14.1 Introduction to the Oceans

Lesson Objectives

• Describe how the oceans formed.
• State how the oceans influence Earth.
• Describe the makeup of ocean water.
• Identify ocean zones.

Vocabulary

• aphotic zone
• benthic zone
• intertidal zone
• neritic zone
• oceanic zone
• photic zone

Introduction

Much of Earth’s surface is covered with oceans. That’s why Earth is called the “water planet.” Without all that water, Earth would be a very different place. The oceans affect Earth’s atmosphere and influence its climate. An incredible diversity of living things inhabit the ocean as well. You might think that oceans have always covered Earth’s surface, but you would be wrong!

How the Oceans Formed

When Earth formed 4.6 billion years ago, it would not have been called the “water planet.” There were no oceans then. In fact, there was no liquid water at all. Early Earth was too hot for liquid water to exist. Earth's early years were spent as molten rock and metal.

Water on Early Earth

Over time, Earth cooled. The surface hardened to become solid rock. Volcanic eruptions, like the one in Figure below, brought lava and gases to the surface. One of the gases was water vapor. More water vapor came from asteroids and comets that crashed into Earth. As Earth cooled still more, the water vapor condensed to make Earth’s first liquid water. At last, the oceans could start to form.

Volcanoes were one source of water vapor on ancient Earth. What were other sources?

Ancient Oceans

Earth’s crust consists of many tectonic plates that move over time. Due to plate tectonics, the continents changed their shapes and positions during Earth history. As the continents changed, so did the oceans. About 250 million years ago, there was one huge land mass known as Pangaea. There was also one huge ocean called Panthalassa. You can see it in Figure below.

At the time shown, there was one vast ocean and two smaller ones. How many oceans are there today?

By 180 million years ago, Pangaea began to break up. The continents started to drift apart. They slowly moved to where they are today. The movement of the continents caused Panthalassa to break into smaller oceans. These oceans are now known as the Pacific, Atlantic, Indian, and Arctic Oceans. The waters of all the oceans are connected. That’s why some people refer to the oceans together as the “World Ocean.”

The Oceans’ Influence

Oceans cover more than 70 percent of Earth's surface and hold 97 percent of its surface water. It’s no surprise that the oceans have a big influence on the planet. The oceans affect the atmosphere, climate, and living things.

Oceans and the Atmosphere

Oceans are the major source of water vapor in the atmosphere. Sunlight heats water near the sea surface, as shown in Figure below. As the water warms, some of it evaporates. The water vapor rises into the air, where it may form clouds and precipitation. Precipitation provides the freshwater needed by plants and other living things.

The oceans and atmosphere exchange gases. Why does water vapor enter the atmosphere from the water?

Ocean water also absorbs gases from the atmosphere. The most important are oxygen and carbon dioxide. Oxygen is needed by living things in the oceans. Much of the carbon dioxide sinks to the bottom of the seas. Carbon dioxide is a major cause of global warming. By absorbing carbon dioxide, the oceans help control global warming.

Oceans and Climate

Coastal areas have a milder climate than inland areas. They are warmer in the winter and cooler in the summer. That’s because land near an ocean is influenced by the temperature of the oceans. The temperature of ocean water is moderate and stable. Why? There are two major reasons:

1. Water is much slower to warm up and cool down than land. As a result, oceans never get as hot or as cold as land.
2. Water flows through all the world’s oceans. Warm water from the equator mixes with cold water from the poles. The mixing of warm and cold water makes the water temperature moderate.

Even inland temperatures are milder because of oceans. Without oceans, there would be much bigger temperature swings all over Earth. Temperatures might plunge hundreds of degrees below freezing in the winter. In the summer, lakes and seas might boil! Life as we know it could not exist on Earth without the oceans.

Oceans and Living Things

The oceans provide a home to many living things. In fact, a greater number of organisms lives in the oceans than on land. Coral reefs, like the one in Figure below, have more diversity of life forms than almost anywhere else on Earth.

Coral reefs teem with life.

Makeup of Ocean Water

You know that ocean water is salty. But do you know why? How salty is it?

Why Is Ocean Water Salty?

Ocean water is salty because water dissolves minerals out of rocks. This happens whenever water flows over or through rocks. Much of this water and its minerals flow in rivers that end up in the oceans. Minerals dissolved in water form salts. When the water evaporates, it leaves the salts behind. As a result, ocean water is much saltier than other water on Earth.

How Salty Is Ocean Water?

Have you ever gone swimming in the ocean? If you have, then you probably tasted the salts in the water. By mass, salts make up about 3.5 percent of ocean water. Figure below shows the most common minerals in ocean water. The main components are sodium and chloride. Together they form the salt known as sodium chloride. You may know the compound as table salt or the mineral halite.

What percentage of the salts in ocean water is sodium chloride?

The amount of salts in ocean water varies from place to place. For example, near the mouth of a river, ocean water may be less salty. That’s because river water contains less salt than ocean water. Where the ocean is warm, the water may be more salty. Can you explain why? (Hint: More water evaporates when the water is warm.)

Ocean Zones

In addition to the amount of salts, other conditions in ocean water vary from place to place. One is the amount of nutrients in the water. Another is the amount of sunlight that reaches the water. These conditions depend mainly on two factors: distance from shore and depth of water. Oceans are divided into zones based on these two factors. The ocean floor makes up another zone. Figure below shows all the ocean zones.

Distance from shore and depth of water define ocean zones. Which zone is on the ocean floor?

Zones Based on Distance from Shore

There are three main ocean zones based on distance from shore. They are the intertidal zone, neritic zone, and oceanic zone. Distance from shore influences how many nutrients are in the water. Why? Most nutrients are washed into ocean water from land. Therefore, water closer to shore tends to have more nutrients. Living things need nutrients. So distance from shore also influences how many organisms live in the water.

Zones Based on Depth of Water

Two main zones based on depth of water are the photic zone and aphotic zone. The photic zone is the top 200 meters of water. The aphotic zone is water deeper than 200 meters. The deeper you go, the darker the water gets. That’s because sunlight cannot penetrate very far under water. Sunlight is needed for photosynthesis. So the depth of water determines whether photosynthesis is possible. There is enough sunlight for photosynthesis only in the photic zone.

Water also gets colder as you go deeper. The weight of the water pressing down from above increases as well. At great depths, life becomes very difficult. The pressure is so great that only specially adapted creatures can live there.

Lesson Summary

• Early Earth was too hot for liquid water to exist. Eventually Earth cooled. Water vapor from volcanoes and objects in space condensed. Oceans finally formed. The oceans changed size and shape as continents drifted.
• Oceans have a big influence on Earth. They exchange gases with the atmosphere. They prevent very hot and very cold temperatures. They are home to many living things.
• Dissolved mineral salts wash into the ocean. As ocean water evaporates, it leaves the salts behind. This makes the water saltier. Ocean water is about 3.5 percent salts. The main salt is sodium chloride.
• The ocean is divided into many zones. Some are based on distance from shore. Some are based on depth of water. The ocean floor is another zone.

Lesson Review Questions

Recall

1. State why there was no liquid water on ancient Earth.

2. Describe how the oceans influence Earth’s atmosphere.

3. What is the makeup of ocean water?

4. Describe how ocean water changes as you go deeper in the water.

5. What is the benthic zone?

6. Define the intertidal zone.

Apply Concepts

7. Look at the map (Figure below) of Washington State. Washington is on the Pacific coast. Find Raymond and Pullman on the map. Apply lesson concepts to predict how their temperatures compare. Explain your predictions.

Map of Washington State.

8. Describe the causes of high and low tides on Earth.

Think Critically

9. Compare and contrast the photic and aphotic zones.

10. Imagine that you are going down to the bottom of the ocean in a tiny submarine. What might you see as you go down? Where will it be light? Where will you see the most life forms?

11. Relate ocean zones to nutrients and sunlight in ocean water.

Points to Consider

Most nutrients enter ocean water from the land. However, they may be carried far from shore by currents.

• Many large ocean currents have names. Can you name any ocean currents?
• Currents are like rivers flowing through the ocean. Rivers always flow downhill because of gravity. What do you think causes ocean currents to flow?