The Formation Of Earth

Evidence from computer models of the sun suggests that about 5 billion years ago, our solar system was a swirling mass of gas and dust, as shown in Figure 14-4. Over time, most of this material was pulled together by gravity and formed the sun. The remaining gas, dust, and debris circled the young sun. Scientists think that the planets formed through repeated collisions of this space debris. During a 400 million-year period, Earth grew larger as gravity pulled in more debris. The collisions between Earth and space debris also released a great deal of thermal energy. Some collisions would have released enough energy to melt large portions of Earth's surface.

Earth's Age

The estimated age of Earth, more than 4 billion years, is about 700,000 times as long as the period of recorded history. It is about 50 million times as long as an average human life span. How can we determine what happened so long ago? Scientists have explored Earth's surface and examined its many layers to establish a fairly complete picture of its geologic history. Early estimates of Earth's age were made from studying layers of sedimentary rock in Earth's crust. The age of Earth could not be estimated accurately, however, until the middle of the twentieth century, when modern methods of establishing the age of materials were developed.

Radiometric Dating

Methods of establishing the age of materials include the techniques known as radiometric dating. Recall that the atomic number of an element is the number of protons in the nucleus. All atoms of an element have the same atomic number, but their number of neutrons can vary. Atoms of the same element that differ in the number of neutrons they contain are called isotopes (lE-suh-TOHPS). Most elements have several isotopes.

The solar system began to form about 5 billion years ago.

The sun began to form a few million years later.

By 2.2 billion years ago, Earth probably appeared much as it does today.

Earth began to form about 4.6 billion years ago and grew by colliding with space debris.

Volcanoes emitted gas, forming an atmosphere.


The mass number of an isotope is the total number of protons and neutrons in the nucleus. The mass number of the most common carbon isotope is 12. If you recall that the atomic number of carbon is 6, you can calculate that this carbon isotope has six protons and six neutrons. Isotopes are designated by their chemical name followed by their mass number; for example, carbon exists as both carbon-12 and carbon-14.

Some isotopes have unstable nuclei, which undergo radioactive decay; that is, their nuclei release particles or radiant energy, or both, until the nuclei become stable. Such isotopes are called radioactive isotopes. Rates of decay of radioactive isotopes have been determined for many isotopes. The length of time it takes for one-half of any size sample of an isotope to decay to a stable form is called the half-life of the isotope. Depending on the isotope, half-lives vary from a fraction of a second to billions of years.

The quantity of a particular radioactive isotope in a material can be measured to determine the material's age. This amount is compared with that of some other substance whose quantity in the material remains constant over time. For example, organic materials can be dated by comparing the amount of carbon-14, a radioactive isotope, with the amount of carbon-12, a stable isotope. Living things take carbon into their bodies constantly. Most of the carbon is in the form of carbon-12. A very small proportion of it, however, is in the form of carbon-14, which undergoes decay. This ratio of carbon-14 to carbon-12 is a known quantity for living organisms.

figure 14-4

It took about one-half billion years for Earth to form from a swirling mass of gases. Sometime after Earth formed, living organisms began to affect the atmosphere.

Word Roots and Origins isotope from the Greek isos, meaning "equal," and topos, meaning "place"

Modeling Radioactive Decay

Materials shoebox, 50 pennies, 50 paper clips

Procedure Place the pennies in the shoebox. Close the lid, shake the box, and then set the box down. Replace each face-up penny with a paper clip. Record the number of pennies and paper clips in the box. Repeat this process five times. Analysis If the pennies in the box represent an unstable isotope, such as carbon-14, what do the paper clips represent? What does each shake of the box represent?

figure 14-5

This deerskin quiver, with a wooden bow and arrows, is about 3,000 years old. Carbon-14 dating methods can be used for organic materials less than 60,000 years old.

figure 14-5

This deerskin quiver, with a wooden bow and arrows, is about 3,000 years old. Carbon-14 dating methods can be used for organic materials less than 60,000 years old.

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