Radioactive isotopes with longer half-lives are used to date older fossils.
Paleontologists can determine the age of fossils sandwiched between layers of volcanic rocks by measuring the amount of potassium-40 in those layers.
Potassium-40 decays to the chemically unreactive gas argon-40, which is trapped in the rock.
When the rock is heated during a volcanic eruption, the argon is driven out, but the potassium remains.
This resets the clock for potassium-40 to zero.
The current ratio of potassium-40 to argon-40 in a layer of volcanic rock gives an estimate of when that layer was formed.
Magnetism of rocks can also be used to date them.
When volcanic or sedimentary rock forms, iron particles in the rock align themselves with Earth’s magnetic field.
When the rock hardens, their orientation is frozen in time.
Geologists have determined that Earth’s north and south magnetic poles have reversed repeatedly in the past.
These magnetic reversals have left their record on rocks throughout the world.
Patterns of magnetic reversal can be matched with corresponding patterns elsewhere, allowing rocks to be dated when other methods are not available.
Geologists have established a geologic record of Earth’s history.
By studying rocks and fossils at many different sites, geologists have established a geologic record of the history of life on Earth, which is divided into three eons.
The first two eons—the Archaean and the Proterozoic—lasted approximately four billion years.
These two eons are referred to as the Precambrian.
The Phanerozoic eon covers the last half billion years and encompasses much of the time that multicellular eukaryotic life has existed on Earth.
It is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
Each age represents a distinct age in the history of Earth and life on Earth.
The boundaries between eras correspond to times of mass extinction, when many forms of life disappeared.
Mass extinctions have destroyed the majority of species on Earth.
A species may become extinct for many reasons.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-8