Relative and Relative and Absolute DatingAbsolute Dating

Relative and Relative and Absolute DatingAbsolute Dating

Question of the DayQuestion of the DayQuestion of the DayQuestion of the Day

Question: What are the 3 types of rocks and what is the rock cycle?

Answer: … … …Turn In:

-Mineral ID Lab-Rock ID Lab

-Rocks Webquest-p. 582 CYU 1 - 6

Relative and Absolute Dating

What is Dating?

When geologists date rocks, they are determining how long ago they formed.

Two ways to do this:Relative DatingAbsolute Dating

Relative Dating

Determining how old something is compared to something else

Use words like “older” or “younger” instead of exact numbers

Rules of Dating

two rules for dating rocks:1. Stuff on the bottom is oldest and the stuff on

the top is youngest

This is called the

Law of Superposition This only works for sedimentary rocks

(sometimes lava flows)

Order these layers from oldest to youngest

Youngest layer

Oldest layer

Rules of Dating

two rules for dating rocks:2. If a fault or another rock cuts through a layer it

happened after (and is younger than) the layer

This is called the

Law of Cross-Cutting Relations

Order the events from oldest to youngest

Fault

Gray Layer

Multicolored layer

Spotted Layer

OLDEST LAYER/EVENT

YOUNGEST LAYER/EVENT

Absolute Dating

Determining how old something is

Use numbers (in millions of years, mya)

Only works for Igneous Rocks

How Absolute Dating Works

When magma/lava cools, radioactive elements are incorporated into the minerals

Examples:

-- Potassium 40

-- Uranium 235

How Absolute Dating Works

These elements begin to decay at a known rate starting when the rock cools

We can measure how much of the element is left

Tells us how much time has passed since the rock formed.

What can we learn from this?

T-Rex Fossil

Geologist

Basalt, dated to 80 mya

Basalt flow, dated to 100 mya

Sandstone

So when did this T-Rex live?

Between 100 and 80 mya

This may not seem very accurate, but compared to the 4,500 million years the earth has been around it gives us a lot more information than we had before

What can we learn from this?

Absolute DatingHelps us determine the age of the earthHelps us determine when specific events in the

history of the earth happened (ex. Extinction of the dinosaurs)

Relative DatingCan help us estimate the time span between

major earthquakes, storms, tsunamis etcCan help us determine the order that life forms

developed on earth

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