ScienceEarth’s Changing Surface

Chapter 4

Section 3

Radioactive Dating

• Everything around us is made up of matter• Matter is made up of tiny particles called

atoms• When atoms in a particular type of matter

are the same, the matter is an element• There are about 110 elements• Most elements are stable

• They do not change under normal conditions

Continued

• Some elements exist in forms that are unstable• These may break down or decay, & in the process

release particles & energy in a process called radioactive decay

• These unstable elements are said to be radioactive• During this decay, the atoms of one element break down

to form atoms of another element.

The Rate of Radioactive Decay

• As a radioactive element w/i the igneous rock decays, it changes into another element.

• So, the composition of the rock changes slowly over time• The amt of the radioactive element goes down, but the

amt of the new element goes up• *the rate of decay of each radioactive element is

constant—it never changes.• This rate of decay is the element’s half-life.• This is the time it takes for half of the radioactive

atoms to deday

Absolute Ages from Radioactive Dating

• Geologists use radioactive dating to determine the absolute ages of rocks.• First, the amt of radioactive element in a rock is

determined• Then a comparison this amt to the amt of the stable

element into which the radioactive element decays• Types of dating

• Potassium-Argon Dating—used to date rocks• Carbon-14—useful for dating plants & animals up to

50k years ago

Radioactive Dating of Rock Layers

• RA cannot be used in dating other than igneous rocks• For sed rocks scientists date the intrusion & extrusions

nearby.• E has been dated to about 4 billion years• May be older