Lesson 4.2ROCKS PROVIDE A TIMELINE FOR EARTH

LAYERS OF SEDIMENTARY ROCKS SHOW RELATIVE AGE

Geologists got the clues of Earth’s past by the study of Relative Age of sedimentary Rocks.

What is Relative Age?

The age of an event or object in relation to other events or objects.

For example:

The relative age of Agnes can be determined in relation to Mary’s age.

Q: What is Agnes relative age?

Less than 7y, maybe around 5 y

What are sedimentary rocks ?

Sedimentary rocks form from the deposition of material (particles, minerals, etc. ) at the Earth's surface and within bodies of water .

Limestone Sandstone Siltstone

Conglomerate

Law of Original Horizontality

Sediments pile up horizontally in sedimentary rocks.

Law of Superposition

When the horizontal layers of sedimentary rocks are UNDISTURBED, the youngest layer is always on top and the oldest on bottom.

UNDISTURBED SEDIMENTARY ROCK LAYERS

YOUNGER

OLDER

How the layers are disturbed?

The movement of tectonic plates can disturb the layers, causing the rock layers can bend or fold over.

DISTURBED SEDIMENTARY ROCK LAYERS

YOUNGER

OLDER

To determine the correct order of layers, scientists compare the disturbed rock layers to the similar undisturbed rock layers.

YOUNGER

OLDER

In this case the Igneous rock will be younger than the sedimentary rock.

The igneous rocks are formed by the cooling of molten rock erupted from the Earth. Molten rock from the Earth’s crust cuts through the layers of sedimentary layers

DISTURBED SEDIMENTARY ROCK LAYERS

Index fossils are commonly found, widely distributed fossils that are limited in time span.

They help in dating other fossils found in the same sedimentary layer.

Used to compare the ages of rock layers

For example, if you find a fossil from an unknown era near a fossil from a known time, you can assume that the two species were from about the same time.

What are Index Fossils ?

Rock contains index fossil Arnioceras semicostatum( 206 million and 144 million years ago)

INDEX FOSSILS

144 million years ago

The actual age of an object or event.

For example Mary’s absolute age is 7 years

RADIOACTIVE DATING CAN SHOW ABSOLUTE AGE

What are Absolute age?

Q. What is Mary’s EXACT AGE? 7 y 4 m 5 d Q. Can the absolute age be the exact age? No

Rocks and organisms are made up of atoms. These atoms breakdown into another form over a period of time. This breakdown is called Radioactivity.

A particular element breaks down at the same rate into some other form.

What are Radioactivity ?

Scientists use radioactive dating to find the absolute age of rocks.

Radiometric dating methods estimate the age of rocks using calculations based on the decay rates of radioactive elements such as uranium, strontium, and potassium. 

The rate of change of radioactive elements is measured in half life.

“Half-life is the period of time it takes for the amount of a substance undergoing decay to decrease by half.”

Symbol : t½

RADIOACTIVE DATING CAN SHOW ABSOLUTE AGE

What is half life ?

Examples

Carbon-14 has a half-life of 5700 years.

If an object were found in which 25 % of the original carbon-14 remained, the age of the object would be taken as approximately 11,400 years.

Uranium 235 has half life of 704 million years. It is found in igneous rocks. It breaks down to Lead 207.

USING THIS INFORMATION SCIENTISTS ESTIMATE THAT EARTH IS AROUND 4.6 BILLION YEARS OLD

Elements with half life of millions or billions are used to date rocks.

0 HAL

F LIFE

1 HAL

F-LIFE

2 HAL

F-LIVE

S

3 HAL

F - L

IVES

4 HAL

F-LIVE

S

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How scientists use Absolute and Relative age?

Radioactive dating of igneous rocks reveals absolute age

Interpreting sedimentary layers shows the relative order of events

Fossils help to sort out sedimentary record

RADIOCTIVE DATING METHODS

WHAT SHOULD BE THE RIGHT RADIOACTIVE DATING METHOD TO DATE IGNEOUS ROCKS?

Uranium 235 (235U) is used to date igneous rocks.

Why? Igneous rocks contain radioactive elements which break down over time.

WHAT SHOULD BE THE RIGHT RADIOACTIVE DATING METHOD TO DATE FOSSILS?

Carbon 14 (14C) , is an unstable form of carbon. It is used to find ages of fossils that are about 100 to 70,000 years old. Why? Because carbon is found in all living things, which break down over time.