-Introducing our Planet-
(pages 10-11)

Earth's Matter

Our planet has been circling the sun for more than four billion years. During those billions of years, the matter on our planet keeps changing its form. Water evaporates from the ocean, goes into the clouds and falls as snow and rain. Rocks get broken down into dirt that is washed as sediment into rivers. Plants take carbon dioxide gas from the atmosphere and convert it into solid sugars and starches. Why doesn't all the ocean water turn into mountain snow, or all the rocks turn into sediment or all the atmospheric carbon dioxide turn into sugar?

Earth still has oceans, mountains and atmospheric carbon dioxide because they are part of cycles - the water cycle, the rock cycle and the carbon cycle. Water flows in rivers back to the oceans; buried sediments reach the surface again through volcanoes; and animals chemically change sugars into carbon dioxide that goes back into the atmosphere.

Earth is a recycling planet. Essentially all the matter on Earth has been here since the planet was formed. We don't get new matter; old matter does not go away into outer space. The same matter keeps getting used over and over again. From a systems point of view, we say that Earth is essentially a closed system with respect to matter.

*	Of course, Earth is not a totally closed system with respect to matter. For example, we are constantly bombarded by meteorites. Yet, the total amount of matter that has entered the Earth system during the past three billion years is less than 0.000001% of Earth's total mass.