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The Geologic Time Scale
10.15.07 / 10.16.07
Correlation
• Using rock formations and fossil types to
relate geologic materials from different
regions
Essential Question
• How do scientists use the geologic time
scale to understand and describe natural
history?
What is the geologic time scale?
• It is a graphic model of Earth’s history
divided into units based on geologic
events found in the fossil record.
• First developed in the 19th century as a
relative time scale using relative dating
principles
• Later modified, in the 20th century, to
include absolute ages
Geologic Time Scale
Mass extinctions
• Geologic time is divided according to types
of life
• These divisions often occur from mass
extinctions, examples of which include:
– Cretaceous-Tertiary extinction killed dinosaurs
– Triassic-Jurassic extinction (22% of marine
families)
– Permian-Triassic extinction (95% of all
species)
Precambrian Time
• 4.6 billion – 540 million years ago
– Comprises about 88% of Earth’s history
– Divided into three eons
• The further back in time, the less we know
about that time
– Life forms had soft bodies so there are few
fossils
– Many rocks from this time have been recycled
– Surviving rocks are often deeply buried
– Grand Canyon reveals some Precambrian
rocks
Precambrian life
• Early atmosphere had an abundance of
CO2 resulting from cooling magma
• First life was anaerobic (not requiring
oxygen) bacteria
• Bacteria and early plants released oxygen
into the atmosphere
• Fossils exist for life as far back as
3.5 billion years
Paleozoic Era
• Paleozoic means “ancient life”
• About 540 million years ago, many varied
types of fossils appear
• Paleozoic life had shells, bones and teeth,
and was therefore more likely to become
fossilized
• Invertebrates, fish, amphibians and land
plants all live in this time
• Ends with mass extinction (90% marine,
70% land life dies)
Mesozoic Era
• “Middle life”
• Age of reptiles (dinosaurs)
• Dryer environment led to proliferation of
trees and land animals
• Ends with mass extinction
• Impact hypothesis
– Asteroid impact may have caused extinction
– Iridium layer around the world provides
evidence
Is this the K-T Asteroid?
• 6-8 mile wide asteroid
• Left 120 mile wide crater
Cenozoic Era
• “Recent life”
• Mammals take over in colder climate
• Mammals develop in four ways:
– Increased size
– Increased brain power
– Specialized teeth for specific diets
– Specialized limbs for specific living
environments
Cenozoic extinctions
• Over the last 8-20,000 years, many large
species of mammals have gone extinct
(ex. mastodons and mammoths, sabertoothed cats, giant sloths, etc)
• Most likely explanation is that human
hunting of these animals and their food
sources led to extinctions
Life across the geologic time
scale
First life
fish
reptiles
mammals
humans
Summary activity
1. Briefly describe the major distinctions
between the Precambrian time and each
of the three most recent eras.
2. The geologic time scale has been
updated as new information has become
available. Why did the early versions of
the time scale not include approximate
dates for each segment of time?