Chapter 5
Clues to Earth’s Past
This Chapter is made of 3 Sections
Sec 1 Fossils,
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:
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:
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” –
“THE
PRESENT IS THE KEY TO 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.
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
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
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
Stability
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.
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:
|
Chemical |
Type of stuff |
Why |
Actual material |
Half life |
|
Carbon-14 |
good for stuff that was alive |
since most living things are made of carbon |
dating bones, wood |
5730 years |
|
Potassium-40 |
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 Coal Molds
& Casts Original
Remains Trace Fossils Trails & Burrows Index Fossils Fossils & Ancient
Environments |
Superposition Oldest Rocks on the Bottom Relative Ages Other Clues Help Unconformities (3 types) Angular
Conformities Disconformity Nonconformity 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 Uniformitarianism |
|
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 because: 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: Scavengers Agents of physical destruction: Waves Currents 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 minerals. 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. Coal 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 cement) 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 particles 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 |
Superposition Oldest Rocks on the Bottom Relative Ages Other Clues Help Unconformities Angular
Conformities Disconformity Nonconformity 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 Uniformitarianism |
More later