Earth is often called the blue planet because most of its surface is covered by water. About 71% of Earth's surface is water, while about 29% is land. This simple comparison helps explain why oceans play such a big role in climate, weather, and life on Earth.
Understanding the balance between land and water is one of the first big ideas in Earth science.
Water and land affect Earth in different ways because they absorb and release heat differently, support different ecosystems, and shape the movement of air and water around the planet. Oceans store large amounts of heat and help moderate temperatures, while land heats and cools more quickly. The uneven distribution of land and water also influences rainfall, ocean currents, and where people live.
Looking at Earth as a whole makes it clear that water is the dominant surface feature of our planet.
Understanding Land vs Water: Earth Basics
The surface pattern comes from Earth’s history. Early Earth had a hot outer layer that cooled into a solid crust. Water vapor released by volcanoes condensed as the planet cooled, adding to water delivered by icy objects from space.
Over very long periods, plate tectonics built continents from lighter rocks. Ocean crust is thinner, denser, and continually recycled at plate boundaries.
Continental crust is thicker and tends to remain above sea level. This difference in rock type helps create deep ocean basins beside higher continental land.
Water changes temperature slowly because a great deal of energy is needed to warm it. Some energy goes into moving water molecules rather than raising temperature. Water also mixes, carrying heat below the surface.
Land does not mix in the same way, so its surface can become hot during daylight and cool rapidly after sunset. Coastal places usually have smaller daily and yearly temperature changes than inland places at similar latitudes.
In summer, air over warm land rises and can draw moist air in from the sea. This process contributes to seasonal monsoon rainfall in several parts of the world.
The ocean is not a still cover of water. Winds push surface currents, while differences in temperature and saltiness move deeper water. Together these currents shift heat from tropical regions toward cooler regions.
The Gulf Stream system, for example, helps keep parts of western Europe milder than places at a similar latitude in eastern Canada. Ocean water evaporates into the atmosphere, then later forms clouds and precipitation.
Most rain that falls on land began as ocean water. Rivers, groundwater, glaciers, and runoff return much of that water to the sea, linking the hydrosphere with the atmosphere, land, and living things.
A map can make the land area seem larger or smaller depending on its projection. Flat world maps must stretch some parts of a spherical Earth, especially near the poles. A globe gives a more reliable sense of relative surface area.
Students should separate surface coverage from the amount of usable fresh water. Nearly all of Earth’s water is salty, and much of the fresh water is locked in ice or stored underground.
Pay attention to scale when studying maps, climate graphs, and satellite images. A small change in ocean temperature can affect evaporation, storms, fisheries, coral reefs, and rainfall far from the coast.
Key Facts
- Water covers about 71% of Earth's surface.
- Land covers about 29% of Earth's surface.
- water fraction = water area / total surface area
- land fraction = land area / total surface area
- water % + land % = 100%
- 0.71 + 0.29 = 1.00
Vocabulary
- Surface area
- Surface area is the total outside area of an object, such as the outside of Earth.
- Ocean
- An ocean is a large body of salt water that covers much of Earth's surface.
- Continent
- A continent is one of Earth's large continuous landmasses.
- Climate
- Climate is the long term pattern of temperature, precipitation, and other weather conditions in a region.
- Hydrosphere
- The hydrosphere is all of the water on Earth, including oceans, lakes, rivers, ice, and water vapor.
Common Mistakes to Avoid
- Assuming Earth is mostly land because maps show large continents, which is wrong because oceans cover a much greater total surface area than land.
- Thinking the 71% water means Earth is made mostly of water all the way through, which is wrong because the percentage refers only to surface coverage.
- Confusing fresh water with all water on Earth, which is wrong because most of Earth's surface water is salty ocean water.
- Adding land and water percentages incorrectly, which is wrong because the surface categories should total 100% when using the same whole Earth surface.
Practice Questions
- 1 If Earth's surface area is represented by 100 units, how many units are water and how many are land?
- 2 A model globe has a surface divided into 500 equal squares. About how many squares should be colored water and how many should be colored land?
- 3 Explain why Earth is called the blue planet and describe one way the large amount of water affects Earth's climate.