4.2 Rocks provide a timeline for Earth

Big Idea:  Rocks, fossils and other types of natural evidence tell Earth’s story.

Activity:  How old are these cars? CAR 1              CAR 2              CAR 3

*We do not know the exact age of the cars, we will have to use relative ages.


I.  Layers of sedimentary rocks show relative age

A.  Fossils are clues in the story of Earth’s past, but these clues need to be arranged in order for the story to make sense

B.  Relative Age- the age of an event or object in relation to other events or objects

         1.  Used when objects cannot be given exact ages

2.  Today there are still many parts of Earth’s history that cannot be given exact ages

C.  Sedimentary rock layers contain information about the relative age of objects in them

         1.  Over time, these rocks form horizontal layers.

         2.  Bottom layers form first (oldest)

         3.  Top layer forms last (youngest)

         4.  This shows relative age

D.  Undisturbed sedimentary rock

         1.  Straight horizontal layers

         2.  Youngest layer is always above

E.  Disturbed sedimentary rock

         1.  Tectonic plate movements fold rock

         2.  Youngest layer may not be on top


II. Igneous rock and sedimentary layers

         A.  Sedimentary rock layers can be disturbed by igneous rock

                  1.  Molten rock from deep in the Earth can force its way up

                  2.  Igneous rock may be younger than sedimentary layers below it

B.  Plutons- large bodies of igneous rock that form when magma is injected into surrounding rock


III.  Index fossils

A.  Fossils of organisms that were common, that lived in many places, and that existed only during specific spans of time

B.  Most useful index fossils are generally marine organisms

1.  Floaters/Open swimmers:  cephalopod Nautilus (regarded as a “living fossil”)

2.  Bottom-dwellers: brachiopod medusa stage


IV.  Radioactive dating can show absolute age

         A.  Absolute age- the actual age of an event or object

         B.   Radioactivity- the breakdown of unstable chemical atoms over time

1.  Useful as a clock because a particular unstable form of an element always breaks down at the same rate into the same alternate form

C.  Half-Life (Chart pg. 123)

         1.  The rate of change of a radioactive element is measured in half-lives

2.  Definition:  the length of time it takes for half of the atoms in a sample of a radioactive element to change from and unstable form into another form

3.  Different elements have different half-lives ranging from fractions of a second to billions of years

4.  Elements with million and billion-year half-lives are used for dating rocks

5.  Example A:  Uranium 235… Half-life:  704 million years

         a.  Unstable element found in some igneous rock  *Why not sedimentary?

         b.  Over time, breaks down into lead 207

6.  Example B:  Carbon 14… Half-life:  5,730 years

         a.  Unstable form of carbon

         b.  Useful for dating objects around 100 to 70,000 years old

6.  Using radioactive dating, scientists believe that the Earth is about 4.6 billion years old


V.  Piecing together all the information

         A.  Radioactive dating of igneous rocks reveals their absolute age

         B.  Interpreting layers of sedimentary rock shows the relative order of events

         C.  Fossils help to sort out the sedimentary record