Page 130 - Science Course 3 (Book 2)

Mo11-L4b: What Causes Mass Extinction?

Eons are subdivided into smaller units of time called SUMMARY
eras.
How was the Geologic Time Scale Developed?
Eras are subdivided into periods.
Earth’s history is organized into eons, eras,
periods, and epochs.
Periods are subdivided into epochs.

YOUNGEST Cretaceous
Mo11–L4b What Causes Mass Extinction?

Jurassic
KEY CONCEPTS:
Triassic 1. What are some causes of mass extinctions?
2. How is evolution affected by environmental
Permian change?

Carboniferous

Devonian Vocabulary A-Z
OLDEST Silurian Let us learn some vocabulary
Ordovician

Cambrian mass extinction
the extinction of many species on Earth within a
Precambrian
short period of time

When geologists developed the geologic time scale,
they chose the time boundaries based on what they
observed in Earth’s rock layers.

Geologists used sudden changes in the fossil record
to mark divisions in geologic time.

Because changes did not occur at regular intervals,
the boundaries between the units of time in the
geologic time scale are irregular.

Key Concept
How was the Geologic Time Scale Developed?
land bridge
Why are fossils important in the development of a landform that connects two continents that were
the geologic time scale? previously separated.

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