increasing salinity. A second atmosphere formed, at first with little oxygen. As life developed in the oceans, oxygen was released creating today's atmosphere, which extends upwards for several hundred kilometres.

Dating The Earth.

One of the earliest attempts to calculate the age of the Earth was made in the 17th Century, when Anglo-Irish Bishop Ussher, working from Biblical genealogy , estimated the Earth and Universe were created in October, 400 BC.

The first accurate estimate was obtained in 1956 by physicist Claire Patterson (male) who compared radioisotope measurements from meteorites and Earth minerals to give an age of 4,550 million years, (more recently modified to 4,560 million years).

The oldest directly measured date is for zircon crystals taken from Mount Narryer, Western Australia, dating their formation to 4,400 to 3,900 million years ago.( Source DK Publishing: Rocks and minerals. Picture courtesy of Google images).

So Guys, a more complete picture is beginning to build up, not just of

the Earth, but how you came to own your beloved Marble Eggs!

Next week, we'll be looking at the the Earth's crust in greater detail, so stay tuned!