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Earth’s History
Geologic time scale-a timeline that
organizes the events in Earth’s history
Like a calendar extending from Earth’s
formation to the present
Divided into eons, eras, periods, and epochs
Earth’s History
•Eon-the largest group; divided by the Cambrian boundary
(about 550 million years ago) when the variety of life forms
explodes; billions of years long
•Era: mass extinctions mark the boundaries between the eras;
hundreds of millions years long
•Period: blocks of time when a unique rock series was laid down;
tens of millions of years long
•Epoch: divisions of the most recent periods; several million years
•Imprints or remains of organisms that were once alive
•Typically form in sedimentary rock, however:
•May form in rocks
•Frozen Fossils
•Trace fossils-any naturally preserved evidence of animal
•Mold-a cavity in rock where a plant or animal was buried
•Cast-created when sediment fills in a mold and becomes
History of Change
•The fossil record reveals a history of environmental change.
•Studying the relationship between older and younger fossils
•Fossils of organisms that lived during a relatively short,
well-defined geologic time span
•Found in rock layers throughout the world
Ammonites 230-208
Trilobite ~400 MYA
Layers of Rocks
1. Layer A is the oldest
due to the law of
layers are set down first
2. Radiometric dating
because the rock layers
are inorganic
3. The geologist did not
include dates because
they included the age of
the extrusion and
intrusion. They also
used index fossils
4. < 250 million years old but
> 100 million years old
because it is between the
extrusion and intrusion
5. No because the first
vertebrates did not appear
until the Ordovician Period
(505 MYA)
6. A major event in Earth’s
history was the evolution of
dinosaurs and flowering
7. Layer F was deposited
during the Cenozoic Era
because it contains a
whale fossil.