The Structure of the Earth
Senior 1 · Geography
The Structure of the Earth
Lesson 1 of 3
20 min
Look at the photograph of a fishing beach on Lake Kivu. Find (1) something solid, (2) something liquid, (3) something that is a gas, and (4) something alive. You have just found the four "spheres" into which geographers divide the outside of the Earth.
The Earth's surface zone is made of four parts that surround or cover the planet like layers, which is why each is called a sphere (a ball-shaped layer).
Figure 1. The four spheres of the Earth meet in every landscape.
Definition
Lithosphere: the solid, rocky outer layer of the Earth: the crust together with the top of the mantle, about 100 km thick. It includes the rocks, minerals and soils of the continents and the ocean floor, and it is broken into large pieces called plates. Example: the granite hills of central Rwanda and the volcanic rock of the Virungas.
Definition
Hydrosphere: all the water on, under and above the Earth's surface: oceans, lakes, rivers, swamps, ice, groundwater and water vapour. Example: Lake Kivu, the Nyabarongo River and the Rugezi wetland.
Definition
Atmosphere: the layer of air, a mixture of gases, that surrounds the Earth and is held by gravity. It gives us oxygen to breathe, protects us from harmful rays from the Sun and holds the weather. Example: the clouds that bring the long rains between March and May.
Definition
Biosphere: the zone of the Earth where living things are found, together with all the plants, animals and people that live there. It lies where the lithosphere, hydrosphere and atmosphere meet. Example: the mountain gorillas and bamboo forest of Volcanoes National Park.
| Sphere | Made of | Key facts | In Rwanda |
|---|---|---|---|
| Atmosphere | Nitrogen 78%, oxygen 21%, argon 0.9%, carbon dioxide about 0.04%, plus water vapour and dust | Weather happens in the lowest layer, the troposphere, which is about 16 km thick over the Equator. Above it, the stratosphere holds the ozone layer, which absorbs harmful ultraviolet rays. | Air is thinner on high ground: on Karisimbi there is noticeably less oxygen in each breath than in Kigali. |
| Hydrosphere | Salt water, fresh water, ice and water vapour | Water covers about 71% of the Earth's surface. About 97% of all water is salty ocean; of the 3% that is fresh, most is frozen in ice sheets or lies underground. | About 6% of Rwanda's area is water. About 80% of the country drains to the Nile and 20% to the Congo. |
| Lithosphere | Rocks, minerals and soil | Contains the landforms (mountains, plateaus, valleys) and the minerals we mine. | Tin, tantalum (coltan) and tungsten ores are mined; volcanic soils in the north-west are very fertile. |
| Biosphere | Plants, animals, fungi, micro-organisms and people | A thin zone, from a few kilometres into the air to the deepest ocean floor. | Montane rainforest in Nyungwe, savanna in Akagera, farms on nearly every hill. |
Worked example: how much of the Earth is water?
The Earth's surface has an area of about 510 million km², and water covers about 71% of it. Find the area of water and of land.
Check. Land should be of the surface: million km². Both methods agree. So there is about two and a half times as much water surface as land surface.
No sphere works alone. Rain falls from the atmosphere into the hydrosphere; it soaks into the soil of the lithosphere, where plant roots of the biosphere take it up; the plants breathe it back out into the air. On a steep, bare hillside the same rain washes soil (lithosphere) into a river (hydrosphere), and fish and farmers (biosphere) both suffer. A change in one sphere always affects the others.
Important
Case study: Lake Kivu, where all four spheres meet
Rwanda now pumps methane from the deep water and burns it to make electricity, which also slowly lowers the amount of gas stored in the lake.
What is Physical Geography? Crash Course Geography #4, by CrashCourse (11 min). Watch for how the presenter describes the Earth as a system of spheres that exchange energy and matter.
Which sphere do people in Rwanda change the most, and how? Give one example of a harmful change and one example of a helpful change.
Tip
Greek helps: litho = stone, hydro = water, atmo = vapour (air), bio = life. Stone, water, air, life.
Practice
Remember
Sources and credits
The lesson text, diagram, table, worked example and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
