This is a story about our relationship with the planet Earth. As humans, we have only been here a short time compared to how old the Earth is. In that time, science has taught us a lot about the planet itself.
Through that knowledge, we can now look at the Earth in a different way. Originally, we did not care very much if we dumped garbage in the ocean. We did not care if we caught too many fish from the sea, or killed so many animals of one kind that the whole species disappeared.
Gradually, we have learned some lessons about how to live on the planet without hurting it. We know now, for instance, that to be a good steward of the planet, to be responsible for the rich resources the Earth has, we need to treat it far more carefully than we have in the past.
Science gives us tremendous power over our surroundings. The information in this book will help us use it for the health of every living thing—including the planet Earth, itself.
The Earth has many stories to tell because it is always changing. Some of these stories go back over four billion years, when the planet was being formed. One of the stories has to do with the continent known as Pangæa. The Earth has not always looked the way it does now, with all of its beautifully shaped continents.
Pangæa existed over 200 million years ago. It was a supercontinent that was made up of all the continents. Pangæa was like a complete puzzle put together; it had North America, South America, Africa and all of the other continents.
Approximately 180 million years ago Pangæa began to change. The continents we know today, the pieces of the puzzle that formed Pangæa, began to break apart. Volcanic action and lava flow along what is now called the mid-ocean ridge separated the pieces of Pangæa. The oceans filled in between the new continents and the separation was complete. The ocean floor and continents moved apart at a rate of 1-5 inches per year and the spreading continues today.
The ocean floor is formed along the mid-ocean ridge and slowly moves towards the edges of the continent. It takes over a 100 million years for the oldest portion of the sea floor to move from the mid-ocean ridge to the edge of the continent. The old sea floor slowly glides down into the mantle when it comes in contact with a plate boundary or an ocean trench (ocean trenches form the deepest areas of the ocean).
The sea floor and continents move because the Earth’s crust is made up of a series of plates that float above its molten-fluid mantle. The plates move because of volcanic activity along the mid- ocean ridge, deep beneath the ocean’s surface. The mid-ocean ridge consists of a series of active volcanoes that push up molten lava from the Earth’s mantle. As the lava cools, it forms a type of rock known as basalt, which is the bedrock of the new ocean floor.
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