THE GEOLOGIC TIME SCALE: THE HISTORY

OF EARTH

GEOLOGIC TIME

SCALELOOK AT THIS TIME SCALE.TELL ME 5 THINGS ABOUT WHAT YOU SEE.TELL ME ONE THING YOU ALREADY KNOW AND ONE THING YOU WANT TO KNOW.

Rocks Record Earth History

Discovering Earth’s History

Rocks record geological events and changing life forms of the past.

We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today.

They may contain fossils Older rocks are usually below younger

ones. Some rocks contain radioactive

isotopes.

Based on their interpretations of the rock record, geologists have divided Earth’s 4.6-billion-year history into units that represent specific amounts of time. Taken together, these time spans make up the geologic time scale.

How Do Rocks Record Earth’s History?

graptolites, gingko, trilobite

Interpreting Environments

• Fossils can be used to interpret and describe ancient environments.

How they form Answer these questions during the video:1- People who study fossils are called ___________________2- Fossils are at least ________ years old.3- Trace fossils could be _________, _________ or _________________4- A resin fossil is made when small creatures get trapped in tree _____.5- Most fossils form when a dinosaur falls in ___________ and is covered by ___________.

Fossil formation

What is a fossil?

The principle of superposition

The principle of superposition states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.

Relative dating- rock Y is younger than rock Z because Y is above Z. Similar to saying that I am older than you, but younger than your grandparents.

Absolute dating – rock Y is 22,000 years old and rock Z is 1.5 million years old.

Principle of superposition uses relative dating and radiometric dating uses absolute dating.

Relative Vs Absolute Dating

Radiometric Dating

Dating with Radioactivity

Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.

This is the method used to date the oldest rocks.

Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.

It is the time it takes for half of a substance to decay (or change) into a new substance because the original substance is radioactive which means it is unstable. In order for the substance to become stable, it may release alpha, beta or gamma radiation. The release of these particles may change the identity and mass of the original substance.

For example, as uranium-238 decays, it releases alpha and beta radiation to eventually become lead-206. New element, new mass. Scientists compare the amount of U-238 to Pb-206 to determine the age of the rock.

What is “half-life”?

Radioactive Isotopes Frequently Used in Radiometric Dating

Dating with Carbon-14 Radiocarbon dating is the method for

determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.

Dating with Radioactivity

When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.

Used to date the ages of once living things (all living things contain carbon).

Can only date substances up to 60,000 years because the half-life of C-14 is pretty fast.

Found in sedimentary rocks.

Used to date the oldest rocks.

Has a very long half-life (4.5 by).

Dates igneous rocks (those formed from cooled magma/lava).

Carbon and U-238 dating

Geologize

The half-life of potassium is 1.3 billion years. A rock contains 100 grams of potassium and was formed 3.9 bya.

a) How many half lives have passed?3 half lives

b) How many grams of potassium remain?25 grams

c) What percent of the original potassium is now in the rock?

12.5%

Try this problem.