Isaac Newton was one of the first scientists to theorize about the structure of Earth. Based on his studies of the force of gravity, Newton calculated the average density of Earth and found it to be more than twice the density of the rocks near the surface. From these results, Newton realized that the interior of Earth had to be much denser than the surface rocks. His findings excluded the possibility of a cavernous, fiery underworld inhabited by the dead, but still left many questions unanswered.

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 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).

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:

  1. Troposphere (0 km – 13 km)
  2. Ozone Layer (13 km – 25 km)
  3. Stratosphere (25 km – 50 km)
  4. Mesosphere (50 km – 75 km)
  5. Thermosphere (75 km – 150 km)


  • Solid Earth: measured from the surface of the Earth downwards to the core

The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust is made up of tectonic plates, which are in constant motion.

1. Inner Core

  • Temperature: 5,000°C – 6,000°C
  • State: Solid
  • Composition: iron and nickel

Earth’s Core is thought to be composed mainly of an iron and nickel alloy. This composition is assumed based upon calculations of its density and upon the fact that many meteorites (which are thought to be portions of the interior of a planetary body) are iron-nickel alloys. The core is earth’s source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances.

The Earth’s inner core is a huge metal ball, 2,500km wide and the temperature of the ball is 5,000°C to 6,000°C – that’s up to 6,000 times hotter than our atmosphere and scorching enough to make metal melt! The metal at the inner core stays solid because of the incredible pressure surrounding it.


2. Outer Core

Temperature: 4,000°C – 6,000°C

State: Liquid

Composition: iron, nickel, sulphur and oxygen

This liquid layer of iron and nickel is 5,150km deep. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy. The outer core flows around the centre of the Earth, and the movement of the metals creates our planet’s magnetic field(As the Earth rotates, the liquid outer core spins, creating the Earth’s magnetic field).


3a. Lower Mantle

  • Temperature: 3,000°C
  • State: solid
  • Composition: iron, oxygen, silicon, magnesium and aluminium

The lower mantle is found between 670km and 2,890km below the surface, and is made from solid rock. The rock is hot enough to melt, but is solid because of the pressure pushing down on it.

Earth’s mantle is thought to be composed mainly of olivine-rich rock. It has different temperatures at different depths. The temperature is lowest immediately beneath the crust and increases with depth. The highest temperatures occur where the mantle material is in contact with the heat-producing core. This steady increase of temperature with depth is known as the geothermal gradient. The geothermal gradient is responsible for different rock behaviors, and the different rock behaviors are used to divide the mantle into two different zones. rocks in the lower mantle are hot and soft (but not molten).


b. Upper Mantle

  • Temperature: 1,400°C – 3,000°C
  • State: liquid / solid
  • Composition: iron, oxygen, silicon, magnesium and aluminium

This layer is up to 670km below the Earth’s surface. The lower part of the upper mantle is made from both solid and melted rock (liquid), while the rock in the upper region is stiffer, because it’s cooler.  The mantle is thought to be made up mostly of peridotite, a type of rock composed of iron, magnesium, silicon, and oxygen. Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes.

The mantle contains 83% of the total volume and 68% of the total mass of the earth.  The mantle is separated from the core by Gutenberg discontinuity. It is largely composed of silicon and magnesium called sima. The rocks in this layer may be in the glassy state.

Rocks in the upper mantle are cool and brittle, while However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking.

The lower limit of brittle behavior is the boundary between the upper and lower mantle.


  • Asthenosphere – the layer or shell below the lithosphere, about 2000 km thick. It is composed mostly of Ultramafic rocks (or see Igneous Primer) such as peridotite and dunite that are weak and plastic, and flow slowly under stress. The asthenosphere (from the Greek, ἀσθενής, asthenēs, devoid of force) is the part of the mantle that flows (called plastic behavior). This can be compared to the movement of toothpaste in a tube.

The sharp boundary between the crust and mantle is called the Mohorovičić discontinuity or Moho. While the Gutenberg discontinuity separates the mantle from the core.


4. Crust

Temperature: Around 22°C

State: Solid

Composition: Oceanic crust made up of iron, oxygen, silicon, magnesium and aluminium. Continental crust made up of granite, sedimentary rocks and metamorphic rocks.

The outermost layer of  the geosphere is called the crust. The Earth’s surface is covered by its thinnest layer, the crust. Land is made of continental crust, which is 8km to 70km thick and made mostly from a rock called granite. The layer beneath the ocean bed is made of oceanic crust, which is about 8km thick and made mainly from a rock called basalt.

Lithosphere –  The lithosphere (from the Greek, λίθος, lithos, stone) is the rigid outermost layer of the geosphere. The upper layer of the lithosphere is the crust.

There are actually two types of crust, continental and oceanic.

  • Continental crust is made up of volcanic lava flows, huge granite blocks, and sediments laid down in shallow water or continental seas. It is quite thick, averaging 30 to 40 km and beneath parts of mountain ranges reaching 80 km. But it is made of lower density rocks, such as andesite and granite.
  • Oceanic crust is thin, only about 5 to 10 km deep, and made of relatively dense rock called basalt, possibly underlain by gabbro (a similar but coarser grained rock). It also contains a greater proportion of magnesium. The composition of the lower crust is not known, but it is probably gabbro. Seismic waves pass through it at a more rapid rate than they do the upper crust.

Scientists would say you are living on the sial. The sial is the part of the crust that is above water. It’s a continental plate floating over the globe. Right underneath the sial is the sima. The sima is the layer of the Earth’s crust that covers the entire planet. The sial is different in that it can begin and end where the plates do. You can think of the sima as the ocean floors. Under the sima is the mantle.

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.

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