The Outer Zone of the Earth

The Earth has four spheres (or subsystems) which make up the outer structure of the Earth, these are;

1. Atmosphere
2. Hydrosphere
3. Lithosphere
4. Biosphere

Now, we will discuss these four spheres (or subsystems) in more depth;

1. The Atmosphere:

The Earth is enveloped by the atmosphere. The Atmosphere is a mixture of gases with different characteristics and proportions. Most of the atmosphere exists close to the Earth’s surface with 99% of the atmosphere lying within 30 km from the Earth’s surface. The atmosphere is not just oxygen, it also contains water vapour and other gases. The densityDensity is the measurement of how tightly a material is packed together i.e. how closely the particles are packed in the material. The tighter the material is packed the more its... More of the atmosphere decreases as one travels farther away from the Earth’s surface. The atmosphere in its optimum state is made up of the following;

• Oxygen 21%
• Nitrogen 78%
• Carbon dioxide 0.03%
• Argon 0.93%
• Helium and other gases 0.01%

The nitrogen in the atmosphere serves many purposes; it dilutes oxygen and is very important to living things. Plants absorb carbon dioxide and produce oxygen, whereas, animals take in oxygen and release carbon dioxide.

The atmosphere contains water vapour and dust particles. Water bodies provide water vapour in the atmosphere, and the amount of water vapour in different locations varies. Particles of water vapour combine to form clouds and subsequently rain. Volcanic activities, wind actions, smoke from fires, construction, factories, and other forms of pollution supply dust particles to the atmosphere. 

The atmosphere consists of four main zones; the troposphere, the stratosphere, the mesosphere and the thermosphere (or ionosphere). 

Let’s discuss these four main atmospheric zones briefly:

i. The Troposphere: this is the lowest layer of Earth’s atmosphere. It is 10 km high and in some cases 20 km. Most of the mass (about 75-80%) of the atmosphere is in the troposphere. Most clouds are found in the troposphere, and almost all weather occurs within this layer.

ii. The Stratosphere: this is the upper portion of the atmosphere above the troposphere where temperature changes little and clouds rarely form. It extends to about 50 km above the Earth’s surface.

iii. The Mesosphere: this is the layer of the atmosphere above the stratosphere and below the thermosphere. It is the highest layer of the atmosphere in which the gases are all mixed up rather than being layered by their mass. This layer is between 50 km to 80 km in altitude.

iv. The Thermosphere (or Ionosphere): this is the layer of the earth’s atmosphere which contains a high concentration of ions and free electrons. Due to the ions and electrons, this layer can reflect radio waves. It lies above the mesosphere and extends from about 80 to 1,000 km above the Earth’s surface.

See the diagram below for clarity

Please Note: When you examine the graph above, you will observe that different terms are used like Tropopause, Stratopause, Mesopause and Thermopause. Do not be confused, they are used to describe the upward limit of each of the spheres. The Troposphere has Tropopause, the Stratosphere has Stratopause, the Mesosphere has Mesopause and the Thermosphere or Ionosphere has either Thermopause or Ionopause.

2. The Hydrosphere:

The surface of the Earth is 70% water bodies and 30% landmass. The Hydrosphere is the liquid portion of the Earth, it consists of all water bodies from oceans, seas, rivers, streams, lakes, lagoons etc. The different Oceans make up 97% of the Hydrosphere. It also consists of water in various forms on the Earth’s surface, that is, ice and even the gaseous state in water vapour.

Water from various water bodies evaporates to form clouds and eventually rain. Water from the rivers, streams, and even rainfall contribute to the water in seas and oceans.

3. The Lithosphere:

This is the rigid, outermost layer of the Earth’s rocks and minerals, it consists of the crust and upper mantle. It is the solid portion of the Earth and consists of rocks and minerals. The lithosphere is the Earth’s land mass, it is 10 km to 50 km thick and makes up 30% of the Earth’s surface. The lithosphere forms the basis of all human settlements, farming activities, mining, trading and so on. All mineral resources are also derived from the lithosphere. All forms of transportation are aided by the lithosphere from roads to railways to even airports.

4. The Biosphere:

This portion of the Earth refers to various forms of living organisms, it includes all living things on the Earth’s surface (lithosphere) and in the atmosphere and hydrosphere. These living things include animals, man, plants and minute organisms such as fungi and bacteria. These organisms obtain their energyEnergy is the ability to do work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy. Units of Energy: The SI unit... More from the sun. They interact and depend on one another for their existence. Based on their mode of feeding; some animals are plant eaters known as herbivores, some animals eat other animals these are carnivores, while others are omnivores because they eat both plants and animals.

When living organisms die, they are decomposed by bacteria and fungi. As they decompose, locked-up minerals are released into the soil. These minerals are then absorbed and used by plants. Decomposed animals can also be used as manure, this is beneficial for plants and crops. Plants are referred to as producers, animals which eat up the food produced by plants are called consumers and the bacteria and fungi that break down plants and animals when they die are called decomposers.

The diagram below depicts the interactions between the lithosphere, atmosphere and hydrosphere which gives rise to the existence of the biosphere.