Download Getting to Know: Formation of the Earth

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
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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
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.