Dating and the Past

Both can be painful

Why is dating rocks important?

• It helps tell us how the Earth has changed– Ex. Continental Drift rocks that are the same age

on different continents, fossils that are similar and around the same age…..

• It helps tell us how life has changed over time– Things haven’t always been the same…!

Dating

There are 2 types of dating:

1. Relative Dating2. Radiometric Dating

Relative Dating

Relative Dating: estimating the age of a rock or fossil as compared to the rocks around it

While it’s useful in comparing layers of rocks, it WON’T give you an actual date

Relative DatingYou use the Law of

Superposition to relatively date a layer

of rocks:

Ex. Which fossils are younger?

Answer in your notes!

Radiometric Dating

Radiometric Dating: a dating process used to calculate the absolute age of a rock by measuring radioactive elements in a sample

Okay….so this is a little more complicated….

Radiometric Dating

In measuring the radioactive elements, you need to know the half-life of that element:

Half-Life: how long it takes for half of the atoms in an element to break down

- most take pretty long – over 5,000 years for just one half life!

Radiometric Dating

By using this information about an element, you can measure how much of an element is left in a rock sample and get a pretty accurate estimate of how old that sample is!

This is the type of dating where you get an ABSOLUTE age…not just a rough estimate

Now that you know how to date…

When you start to figure out how to ‘date’ certain items, this is when you begin looking back in time…..when things were….different….

When looking back in time, you need to understand how things are broken up on the oh-so-awesome GEOLOGIC SCALE!

GEOLOGIC TIME SCALE

Geologic Time Scale: the division of Earth’s history

4 main divisions of time: Eons Eras

Periods Epochs

EONS

Eons: the longest division and are based on huge groups of certain fossil groups

ERAS

Era: 2nd largest division that is based on major worldwide changes in the types of fossils

PERIODS

Period: 3rd largest division where certain life forms existed all over the world

EPOCHS

Epoch: smallest division of time divided by different life forms showing up at different times

(pronounced ‘epic’)

Geologic Time

Things have changed A LOT over time

Using the geologic scale given to you, answer the following questions about WHAT has happened over time……

So What’s Left Behind?

Fossils!

Fossils: the preserved traces, imprints or remains of living things

- usually form from the harder parts of organisms (bones, skulls, etc)

Fossils

Some fossils can tell us A LOT about the rocks that they are in, especially if they are an index fossil!

Index Fossil: a fossil of an organism that lived over a large area but for only a short period of time - so it will only be found in rocks formed during those times!

Fossils

Ex. Trilobites!

Only around between 250 MYA and 540 MYA (this is a short time for a fossil) and they were found everywhere!

Makes a good index fossil!

Index Fossils

How to use index fossils…. - you can observe them in a rock and

estimate what time period it was based on the fossil

- you can see what layers are the same ages based on what fossils are in them

Example – Index Fossils

What layers are the same age according to the fossils in them?

Let’s practice!

Use the sheet to practice using index fossils to identify what rocks layers are the same

AND

What time period they are from?