HOW OLD IS THE EARTH ANYWAYS? EXAMINING THE CONCEPT OF GEOLOGIC TIME Did you ever wonder about all of the things that have ever happened on Earth and how old it really is? As it turns out, Mother Earth was a bit reluctant to let out her true age. It took us a long time to figure it out, but as near as we can tell, Mother Earth is four thousand, six hundred million years old (4,600 million = 4.6 billion), give or take a few hundred million years. How old is that, really? It is older than anything you can see around you, like trees, buildings, or roads. It is even older than the hills, valleys, and rivers around where you live. It is so old that the world's mountains have been built up and worn down many times, the continents have wandered across the face of Earth, and plants and animals have changed many times from amoebas to dinosaurs to people. The age of Earth is so long compared to all periods of time that we humans are familiar with, it has been given a special name: Geologic Time. The age of Earth is as vast in time as the universe is vast in space. It is not easy to really get a "feel" for 4.600 billion years! One way to try to get a "feel" for how big is to break the number down into smaller pieces that we can understand. For example, you might to think about what is a million? Then try to think about 4,600 millions that makes 4.6 billion! Another way we try to understand the age of Earth is to make comparisons. Here we use a comparison to help show the span of time since the formation of Earth: "The Stairway of Time." The bottom of the Stairway represents the formation of Earth 4.6 billion years ago. The top represents the present or today. The different steps represent different segments of time in Earth's history. Something different was happening during each segment.
PRECAMBRIAN ERA The name means: "before the Cambrian period." This old, but still common term was originally used to refer to the whole period of Earth's history before the formation of the oldest rocks with recognizable fossils in them. In the last few decades, however, geologists have found that there are some very small fossils in some Precambrian rocks. This era covers almost 90% of the entire history of Earth and began about 4.6 billion years ago with the formation of Earth from dust and gas orbiting the Sun. During the early part of this era, the surface of Earth is like a popular image of hell might be with oceans of liquid rock, boiling sulfur, exploding volcanoes, and impact craters everywhere! The air is hot, thick, steamy, and full of dust and crud. But you can't breathe it anyway: it s made of nothing but carbon dioxide and water vapor! About a billion years after the formation of Earth, things changed a lot, and everything has mostly cooled down. Most water vapor in the air has cooled and condensed to form a global ocean or water world. Even most of the carbon dioxide is gone, and the sky is filled with normal clouds and rain. Volcanoes form many small islands in long chains. The islands are the only visible land because the continents have not formed yet. What about life? If you look closely, you will see evidence of blue-green algae (actually simple bacteria) floating in the ocean. That's all there is, just single-celled bacteria in the ocean! Then, 700 million years ago, we near the end of the Precambrian Era. At this point, there is much more land to be seen. In fact, there are two supercontinents, one on each side of Earth near the equator. These huge land masses are formed by collisions of the many islands made by volcanoes. Life has not changed much during the last two billion years, but the few changes are significant. Life is still found only in the ocean, but sometime around 1.7 billion years ago, single-celled creatures began to change and true multi-celled life appeared! The atmosphere is still unbreathable, but oxygen released by the algae floating in the oceans is beginning to collect in the air! By now, these single-celled plants have been producing oxygen for about two billion years. Finally, the Earth at this time is also becomes very cold, with huge, bluish glacial ice sheets visible across the supercontinent, even in the normally warm equatorial regions turning the Earth in to a giant ball of ice and wiping out almost all of the life that has developed so far!
PALEOZOIC ERA This is the first of three geologic eras squeezed into the last 10% of Earth's whole geologic history. But things are really starting to happen! The Paleozoic period lasted about 325 million years, from about 570 million years ago to about 245 million years ago. So much happened during the Paleozoic era that it is divided into seven time periods, shown on the steps of the "Staircase of Time." Here in the Paleozoic, Earth's interior has cooled down to something like modern levels, so that volcanic activity is usually about as humanity experiences it: a few minor eruptions each year, and major ones every century or so. However, gigantic "hot-spot" type eruptions still occur every hundred million years or so. Plate tectonics continues to push land masses across Earth's surface. The composition of the atmosphere has continued to slowly change, mostly due to the increase of oxygen produced by algae floating on the ocean. By the Paleozoic, the air has reached something like what we breathe now. At long last the air is capable of supporting large animals, and almost in response, life explodes into the Paleozoic! At the beginning of the Paleozoic, life existed only in or near the ocean. Trilobites, shellfish, corals, and sponges appeared, followed by the first fish. Slowly, land plants begin to appear and for the first time we see green on land in our global view. Huge forests and swamplands formed because of the warm climate. Animal life also moved onto the land, first the arthropods (spiders and insects to you), then the amphibians, and later the reptiles. The Paleozoic was also marked by several mass extinctions: geologically short periods of time during which large numbers of life forms died out. The worst mass extinction of all was called the Permian Extinction, when about 95% of all life on Earth died! This kind of mass destruction was probably caused by climate changes and periods of giant volcanic eruptions.
MESOZOIC ERA This is the second of three geologic eras squeezed into the time that covers the last 10% of Earth's whole geologic history. This is the era we all think of when we imagine the Ancient Earth! Rampaging dinosaurs! Dive-bombing pterodactyls! Endless forests of giant ferns! Erupting Volcanoes! (But sorry, no cave men they didn't show up until the end of the Cenozoic era.) The Mesozoic Era lasted about 180 million years, from about 245 million years ago to about 65 million years ago. The Mesozoic is divided into three time periods you may know: the Triassic, the Jurassic, and the Cretaceous. The land on Earth has collided some time back to form a single super-super continent called Pangea (meaning "All-Earth"). But the land continues to move, and Pangea is beginning to break up into the continents we know now. North America is just breaking away from the northwest coast of Africa, and the Atlantic Ocean and Gulf of Mexico are beginning to form. The Appalachian Mountains of the eastern United States are a high, rugged mountain range, something like the Rocky Mountains of today. Finally, over the next fifty million years or so, South America, India, and Antarctica will all break away from Africa and move toward their present positions. Life is expanding rapidly and beginning to look familiar. The dominant animals on both land and sea are reptiles, the most famous of which are the dinosaurs. Dinosaurs began developing in the Triassic, spread during the Jurassic, and dominated Earth in the Cretaceous period. They are so prominent that the Mesozoic is also called "The Age of Reptiles." But dinosaurs are not the only life form around: birds and mammals also appear during the Mesozoic, as well as leafy trees and flowering plants! The climate during the Mesozoic is so warm that there are no glaciers at all, even at the poles! Plants grow like crazy in the warmth and moisture, so there is food everywhere for your average hungry 10- ton Tyrannosaurus Rex! So what happened to this Dino-Paradise? More change! A mass extinction like those in the Paleozoic ended the Mesozoic Era. More than half of all existing life forms disappeared, including virtually all of the dinosaurs. Why? There are many theories, but the most widely accepted view is that a giant meteor impacted the Earth, triggering volcanic eruptions and an ice age.
THE CENOZOIC ERA This is the last of three geologic eras squeezed into the last 10% of Earth's whole geologic history. This is the "Age of Mammals" in which whales took over the oceans, saber-tooth cats shared the land with elephants and giant sloths, and humans finally appeared. (Yay humans!) The Cenozoic period began about 65 million years ago with the extinction of the dinosaurs and continues through today. The view of the Earth from above is that of today. During the last 65 million years, Pangea (the supercontinent) has broken up into the continents we know, and they have moved into the positions as we see them on the map now! The global climate has turned somewhat colder, and the last few million years have seen the return of giant glaciers and ice caps and a continuous cycle of ice ages and thaws. After the disappearance of the dinosaurs, there were suddenly many empty places on Earth where animals could live. Mammals, which were small, mouse-like animals at the beginning of the Cenozoic, quickly spread out, diversified, and grew in size. Soon the plains and forests of Earth were occupied by giant rhinos and elephants, lions and saber-tooth cats, horses, and deer. The skies filled with bats and birds, and the seas filled with whales, dolphins, sharks, fish and many other creatures! Finally, humanity initially looking more like primates (monkeys) than humans appeared during the last two million years. In the last 10,000 years, a blink of the eye in geologic time, humanity has spread across the lands and seas of Earth, altering the face of Earth with cities and farms, destroying some plants and animals and domesticating others. Humans have become the dominant life form: more numerous than any other large animal, and more fearsome than the most terrible of dinosaurs!