Introduction | A LONG LONG LONG time ago... | A LONG LONG time ago...
A LONG time ago... | Not so long ago...

A LONG LONG time ago...

As much as 540 million years ago, the rock cycle and the water cycle helped control the amount of carbon dioxide in the atmosphere. Carbon dioxide from Earth’s interior spewed into the atmosphere through volcanoes. The rocks of newly formed mountains were broken apart by weathering, which removed carbon dioxide from the atmosphere. The sediments and dissolved carbon dioxide traveled down rivers and washed into oceans, where marine animals used it to make shells and skeletons.

About 300 million years ago, swamps formed along shallow coasts. They looked somewhat like swamps today, with murky waters, plants, and insects. But the swamps that formed back then were full of horsetails 50 feet high and ancient trees with strange, scaly bark. Dragonflies the size of model airplanes and cockroaches larger than Frisbees made the swamp their home. Over millions of years, some of the swamps were buried and became coal.

Dinosaurs and many other animals and plants disappeared around 65 million years ago. They all became extinct at about the same time. Scientists call that a mass extinction. What caused the mass extinction? There is evidence that a giant asteroid hit the Earth at that time and there is also evidence of huge volcanic eruptions. Both events may have filled the atmosphere with large amounts of dust and ash, causing acid rain, and hiding the Sun for many years. This climate cooled, plants died, and animals like dinosaurs were left with nothing to eat.