Rocks on Earth go back 3800 million years the oldest are in Greenland, followed by ones in Australia, Canada and South Africa. Most present day continents formed 3000 to 2500 million years before and have broken apart and regrouped. Most great mountain ranges were formed by collisions of continents, and occurred in phases. The first was the Caledonian (460 million years old) as Europe collided with North America; next was the Appalachian uplift in the eastern USA; about 300 million years ago the collision between Europe, America and Gondwanaland saw the Hercynian phase; and in the last ten million years the Alps were formed by the collision of the Eurasian and African continental plates, and were then deformed by faulting and thrusting.
From their rise 235 million years ago, dinosaurs dominated the land. More than 800 species of dinosaurs have been identified. The biggest, Brachiosaurus, grew to the height of 28 m and may have weighed as much as 100 tones. There were two main groups of dinosaur: the Saurischian, or lizard-hipped, and the ornithischian, or bird hipped. Though the dinosaurs are long gone, their descendants, the birds, are still abundant.
Geological history is peppered with catastrophes that may have been the reason why many species suddenly became extinct. One such species was the dinosaur which disappeared 65 million years ago. The most popular theory is that a large asteroid collided with the Earth, throwing dust and steam onto the air, blotting out the sun and changing the climate. A large impact crater off the Yucatan Peninsula in Mexico is often cited as possible evidence of this.