Figure7.31: Themostreliablewaytofigureouttheearth’sageistomeasuretheradioactivity
of certain minerals found in rocks (called radiometric dating). This mass spectrophotometer
can also be used to measure age of fossils from the level of radiation in minerals surrounding
the fossil. ( 1 )
Over 4 Billion Years
The earth is at least as old as its oldest rocks. The oldest rock minerals found on Earth
so far are zircon crystals that are at least 4.404 billion years old. These tiny crystals were
found in the Jack Hills of Western Australia. Since the earth is at least as old as the oldest
minerals found on Earth, geologists estimate that the minimum age of the earth is 4.404
billion years.
Likewise, the earth cannot be any older than the solar system. The oldest possible age of the
earth is 4.57 billion years old, the age of the solar system. Geologists and geophysicists based
the age of the universe on the age of materials within meteorites that are formed within the
solar system.
Origin of Life on Earth
There is good evidence that life has probably existed on Earth for most of Earth’s history.
Some of the oldest fossils of life forms on Earth are at least 3.5 billion year old fossils of blue
green algae found in Australia (Figure7.32).
The next step is to determine exactly how life formed billions of years ago. First, scientists
need to know what the environment was like 3.5 to 4 billion years ago; they need to know