Chapter 5

Clues to Earth’s Past

This Chapter is made of 3 Sections


Sec 1 Fossils,

Sec 2 Relative Ages of Rocks and

Sec 3 Absolute Ages of Rocks


Dr. Drantch’s big picture intro and discussion

This should help to frame and help you understand why we are studying this material and how it is just another application of the way scientists work and answer questions


The Big Picture

Obviously scientists want to understand Earth’s past.

Three tools, each separate & needed to complete testing the hypothesis - answering the question about how Earth  has changed and how organisms evolved during those changes.


Since we (modern man) weren’t  there when these events were occurring and man himself wasn’t there when these events were occurring,  modern scientists, have to use whatever evidence is left behind to fit the pieces of the puzzle together. The puzzle pieces are the clues.


1.Fossils are hard touchable pieces of evidence.


2.Relative age creates a timeframe by comparing (without knowing a real time) things that happened at one place to things that happened in another place. This is an approximation, an bigger estimation


3.Absolute age is a scientific and chemical way to assign a real date of when a fossil, an artifact of the past, existed. This is a pretty specific time. For each material you date you must choose a chemical within it that has a long or short enough half life. From Potassium-40 which is 1.25 billion years to Carbon –14 for items that are no older than 75,000 – a very recent time when we talk about hundreds of millions of years of earth age

When we put all these clues together it helps to give a better answer to what happened at about (give or take a few million years) when it happened.


One of the questions that has scientists still arguing depending how they interpret the clues is “What caused the dinosaurs to die?”


Another question “Were there other times of global warming?” and

                             “How many ice ages were there? “

                              “Will there be others?”


We can only hypothesize our answers and use the fossils before us to put up strong answers but they are not laws they are theories at best. As and we know about theories and hypothesis we must change them as better technology gives us new and more accurate evidence.


Taking a further step back – (remember we want you to keep zooming in and stepping back. Big picture, bigger picture then closer to the details then back to a bigger picture) –


You have learned that science answers questions and the only acceptable answers are with evidence. The way we solve the mysteries questions is by applying our critical tool the scientific method. ORHEOAC



Now lets look at the tools we use to build our answers and ask more questions.


Lets start this Chapter looking at the end first:

Section 3 Absolute Ages of Rocks


Dr. Drantch’s intro to this section:

The past – that which happened already is long distance of time (one second ago to 400 million years ago)– when talking about earth age, and so the past is even divided into sections of pasts. (all related to earth age). When we talk in relation to people age we talk about one hour ago, yesterday, last month, when I was 9, etc. All relative ages of people.


When talking about earth we talk about: (these are made up numbers just to help you understand)

A. the immediate past –               

lets say 50 million to 100 million years ago,

B. the not so immediate past-       

lets say from 101 million years ago to 200 million years ago

C. the distant past

            lets say anything from 200 million to 400 million years ago


We are currently really dating rocks from earth’s distant past that are older than 3.96 Billion years.

 Who knows? As our tools get better, maybe we will be able to have a tool that names the incredibly distant past which was – (I don’t know what a number is older then 4.5 billion years ago. )  


Notes from the book:

Uniformitarianism(can u spell it backwards without looking?)

Principle created by the Scotsman, James Hutton, which stated “ The processes happening on earth today are the same ones that changed the earth in the past”


Ask yourself and if you can answer correctly – What are those processes? – then you know your stuff!!!!    If not – LOOK IT UP and Learn IT.

His thoughts were that

       – current earth processes are slow – ie erosion – and so they must have been slow throughout earth’s history.  And he said that 309 years ago – people were even smart then – and you think 80 or 90 years is ancient history .

(Hey, the book said he inferred – that reminds me – what is an inference?)


Our current scientists recognize –( that’s a conceited but polite fancy way to say – “we only partially agree”)

There are really two process 1. the slow everyday kind that change things over millions of years and

                                               2. the violent, rapid and cataclysmic events that make major widespread changes quickly and then over long term – such as an asteroid strike – think dinosaur extinction – still a theory though.


Absolute Ages

The real age in years of a rock or other object.

            This is figured out using knowledge of atoms that make up materials

                        We will study chemistry later in detail but we must now talk a little chemistry – 275 in txt for picture of atoms

Radioactive decay

Atoms have 2 areas  which hold the 3 things they are made of:

            Area 1 is the nucleus

                        The place where the

 protons and neutrons stay

             Area 2 is the electron ring

                        The place where the electrons stay


            Isotopes are versions of something – they are nearly identical but have aged or changed, they get named as different chemicals for instance:

carbon –14 ages and creates an isotope called nitrogen - 14      (carbon –14                  half life is            5,730 years )                      

potassium-40 ages and creates an isotope called argon - 40    (potassium-40            half life is            1.25 billion years)       


As an example

If you write a story that is your first version – isotope one –        carbon-14

If you go back and edit it and take out some things – it is similar, another version – isotope two   -        nitrogen-14

If you go back again and edit – create a third version maybe add or subtract from the story it is similar to the two before but may have small differences plus or minus – isotope 3

And this keeps going on



            Something that is unstable loses control

In chemistry - Stability is graded by how easily something loses control and loses its part or parts

       Very stable – holds tight doesn’t easily change

        Very unstable – has little control of itself and easily changes – an old idiom was “a person would fly off”- which meant the person would lose their temper and get angry easily

Some atoms are unstable and they lose control of their parts


When a material ages it “ breaks down” it loses some of its parts – it creates isotopes

This process of chemical aging and losing some of your chemical parts  is called  radioactive decay.


Half life

Really not that hard to understand

Each chemical (stable or  unstable)  material has its own half life

Lets make an analogy – for instance the normal life of  a:

                         Belgium water dog is 14 years      so its normal half life is 7 years   (we will consider that relatively stable)               (compared to other dogs)

                         Galapagos turtle     is 120 years                         half life is 60 years                                     very stabile     (much more stabile when compared to a dog)  

                         Common house fly  is 10 days                            half life is  5 days                                        unstable                    (when compared to any of the above)




Practical uses of radiometric dating:



Type  of stuff


Actual material

Half life


good for stuff that was alive

since most living things are made of carbon

dating bones, wood


5730 years


Old rocks

Has a long half life

Old rocks

1.25 billion years


When can’t radiometric dating be used? 

Can’t be used on sedimentary rocks because they are really composed of an eroded other rock and the eroded rock is the one that will be aged

                        For instance – a sedimentary rock composed of igneous rock particles – the sedimentary rock is really younger then the igneous rock it is made of but the date /age will reflect the igneous rock and so the sedimentary rock will appear very old when it isn’t




OK and now from the beginning!:

A big picture statement

       The evolution of organisms throughout geologic time as shown by the fossil record is our major chapter focus.

       The changes in organisms on earth can be used to obtain relative ages of rocks and to date geologic events.


Keep these things in the back of your mind and

be able to explain how what you are learning does this and

you’ve got it beat!



Chap 5 objectives: (there are 9 total)

Section 1

·       list the conditions necessary to form fossils

·       describe several processes to fossil formation

·       explain how fossil correlation is used to determine rock ages

·       determine how fossils can be sued to explain changes in earth’s surface, life forms, and environment


Section 2

·       describe methods used to assign relative ages to rock layers

·       interpret gaps in the rock layers

·       give an example of how rock layers can be correlated with other rock layers

Section 3

·       identify how absolute age differs from relative age

·       describe how the half lives of isotopes are used to determine a rock’s age





Section 1 Fossils

Section 2 Relative Ages of Rocks

Section 3 Absolute Ages of Rocks

Traces of the Distant Past

Formation of Fossils

             Condition Needed for Fossil Formation

Types of Preservation  (6 types)

            Mineral Replacement

            Carbon Films


            Molds & Casts

            Original Remains

            Trace Fossils

            Trails & Burrows

Index Fossils  

Fossils & Ancient Environments


          Oldest Rocks on the Bottom

Relative Ages

          Other Clues Help

Unconformities (3 types)

           Angular Conformities



Matching Up Rock Layers

           Evidence Used for Correlation





Absolute Ages

Radioactive Decay

          Alpha & Beta Decay

          Half Life

Radiometric Ages

          Radiocarbon dating

          Age Determination

          The Oldest Known Rocks






Section 1 Fossils

Section 2 Relative Ages of Rocks

Section 3 Absolute Ages of Rocks

Traces of the Distant Past

              Paleontologists –scientists that study fossils

                      Learn about extinct animals from their fossil remains.               .  

Formation of Fossils

Fossils are the remains, imprints or traces of prehistoric organisms.

       They have helped us understand when :

                     life 1st appeared 

                    plants & animals lived on land

                    organisms became extinct

       Fossils are evidence of when & how an organism lived

            Most of the time the remains of dead animals disappear quickly 


                      Scavengers eats and scatter their remains

                      Bacteria and fungi cause the thing to rot (decay) & disappear                 

             Conditions Needed for Fossil Formation

                     Fossilization depends on how well an organism is:

                                    Protected from:


                                             Agents of physical destruction:



                    Protection can come from sediment burying the body quickly

                        Ie: fish in creek or lake gets sediment stopping the waves or

                            from moving or eating it.

                    Quick burial alone is NOT enough, they do better if:

                              They have hard parts such as:

                                            Bones, shells or teeth

                        Because: scavengers are less likely to eat hard parts

                                       hard parts decay (rot) slower then soft parts.                           

Types of Preservation

            Mineral Replacement

                Most hard parts of organisms have tiny spaces within them.

                           Filled with cells, blood vessels, nerves, air

                           After death these spaces become empty

                    If buried underground the groundwater seeps in deposits minerals in the spaces

                    Permineralized remains are fossils where the spaces are filled with


                                   Sometimes DNA might be preserved in encased within the mineral.

                                                 Some of the original organisms hereditary genetic code can be

                                                 harvested. (The movie Juassic Park did it from amber)

                    Sometimes the hard parts are actually replaced by the mineral

                             A silicon solution (like dissolved quartz or feldspar) creates an

                            exact replica but made of stone. (think petrified wood-Petrified Forest)     

                    Sometimes the solution may dissolve the original shell & replace it

            Carbon Films

                  Tissues of organisms are made of carbon & other chemicals

                  Some fossils contain only carbon

                  Fossils can be made when an organism dies is covered by sediment

                        Heavy sediments heats and squeezes the remains

                         The liquids and gases in the organism are pushed out

                               Only  a thin carbon silhouette (a ribbon, streak, shadow)

                                  remains called a carbon film.


                   Swamps cause lots & lots of dead plant life to build up

                   These remains become completely carbonized

                              Carbonized  =  (squeezed & cooked down just to carbon only)

                                 Coal is an important (fossil fuel – think 6th grade!) fuel source

                    Since it is completely carbonized all fossil remains are destroyed                  

            Molds & Casts

                    Impressions form when something is pushed into a clay or

                      dough like material (think of putting your handprint in mud or wet


                          In nature if something falls into soft sediment or mud it can

                                  they be entirely covered and buried in more sediment.

                         .Compaction and cementation turn the surrounding sediment into

                          rock  (this has now encased the organism in rock ie limestone.)

                                      cementation = minerals fill and glue the air spaces between the tiny sediment  


                                Pores (tiny holes in the surrounding sedimentary rock) in the

                            other rock layers (above & below) now allow water and air  

                            to decay or dissolve the organism (the hard and soft parts).

                         This dissolving and/or decaying matter washes away and

                            leaves a hollow (empty space) called a mold.

                                      (as in how you made play dough shapes )

                               The empty space (mold) gets filled in with mineral rich water

                             or other sediments.

                         This new filler material hardens and is called a cast.

                Cast = a rock fossil that forms when either a liquid or sediment fills

                     in a mold.                           

           Original Remains

                 (This speaks for itself. )

                 When the actual dead organism is surrounded and protected by:

                      a.  a tree resin and forms amber or

                      b. it dies and is frozen in the ice or

                      c. falls in an ancient tar like mush and is trapped & protected  

                                     (the La Brea Tar Pits – they’re really cool)

            Trace Fossils

                       Think about how the movie stars put their hands in concrete in

                           Hollywood – they have made a trace fossil

                        Trace fossils information can be interpreted to tell about animal

                          speed, size, behavior and more.

            Trails & Burrows

                         Animals dug holes or slithered thru areas and left their imprint

                              which stuck as the material became rock.

Index Fossils

         Fossils can tell us how:

                    how an organism changed over time

                    how long it stayed on earth without or before it evolved

                    how quickly it changed in a short period of time

         Special fossils are chosen to act as a place mark or index

Index fossil = fossil of a species that existed on Earth:

                For a relatively short period of time

                Were plentiful (abundant, very many of them)

                Were spread in very large areas of the map (geography)

          When an index fossil is identified it helps geologists to:

                 Estimate the ages of rock based on that index fossil.

If an index fossil is not found – the range chart is used (study photo in book p125)  

Fossils & Ancient Environments

    Fossils have helped us understand that huge parts of a country

       (which are now mountains or deserts)

        were once areas covered by oceans or seas (see book photo p126)

     Shallow Seas

               Fossils can provide information about:

                  The ages of rocks

                   History of the rock layers

                   The climate and type of environment when the rock was formed




          Oldest Rocks on the Bottom

Relative Ages

          Other Clues Help


           Angular Conformities



Matching Up Rock Layers

           Evidence Used for Correlation





Absolute Ages

Radioactive Decay

          Alpha & Beta Decay

          Half Life

Radiometric Ages

          Radiocarbon dating

          Age Determination

          The Oldest Known Rocks







 More later