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Getting to Know: Formation of the Earth Scientists believe Earth is approximately 4.6 billion years old. A lot can happen over the course of 4.6 billion years. How do scientists reconstruct Earth’s past? Geologists use several important principles to understand how Earth formed, when it formed, and how it has changed over time. One of the key ideas in geology is that the processes occurring on Earth today are the same processes that occurred in the past. Earthquakes, volcanoes, and plate tectonic activity occur today, and those same processes also occurred in Earth’s distant past. This means that we can understand the rock formations, mountain ranges, and ancient lava flows that we see on Earth’s surface today by considering the geologic processes that formed them. How do geologists date rocks? Geologists attempt to reconstruct the past using several methods for dating rocks. Relative dating is the process of comparing the ages of rock layers. Absolute dating uses special scientific instruments to determine the actual age of a rock layer. Geologists study rock formations to understand Earth’s history. Relative dating involves comparing the ages of rock layers. This method relies on the law of superposition and the law of original horizontality. Geologists agree that rocks are usually deposited in parallel layers, with the oldest layers generally on the bottom. An undisturbed layer closest to the surface is youngest, and each layer below it is older. This is called the law of superposition. The law of original horizontality also helps geologists study rock formations. It states that rock layers form from material that is originally deposited horizontally. After rock layers are deposited, they can be folded, fractured by faults, or tilted. However, the original orientation of the rock layers was horizontal. Any change to the layers must have occurred after the layers formed. These laws might seem like simple ideas, but they are fundamental to the way that we interpret the past. For example, layers of rocks in the Appalachian Mountains are folded; scientists know that this folding happened after the initial rock layers were deposited. That means the Appalachian Mountains are younger than the rock layers. Concept: Formation of the Earth Getting to Know www.discoveryeducation.com 1 © Discovery Education. All rights reserved. Discovery Education is a subsidiary of Discovery Communications, LLC. How is relative dating different from absolute dating? Absolute dating determines a more exact age for rocks than relative dating. Rocks and minerals are made of elements and compounds that decay over time. Because the rate of decay of elements such as potassium, carbon, argon, and uranium is highly predictable, scientists can determine how old a rock is by analyzing the elements within it and performing mathematical calculations. Measuring the amount of a radioactive isotope and its decay product contained in certain minerals is called radiometric dating. Radiometric dating is often used to determine the age of igneous rocks. Misconception 1: Earth formed very quickly in the past, but now the rate of geologic change has slowed. That is incorrect. There is no evidence that geologic change happened more quickly in the past. Earth formed very slowly over time; the processes that shape Earth’s surface today are the same as the processes that occurred in the past. Do other processes change Earth’s surface? Sometimes it can be difficult to study rock formations because they are weathered and eroded. Weathering is the breakdown of rock material by wind, water, ice, plant and animal activity, and pressure changes. Erosion is the removal of weathered rock material from its original location. The breakdown and removal of rock material shapes Earth’s surface, but it also makes interpreting rock formations challenging. Keep exploring this lesson to learn more about how geologists study Earth’s past, present, and future! Weathering and erosion shape rock formations into many beautiful structures. The arches shown here are in Arches National Park in Utah. (Image from the National Park Service) Misconception 2: Earth looked different in the past. That means the processes that change Earth’s surface were different in the past. The same geologic processes at work today have been shaping Earth’s surface for millions of years. Volcanoes, earthquakes, and the movement of tectonic plates also occurred in the past. Weathering and erosion also affect Earth’s surface. These processes remain unchanged throughout Earth’s history. Concept: Formation of the Earth Getting to Know www.discoveryeducation.com 2 © Discovery Education. All rights reserved. Discovery Education is a subsidiary of Discovery Communications, LLC.