Geologic Time

Vers. 08/2020 © Kesler Science, LLC

Geologic Time
Scale

© Kesler Science, LLC

Geologic Time Scales

Essential Questions:

Reflect on the
Essential Questions
before you dive in…

1. If you were quizzed
today, which questions
would you know the
answers to already?

2. Which questions would
you need to learn more
about to answer
confidently?

1.

How does the relative and
absolute age of rocks and the
fossil record provide evidence
to Earth’s geologic history?

2.

How is Earth’s geologic
history classified?

3.

What are some major events
in Earth’s geologic history?

Geologic Time Scales

Earth Scientists

•Geologist – a scientist
who studies Earth’s
crust as well as the
processes and history
that shaped it

•Paleontologist – a
scientist that studies
fossil remains found on
Earth’s surface in order
to study primitive life
forms such as: plants,
animals, fungi, and
bacteria

What is the difference

between a geologist and

a paleontologist?

Geologic Time Scales

Stratigraphy

• A branch of geology
dealing with the arrangement of sedimentary rock layers or strata.

• Geologists assume the newest rock layers are on top of the older ones, unless some type of disturbance occurs.

• Called the Law of
Superposition

Multiple Choice

True or False: Law of Superposition states that newest rocks are found below older rocks.

• True

• False

Multiple Choice

According to the Law of Superposition, which layer is the oldest?

• A

• B

• C

Multiple Choice

According to the Law of Superposition, which layer is the youngest?

• A

• B

• C

Geologic Time Scales

Relative Age

• The strata of sedimentary
rocks is important in
determining their relative
age.

• Relative age determines the “relative” order of past events but not the absolute age.

• It is like saying you’re relatively younger than your grandparents.

Sedimentary Rock Layers

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Geologic Time Scales

Relative Age

• Strata is sometimes disturbed.

• Here we see a fault (E) and an igneous intrusion (D)

• Determining relative age in the field is sometimes very difficult because of these disturbances.

Sedimentary Rock Layers

Which rock is older, A or B?

By Kurt Rosenkrantz -http://cafreetextbooks.ck12.org/science/CK12_Earth_Science_rev.pdf (page
420) CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11017657

Multiple Choice

Which layer is the youngest?

• F

• D

• C

• A

Multiple Choice

Which layer is the oldest?

• B

• D

• C

• A

• E

Multiple Choice

Which layer is the older? D or B

• B

• D

Geologic Time Scales

Fossils

• Preserved remains or traces of animals, plants, and other
organisms from the remote past.

• Provides a snapshot of past events in Earth’s geological history.

What is a fossil record?

• FOSSIL Records - history of life as documented by all fossils, preserved in sedimentary rock strata.

Geologic Time Scales

In this example,
ammonite would
be a good index
fossil because it
lived in a wide geographic area,
but only lived for a short time.

How can you tell
ammonite only
lived for a short time?

Index Fossils

• Special fossils called “index fossils” indicate to geologists the boundaries in geological time.

• This is another tool to determine the age of rocks.

• Characteristics of Index Fossils

• Easily recognizable

• Abundant

• Wide geographic
distribution

• Live a short time

Multiple Select

Which of the following is NOT a characteristic of Index Fossils?

• Easily recognizable

• Live a long time

• Abundant

• Wide geographic distribution

Geologic Time Scales

Fossil Record and
Rock Strata

• Index fossils are used to correlate the age of the rock strata.

• If two different rock strata in different areas on Earth contain the same index fossils, then the strata are probably the same age.

Which fossil would make the best index fossil, and why?

Think About It

In order for a layer of rock to be considered a formation useable for relative dating, it must spread across a relatively large area that can be depicted on a geologic map.

Why is this important?

Geologic Time Scales

Absolute Age

• Most accurate form of
dating, also call radiometric dating

• Uses the decay of radioactive elements to find the absolute age of a rock or fossil

•This relies on the property of half-life, which is the predictable time an element takes to decay

What is used to find
the absolute age of a rock or fossil?

Geologic Time Scales

This shows how uranium breaks down into lead. The time it takes to do this is predictable.

Absolute Age

• Geologists tend to mix and match relative age and absolute age dates to piece together a geologic history.

• This is like saying you are 12 years old and your grandfather is 72 (absolute) instead of
you are younger than
your grandfather (relative).

Multiple Choice

How many half-lives of C-14 have occurred if the sample is determined to be 17,190 years old.

• 1

• 2

• 3

• 4

Multiple Choice

How old is a sample of Uranium-238 if it has had TWO (2) half-lives?

• 4.5 billions years old

• 9 billion years old

• Mr. Shafir's age

• 8 days old

Think About It

Why are both relative and radiometric dating important?

Why can't we use only one type?

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Geologic Time Scales

• All of Earth’s geological history represented on a chronological time chart.

• Based on rock strata and the fossil record

• Broken into different time chunks, the largest being the Eon, then Era, Period, and Epoch.

m.y. stands for millions of
years. This diagram is not
to scale.

What is the
difference between absolute age and relative age?

Geologic Time Scales

• Look how much of the Earth’s geologic history falls into Hadean, Archean, and Proterozoic (together called
Precambrian) - 88%

• If this were a clock, humans show up 11:58:43pm.

• Notice some of the important events which happened over 4.6 b.y.

Hadean

Archean

Proterozoic

b.y. stands for billions of years.

Geologic Time Scales

Precambrian

• Broken into 3 parts Hadean,
Archean, and Proterozoic

• Marks the beginning of Earth approximately 4.6 b.y.

• Oldest rock fragment on Earth contains a mineral, zircon, that is 4.4 m.y. (found in Australia)

• Very primitive life forms: one-celled animals, bacteria, jellyfish

When did the Precambrian start?

Multiple Choice

When did the Precambrian start?

• 4.6 Billion Years Ago

• 4.6 Years Ago

• 4.6 Million Years Ago

• 4.6 Hours Ago

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Geologic Time Scales

Paleozoic Era (early life)

• Marks the formation of the super continent, Pangaea

• Life developed rapidly
• Marine plants and invertebrates

• Fish and ferns
• Amphibians
• Mass extinction at the end of Paleozoic

Compare life in the Precambrian with life in the Paleozoic Era.

Geologic Time Scales

Mesozoic Era (middle life)

• Increased complex life forms including:

•Dinosaurs

•Small mammals

•Birds

•Conifers and flowering
plants

• Rocks in the Petrified Forest in AZ were deposited during this time

• Another mass extinction at end of Mesozoic

Geologic Time Scales

Cenozoic Era (late
life)

• Current Era
• Development of:

• Large mammals
• Human beings

• Periods of ice ages played a role in the formation of
the geological features
visible on Earth today.

Compare life in the Mesozoic Era with life in the Cenozoic Era.

Living Organisms

• Fossils of previous living

organisms are used to
further divide Earth’s
geologic history.

• Life on Earth has changed

over time or evolved quite
a lot in 4.6 b.y.

• We live in the Cenozoic Era

which is further broken
down into epochs.

In which epoch do we live?

Multiple Choice

In which epoch do we live?

• Holocene

• Pliocene

• Eocene

• Paleocene

Geologic Time
Scales

• An extinction is the end of a group of organisms,
usually a species.

What does extinction mean?

• During the Earth's 4.6 billion years, it has suffered 5 major mass extinctions.

Think About It

Archeologists found evidence of autumnal fruits
and heating contraptions often used to keep
warm discovered in the ruins.

Historians have long said that Mount Vesuvius erupted on 24 August 79 AD, destroying the nearby Roman city of Pompeii. However, there has been some speculation that the eruption actually happened much later - possibly into October.

How is this like dating techniques
used in the Geologic Time Scale?

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Geologic Time Scales

Dinosaur Extinction

• Most recent extinction is called the Cretaceous-Tertiary
extinction event.

• Happened 65.5 m.y. ago
• Climate and geologic changes are thought to have weakened the dinosaurs.

• A meteor strike was probably the final blow.

Give your best answer to...

1.

How does the relative and absolute age of rocks and the
fossil record provide evidence to Earth’s geologic history?

2.

How is Earth’s geologic history classified?

3.

What are some major events in Earth’s geologic history?
Check for

Understanding

Still have
questions?

Which essential questions do you still need help to understand?