Dating Rocks Using Radioactive Decay

الأحد مايو 17, 2015 6:17 pm

Dating Rocks Using Radioactive Decay

Scientists use geologic evidence for many events in Earth’s history.

Mountains provide evidence of plate collisions or volcanic activity.

The shields that form each continent tell about how Earth’s crust

evolved. Special iron-rich rocks formed in the oceans tell about how the

atmosphere was formed. Changes in fossil groups indicate changes in past

climates. When looking back at events in Earth’s geologic past, scientists

want to know the dates when these events occurred.

You have already looked at one way of dating rocks. You used a basic

geologic principle in an earlier chapter. It is called the Principle of

Superposition. It describes how younger layers of sediments are deposited

on top of older layers. This gives a relative age of a layer of rock. You can

identify a rock layer as being younger or older than the layers next to it.

Knowing the exact ages of rocks is also important to scientists. It helps

them to unravel further some of the secrets of Earth’s history. It can

help them to answer questions like the following. How old is Earth?

When did the first continents form? How long does it take for ocean

crust to be recycled in the mantle? Dating is important for correlating

rocks in different locations as well. It can also allow events to be placed

in sequence. This can be especially useful when trying to find out the

effects of one event on another.

To determine the absolute age of a rock, scientists look for radioactive

minerals. These minerals contain radioactive elements. The nuclei of

these elements are unstable. As a result, they break apart over time.

This process is known as radioactive decay.

The nuclei of radioactive elements

break apart because they are unstable.

As particles are released, the original

element changes into a different one.

The new element has slightly lighter

properties. The atom that undergoes

decay is called the parent atom. The

product is called the daughter atom.

Scientists look at the rate at which

a radioactive element in a mineral

decays to determine the age of the

rock in which it is contained. The time

it takes for half of the parent atoms

to decay into daughter atoms of a

different element is called a half-life.

You used a model to illustrate this in

the Investigate. In your model, the

half-life was 30 seconds. Knowing the

half-life of an element and the fraction

of parent atoms left, a decay graph is

used to determine when the parent

atoms were fully intact. The date when

the rock was formed can then be

determined.

Different elements have different halflives. Ones that have very long halflives decay very slowly. For instance,

the radioactive isotope of rubidium has

a very long half-life. It takes about 48.8

billion years for half of it to change to

strontium. Elements such as this, with

very slow rates of decay, are good for

finding the ages of very old rocks.

Using radioactive decay, scientists have found Earth’s oldest mineral to be

zircon. You read about this mineral when you learned about the evolution

of the geosphere. Zircon contains small traces of the radioactive element

uranium. (Uranium decays to form lead.) Zircon samples have been found

that date as far back as 4 billion years. Zircon is highly resistant to

weathering and erosion. Most other minerals that may have formed in a

rock with an ancient zircon would probably have worn away long ago.

Some elements have short half-lives.

The half-life of radioactive carbon atoms

is only 5730 years. Consider a geologically

young sample of carbon that formed

50 million years ago. It would have gone

through about 100,000 half-lives of decay

by now. The amount of non-decayed

carbon left would be tiny. It would be

too hard to measure accurately. Due to

this, radiocarbon dating is not good for

all samples. It is only good for ones that

are 70,000 years old or younger.

Radioactive dating is done mostly on igneous rocks. Radioactive elements

are trapped in certain minerals when magma cools and hardens to form

rock. There are a few types of sedimentary rocks that contain radioactive

elements. Many sedimentary rocks are made from older rocks that have

been broken down. Therefore, radioactive dating is not always the

best method for finding the age of these rocks. The same is true for

metamorphic rocks. The radioactive “clock” is reset in new minerals that

form when rocks are deformed. These minerals give a younger age for

the rock than when it originally formed.