Investigating Rock Strata

The Fossil Record

By Juanita Rodríguez Serrano

Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth’s 4.6-billion-year-old history​

Learning Intentions​

​

Success Criteria

Some text here about the topic of discussion

Identify the different eons, eras, periods and events that are represented by the rock layers.

Investigate the different ways to determine the relative and absolute ages of rock layers, as well as events that can affect those layers.

​I will be successful when I can . . .

You might already know that fossils are the remains or evidence of once-living organisms. The fossil record is made up of all the fossils ever discovered on Earth. It contains millions of fossils that represent many thousands of species. Most of these species are no longer alive on Earth.

What is a fossil?

Patterns in the fossil record document the existence, diversity, and changes of life forms throughout Earth's history. The fossil record is enormous, but it is still incomplete. Scientists think it represents only a small fraction of all the organisms that have ever lived on Earth.

​

The Fossil Record​

Multiple Choice

The fossil record is made up of ____________.

• prehistoric dinosaur fossils.

• all scientific data about fossils.

• all animal fossils in America.

• all fossils ever discovered

Multiple Choice

Most organisms who have lived on earth have left fossils.

• True

• False

Most dead animals do not become fossils. When an animal dies, any soft tissues animals do not eat, bacteria break down. Under rare conditions, some of the hard parts that remain become fossils. The soft tissues, such as skin, muscles, or leaves, can also become fossils, but only rarely. Let's look at five ways fossils form.​

How Fossils Form?​

​Five ways fossils form:

Petrified: material is buried under sediments. The minerals replace the original material and form a rock.

Molds: when sediment buries an organism, the organism decays and leaves a cavity in the shape of the organism

Casts: forms when a mold is filled with sand or mud which turns into rock in the shape of the organism

Trace Fossils: evidence of an organism's behavior (like a footprint)

Preserved: when even soft tissue is preserved, like with animals frozen whole or others that fell into tar pits

Multiple Choice

What kind of fossil is evidence of an organism's behavior?

• Trace

• Preserved

• Mold

• Petrified

• Cast

Multiple Choice

What kind of fossil includes soft tissue?

• Trace

• Preserved

• Mold

• Petrified

• Cast

Multiple Choice

In what kind of fossil do minerals replace the original material and form a rock?

• Trace

• Preserved

• Mold

• Petrified

• Cast

Scientists cannot date most fossils directly. Instead, they date the rocks the fossils are embedded inside. Rocks erode or are recycled over time. However, scientists can determine ages for most of Earth's rocks.

​

How old are fossils?

Relative age dating: scientists look at the order of rock layers. In an undisturbed rock formation, they know that the bottom layers are oldest and the top layers are youngest.

​

Absolute age dating: scientists look at radioactive decay to learn a rock's absolute age, or its age in years. This process is called radiocarbon dating.

​

How to date fossils?

Multiple Choice

If you wanted to know the age of a rock in years, which would you need to use?

• Absolute Dating

• Relative dating

Multiple Choice

Rock layers closer to the surface are _____________ than those deeper down.

• Younger

• Older

The Law of Superposition states that each sedimentary rock layer is older than the rock layer above it and younger than the layer below it. Rock strata (layers of rock) are formed as horizontal layers called beds and may take millions of years to form. This is what makes relative dating possible.

​

Relative Age Dating​

Sometimes the layers are not laid down perfectly. A break in the sedimentary geologic record is known as unconformity. A lot of things can cause this. Maybe the rock was exposed to the elements, or perhaps volcanic processes destroyed this section. Tectonic plate motion (such as folding, breaking or uplift) can distort these layers as well.

​

​Relative Age Dating

Multiple Choice

What law states that each sedimentary rock layer is older than the rock layer above it and younger than the layer below it?

• Law of Superposition

• Law of Deposition

• Law of Layeration

• Law of Sublimation

Absolute dating is sometimes possible through radiometric dating. Here's how it works: radioactive elements lose their radioactivity at a steady rate. This is called radioactive decay. By measuring the amount of radioactive decay in a layer where fossils have been found,scientists can learn the age of the fossils.

​

​Absolute Age Dating

​

Multiple Choice

What term refers to the steady loss of radioactivity in elements?

• radiometric dating

• radiology

• radioactive decay

​So who cares?

The information found in fossils and rocks have given scientists insight into the Earth's history. Slow changes in fossils help us show the long process of evolution.

Disappearances in the fossil record show extinctions. Altogether, fossils help us get a sense of the geologic time scale.

This series of horse fossils in suggests that the modern horse is related to other extinct species. These species changed over time in what appeared to be a sequence. Change over time is evolution, and fossils show us not only how but when this occurred.

​

​Fossils and Evolution​

Sudden disappearances of fossils in rock layers are evidence of extinction events. Extinction occurs when the last individual organism of a species dies. A mass extinction occurs when many species become extinct within a few million years of each other. The fossil record contains evidence that five mass extinction events have occurred.

​

​Fossils and Extinction​

Multiple Choice

How many mass extinction events have their been?

• 1

• 2

• 3

• 4

• 5

Multiple Select

What are two ways that fossils help scientists?

• Learn about how animals evolved

• Learn about how oil is made

• Learn about when extinctions happened

• Learn about the rock cycle

You might be surprised to learn that evidence of microscopic organisms has been found in rocks 3.4 billion years old.

It can be to keep track of time that is millions or billions of years long.

To help, scientists organize Earth's history into a time line called the geologic time scale.

The geologic time scale is a chart that divides Earth's history into different time units.

​

Geologic Time Scale​

The time scale is divided into eons, really long units of time. The length of each eon on the time scale is not always equal.

The presence or absence of fossils in one layer of rock helps scientists determine when one eon ends and another beings.

​

Geologic Time Scale​

Multiple Choice

What is the geologic time scale?

• a chart that divides Earth's history into different time units

• a really long unit of measurement for time

• the presence or absence of a fossil in a layer of rock

Multiple Choice

The geologic time scale is divided into eons. Which of the following are true of eons?

• they spell "nose" backwards

• they are really long units of time

• they are not necessarily equal to each other

• these has been seven of them

Open Ended

Explain multiple ways that the fossil record helps scientists study life on earth. (Use at least two complete sentences.)