What is Earth? | What is Earth as a planet? | Planet Earth: Facts about our home planet |

Introduction about our planet Earth.

Earth and its moon, according to scientists, formed around the same period as the rest of the solar system. They believe it happened around 4.5 billion years ago. Earth is the solar system's fifth-largest planet. It has a diameter of around 8,000 kilometers. Furthermore, Earth is the third-closest planet to the sun. It is around 93 million miles from the sun on average. Only Mercury and Venus are more distant.

If we could film the history of Earth from its beginning 4.6 billion years ago to the present, and show it as a feature-length movie, we would witness a planet undergoing remarkable change. We would see the geography of the planet continuously changing as continents repeatedly collided and broke apart. As a result of these collisions and breakups, it would rise up, only mountain ranges to eventually be worn down, while ocean basins would open and close between the moving continents. Oceanic and atmospheric circulation patterns would shift in response to the movement of continents and hence would affect global climate and weather patterns. At various times we would see massive ice sheets appearing, growing, and then melting away. At other times, the landscape would be dominated by vast interior deserts or extensive swamps. As our imaginary camera focused closer to Earth's surface, it would capture a breathtaking panorama of life.

 We would witness the first living cells evolving from a primordial organic soup sometime between 4.6 and 3.5 billion years ago. About 2 billion years later, cells with a nucleus would make their appearance. It would not be until around 700 million years ago that we would see the first multicelled soft-bodied organisms evolving in the oceans, followed by small animals with skeletons, and then animals with backbones. Meanwhile, the various continents would appear barren and seemingly devoid of life until about 450 million years ago. At that time, the landscape would change forever with the evolution of land plants. In a relatively short period of time, insects would make their appearance as well as primitive amphibians, and then reptiles. Soon the continents would be teeming with life.

 However, some 250 million years ago, near the close of the Paleozoic Era when the continents were coalescing into one supercontinent called Pangaea, a major catastrophe occurred, resulting in the greatest mass extinction event the world has ever seen. About 90% of all species in the oceans became extinct, while 30% of insect orders died out, and more than 65% of all amphibian and reptile families disappeared. During the next period of Earth’s history, known as the Mesozoic Era, new organisms evolved, filling and expanding the niches left vacant by the Permian mass extinction event. The stars during this part of our movie are the dinosaurs who ruled the land while flying reptiles circled overhead and marine reptiles dominated the seas. Mammals, as well as birds, made their debut during the Mesozoic but were relegated to supporting roles. Also during this time, Pangaea broke apart into various continental landmasses and the Atlantic Ocean came into being. Just as at the end of the Paleozoic Era, another mass extinction event occurred at the end of the Mesozoic Era.

 This global catastrophe wiped out the dinosaurs, flying reptiles, and marine reptiles, as well as various marine invertebrate animals. The last part of our epic movie features the rise and dominance of mammals and flowering plants, and in the grand finale, the evolution and appearance of humans. During this time, known as the Cenozoic Era, the continents continued moving toward their current locations while the landscape was being sculpted to its present-day topography. In the last second or so of the movie, we would see multiple advances and retreats of glaciers in the Northern Hemisphere, and the migration and settlement of humans to all continents. As the movie ends and fades to the closing credits, the impact of human activity on the global ecosystem will be quite evident. It seems only fitting that the final image in the movie will be of Earth, a shimmering blue oasis in the black void of space. In our analogy of Earth history as a feature-length film, three themes stand out. The first is that Earth's outermost part is composed of a series of moving plates whose interactions have affected the physical and biological history of the planet. Second, the earth’s biota has grown, or changed, throughout its history. The third is that the physical and biological changes have taken place over long periods of time.

 The first of these themes, plate tectonics, provides geologists with a unifying theory that explains Earth's internal workings and accounts for how many seemingly unrelated geologic features and events are connected. The second theme, the theory of organic evolution, illustrates how life has changed through time, based on the idea that all living organisms are the evolutionary descendants of life forms in the past. The last theme, the concept of geologic time, shows how small, almost imperceptible changes over vast periods of time have resulted in significant changes in Earth. These three interrelated themes-plate tectonics, organic evolution, and geologic time-are central to our understanding and appreciation of the workings and history of our planet. As you read this book, keep in mind that the different topics you are studying concern parts of dynamic interrelated systems and are not isolated pieces of information. For example, volcanic eruptions are the product of complex interactions involving the Earth's interior and surface. The impact of these eruptions is felt not just locally, but also globally.

This is because the tremendous amount of ash and gases are thrown into the atmosphere by volcanic eruptions reduces the amount of solar radiation reaching Earth, thus contributing to climatic changes that affect the entire planet.



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