Structure and composition of the Earth

What makes Earth unique?

Soon after the Earth formed, unique processes occurred - division into metallic core, silicate mantle and crust - which, along with surface water, made it different from the other planets in our Solar System. The formation of the early mantle was important as it consisted primarily of ferromagnesium silicate minerals, some of which contained water as an essential component (e.g. amphibole group minerals). Water-bearing magmas (molten rock) from deep in the lower mantle then rose towards the surface (being liquid, they were lighter than the surrounding solid rock) and emerged as volcanic eruptions.

The Earth's atmosphere and hydrosphere developed from the degassing (loss of gaseous elements such as carbon, hydrogen and oxygen) of the early-formed core and mantle during this volcanic activity. In the present, abundant gases are still released from the Earth during volcanic eruptions and these are mainly composed of water (77%), carbon dioxide (12%), sulfur dioxide (7%), and nitrogen (3%), with minor amounts of hydrogen, carbon monoxide, sulfur, chlorine and argon.

The Earth can be divided into two main parts.

• atmosphere: measured from the surface of the Earth upwards to 150 km (anything above this is called space)
• solid Earth: measured from the surface of the Earth downwards to the core

The atmosphere

The atmosphere makes up less than one millionth of the total mass of the Earth, and contains mainly nitrogen and oxygen (99% of the total) as gases. Other important components of the atmosphere are hydrogen, carbon dioxide, and inert gases such as argon and helium.

The Earth's atmosphere and climate have changed since the Earth first formed more than four billion years ago. We know this from the geological record of the earliest known sedimentary rocks which could only have been deposited in water. Water boils above 100° C under normal atmospheric conditions. The presence of these early sedimentary rocks indicates that by four billion years ago, the surface temperature of the Earth must have cooled below 100° C. Also, some of these early sedimentary rocks are carbonate rocks (mainly dolomitic), indicating these rocks were formed in water that contained dissolved bicarbonate ions. The early atmosphere of the Earth must have contained more carbon dioxide than today's atmosphere. Currently, the early atmosphere is understood to have consisted of nitrogen, carbon dioxide, carbon monoxide, water and hydrogen.

The atmosphere is divided (measured from the surface of the Earth) into:

• Troposphere (0 km - 13 km)
• Ozone Layer (13 km - 25 km)
• Stratosphere (25 km - 50 km)
• Mesosphere (50 km - 75 km)
• Thermosphere (75 km - 150 km)

The solid Earth

This can be divided into the:

• Biosphere (water, organic substances and skeletal matter) - solid and liquid - and includes all forms of life (e.g. plants and animals) and their products (e.g. skeletons) both on land and in the sea
• Hydrosphere (fresh and salt water, snow and ice) - mainly liquid, some solid - includes all forms of water
• Internal structure of the Earth, which includes:
  • Crust (normal silicate rocks such as granite and basalt) - solid
  • Mantle (ferromagnesium-rich silicate rocks) - solid
  • Core (iron-nickel alloy) - liquid upper part and solid lower part

Structure of the Earth

Although geologists generally don't study the atmosphere, biosphere or hydrosphere, all three are vitally important in understanding geological processes, particularly weathering and erosion, and the formation of many sedimentary rocks (e.g. formation of coral reefs in shallow warm seas).