Transcript U6-Geologic History Power Point

Earth’ s History
Unit 6
A. Order of Geologic Events
When studying the history of Earth, we must be able to place items
or events in a particular order so we can determine which event
happened first, which happened next, and so on.
We order events in two ways:
1. Relative Dating: Determining whether one event or object is
or younger than another. Example: You are younger than your
Parents, but older than your baby brother or sister.
2. Absolute Dating: Determining exactly how old an object is
or how long ago it occurred. Example: Your teacher is 29 years old.
To determine an order of events using relative ages, we use the
following four principles:
1. Superposition
__________________ Layers of rock on the bottom are older than
the rock layers above them, provided they have not been overturned.
Youngest Rock Layer
Oldest Rock Layer
Cross-cutting relationships
2. __________________________
Rock layers are older than any
faulting are younger
event that cuts across them. Example: Folding&
________________________
than
the surrounding rock. Igneous intrusions are also younger
________________________________________________________.
The sedimentary rocks are
older than the igneous
intrusion. Note the contact
metamorphism.
3. ____________________________________:
Uniformitarianism
Processes that
occur now (erosion, deposition, plate movement, etc.) occur the
same way now as they always have throughout geologic history.
Original Horizontality
4. ____________________________Sediments
that form sedimentary
rocks are usually deposited in flat layers. If we see rock layers that are
not flat and level, we can assume that they were changed (tilted, folded
or faulted) after they formed.
#1
1.___________________________________
2.___________________________________
3.___________________________________
4.___________________________________
5.___________________________________
#2
1___________________________________
2___________________________________
3___________________________________
4___________________________________
#3
1___________________________________
2___________________________________
3___________________________________
4___________________________________
#4
B. Matching Rock Layers
When studying the history of Earth, we must also be able to compare
rock layers found in one place to those found in another.
and matching rock strata (layers) in different
Correlation: Comparing
_______________________________________________
locations to see if they formed at the same time or under similar
_________________________________________________________
conditions.
_________________________________________________________
Correlation of a single rock layer can often be difficult. Often, it is better
to try to correlate a sequence of rock strata. This is often done by a
technique called “walking the outcrop”: __________________________
following along an outcrop
from
one location to another
__________________________________________________________
Walking the outcrop can be limited because the rock strata may be
covered by sediment or other rock. Also, part of the rock strata may be
missing from a location.
Unconformity: _____________________________________________
A gap in the rock record due to rock having been
eroded away or never been formed
__________________________________________________________
There are two other methods used to correlate rock layers; index fossils
and volcanic eruptions.
Index Fossils: _______________________________________________
Fossils used to correlate rock layers. If two rock layers
have the same index fossils, they are of the same age.
__________________________________________________________
To be a good index fossil, a fossil must:
Exist over a wide-spread geographical area
a. __________________________________________________
Exist for a relatively brief period of time.
b. __________________________________________________
Usually form rapid depositional events. Chemistry
Volcanic Eruptions: _________________________________________
of
ash allows precise correlation and placement in time.
__________________________________________________________
C. Absolute Age of Rocks
How can the absolute age of rock strata be determined? Some rocks
contain elements that are unstable.
Elements that are radioactive experience
Radioactive Decay: __________________________________________
break-down of their nucleus to form new elements.
__________________________________________________________
Uranium 238
Radioactive Decay
Lead 206
+ Energy
As the uranium decays, it does so in a very predictable manner and at a
predictable rate.
Half-life: __________________________________________________
The amount of time required for half of the original element
(parent element) to decay to its final element (daughter element).
__________________________________________________________
As each half-life is reached, __________
one half of the remaining radioactive
element has decayed. Notice that you will never reach a point where all
of the radioactive element has decayed. However, there will come a
the amounts remaining are so small they can no longer
point where: _____________________________________________
be measured
_____________.
Radioactive Decay Animation
a fraction of a second or on the order of
Half-lives may be as short as ___________________
millions
or billions of years
_________________________.
The front cover of your Earth Science
Reference Table has radioactive decay data for several radioactive
elements or isotopes.
A radioactive isotope with a appropriate half life must be used to
correctly measure age. If an item that is only several thousand years old
was dated using an isotope with a half life of 5 billion years, not enough
radioactive decay would have occurred to be measurable. If a very old
item was dated using an isotope with a short half life, then not enough
radioactive element would be remaining to measure.
Radioactive Dating Example
QUESTION: A sample is found to contain 100 grams total of the
radioactive element Carbon-14 and its daughter element Nitrogen-14. If
25 grams of that total is C14 and 75 grams is N14, how old is the sample?
ANSWER: First, determine the number of half lives this sample has
undergone.
50
50 grams of N14.
•After one half life, there is _____grams
of C14 & _____
25
•After two half lives, there is ____grams
of C14 & _____
75 grams of N14.
5,700 years (see your ESRT) and there
So, if the half life of C14 is _______
have been ____
2
half lives, then _________
5,700 X _______
2
= __________
11,400
years old!
D. Absolute Age of Rocks
Geologic time is divided into units based upon:
Dominant
life forms on Earth
___________________________________________________
Mass
extinctions
___________________________________________________
Major
tectonic or orogenic (mountain building) events
___________________________________________________
time
The units are not based directly on _______!
Eons
When dividing Earth history, begin with the largest units _______.
They
Eras they are divided into _______;
Periods and they are
are divided into _____;
Epochs
divided into _______.
NOTE: Compared to the overall geologic time scale (about 4.6 billion
insignificant
years), human existence is considered ________________
Evidence found in rock strata and in their fossils allow scientists to infer
what life and the environment were like at that particular time and in that
particular location. For example, coral fossils found in limestone would
warm shallow, ocean water
indicate what type of environment? _____________________________
Of all the species on Earth, over
__________
99% of them have become extinct
since life was first recorded in the rock record.
evolved
Most scientists believe the fossil record indicates life forms have ______
(changed) through geologic time, probably to adapt to changes in their
environment.
The Theory of Evolution
This theory, know as _________________________,
is not a change
from simple life like one-celled organisms to complex like humans. It is
an increase in the variety of life forms.