Download Ch 21 Fossils and the Rock Record

Ch 21 Fossils and the Rock Record
Page Objectives:
1. Describe the geologic time
scale
2. Distinguish among the
following geologic time scale
divisions: eon, era, period,
and epoch.
3. Master the geologic
principles of original
horizontality, superposition,
cross-cutting relationships,
inclusions, and methods for
determining relative age
Section 21.1 The Geologic Time Scale
The rock record ***Take out ESRTs turn to pgs 8+9
The Earth’s history has been divided into time units
based upon fossils contained in the rocks.
Section 21.1 The Geologic Time Scale
Geologic time scale: A record of
Earth’s history from it’s origin
over 4.6 billion years ago to the
present.
The oldest part of the scale is on
the bottom.
The same as the rock layers
(oldest on the bottom).
Eon: Longest time unit and is
measured in billions of years.
Section 21.1 The Geologic Time Scale
Page
There are 3 eons:
1. Archean
2. Proterozoic
3. Phanerozoic
Section 21.1 The Geologic Time Scale
1. Archean
Stromatolites created free oxygen and began to change
the composition of our atmosphere.
Section 21.1 The Geologic Time Scale
2. Proterozoic
Section 21.1 The Geologic Time Scale
3. Phanerozoic
Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
1. Paleozoic
(Paleo “old” zoic “life”
2. Mesozoic
(Meso “middle” zoic “life”)
3. Cenozoic
(Ceno “young’ zoic “life”)
Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
1. Paleozoic
(Paleo “old” zoic “life”
Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
2. Mesozoic
(Meso “middle” zoic “life”)
Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
3. Cenozoic
(Ceno “young’ zoic “life”)
Section 21.1 The Geologic Time Scale
Page
The next largest unit of time is the Period.
Each Era is divided into Periods.
The Precambrian (before the Cambrian period) makes
up about 90% of all geologic history.
The first 2 Eons (Archean and Proterozoic), represent
this Precambrian time period.
All organisms of this time had soft bodies with no
shells or skeletons. Ex Worms and sponges.
Section 21.1 The Geologic Time Scale
Plants and animals evolve
Paleozoic era:
Oceans fill with diversity of life
Cambrian period:
Land plants appear -->land animals
End of Paleozoic marks largest extinction ever
-90% of all marine invertebrates become extinct
Mesozoic: Amphibians left water for land
-large water reptiles emerge
-Dinosaurs emerge
Section 21.1 The Geologic Time Scale
Cretaceous Period:
Dinosaur pop goes down
Mammal pop goes up
Flowering plants and trees emerge.
Section 21.1 The Geologic Time Scale
Mesozoic ends with a large extinction event.
This sharp division in time is the K T boundary
-Dinosaurs and many other groups go extinct here
-65 million years ago
Section 21.1 The Geologic Time Scale
These periods are defined by the life forms that were
abundant or went extinct at that time.
Time range is from 10’s of millions to 100’s of
millions of years.
Many periods named after geographic locations.
Epochs: Are time divisions of a period.
Sect 21.2 Relative-age Dating of Rocks
James Usher (some Irish guy) at the turn of the
century states:
-Earth only 4,000 years old
-Based on a chronology of human history
Sect 21.2 Relative-age Dating of Rocks
James Huton (some Scottish guy) around 1770 states:
Principal of Uniformitarianism : The processes
concerning mountain building, erosion, and sea level
changes that are occurring today have been occurring
since Earth formed.
Sect 21.2 Relative-age Dating of Rocks
Principles for determining Relative age:
Places rocks and events that formed them in order, but
without exact dates.
A) There are four basic concepts for relative age
dating.
1. Principle of original horizontality
2. Principle of superposition
3. Principle of cross-cutting relationships
4. Inclusions
Sect 21.2 Relative-age Dating of Rocks
1. Principle of original horizontality
a) Sedimentary rocks are deposited in
horizontal or nearly horizontal layers
Sect 21.2 Relative-age Dating of Rocks
2. Principle of original Superposition
a) The oldest rocks are on the bottom and the
youngest rocks are on top.
Youngest -->
Oldest layer -->
Sect 21.2 Relative-age Dating of Rocks
3. Principle of cross-cutting relationships
a) An intrusion or a fault is younger than the
rock it cuts across
Sect 21.2 Relative-age Dating of Rocks
4. Inclusions
a) Rock fragments found in sedimentary rocks
are older than the rock itself.
Sect 21.2 Relative-age Dating of Rocks
Other means of determining relative age
-Constant tectonic movement and erosion make it hard
to find an undisturbed sequence of rocks.
Unconformity: A gap in the geologic rock record
Sect 21.2 Relative-age Dating of Rocks
***FYI
Disconformity When sedimentary rocks overlie other
horizontal sedimentary rocks.
Angular Disconformity: When sedimentary rocks
overlie at an angle to horizontal sedimentary rocks.
Nonconformity: Sedimentary rocks overlie nonsedimentary rocks suggesting uplift of the nonsedimentary rocks.
Sect 21.2 Relative-age Dating of Rocks
Correlation of rock
strata
Correlation: the
matching of outcrops
of one geographic
region to another.
-Geologists can use
distinctive fossils or
unique rock and
mineral formations to
help.
Sect 21.3 Absolute-age Dating of Rocks
Absolute dating: The actual age of the rock or fossil is
determined.
Look at your ESRTs cover for carbon
Radioactive decay: The emission of radioactive
particles and the resulting change into other elements
over time.
Once the emission begins the rate of decay is a
constant regardless of the environment, pressure,
temperature, or other physical changes.
Sect 21.3 Absolute-age Dating of Rocks
(parent)
--> (daughter)
Original radioactive material --> New material
ex:
U-238 --> Pb 206
Uranium --> Lead
Sect 21.3 Absolute-age Dating of Rocks
Radiometric dating: Process of calculating the ratio of parent
nuclei to daughter nuclei within a given sample of a rock or a
fossil.
Isotope: Atoms of the same element that have different mass
numbers and the same chemical properties.
Sect 21.3 Absolute-age Dating of Rocks
Mass Number: Combined number of protons and neutrons in
the nucleus of an atom.
Half life: Period of time it takes for a radioactive isotope,
such as carbon -14, to decay to one-half of its original amount.
Page
Sect 21.3 Absolute-age Dating of Rocks
Carbon dating very useful
for dating organic matter
but is limited to about
75,000 years.
The half-lives of various
elements are on the first
page of your ESRT’s
Carbon -14 is 5.7 X 10 the
third power. This = 5,700
years
Sect 21.3 Absolute-age Dating of Rocks
***Carbon dating could not be used to date dinosaur bones
since they went extinct 65 million years ago.
In a similar sense Uranium with a half life of 700,000 years
would not be very useful for dating objects only a couple
thousand years old.
Sect 21.3 Absolute-age Dating of Rocks
Sect 21.3 Absolute-age Dating of Rocks
Original parent
--->
Daughter
First half life
--->
1/1
Second
Third
Fourth
--->
--->
--->
½
¼
50%
25%
1/8
12.5%
???
Sect 21.3 Absolute-age Dating of Rocks
Other ways to determine age
-Tree rings
Dendrochronology: the science of comparing annual
growth rings in trees to date events and changes in
past environments.
-Seasonal climatic changes
Sect 21.3 Absolute-age Dating of Rocks
Other ways to determine age
Varves: Bands of alternating
light and dark color sediments
of sand, clay and silt. These
are caused by seasonal changes
in a lake.
Summer --> more NRG--> thin
layer of light colored deposits
Winter --> Less NRG --> thick
layer of dark (organic rich)
deposits
Sect 21.3 Absolute-age Dating of Rocks
Page
or glossary page
Key Bed: Sediment layer that serves as a time marker
in the rock record and results from volcanic ash or
meteorite impact debris that spread out and covered
large areas of Earth.
Sect 21.4 Remains of Organisms in the
Rock Record
Fossils: Are the evidence of remains of once living organisms.
Evolution: Adaptation of life forms to changing environmental
conditions.
Original preservation: Parts of an organism that have not
undergone any change since the organisms’ death.
Ex: insects trapped in tree sap (now amber) like in “Jurassic
Park”
Sect 21.4 Remains of Organisms in the
Rock Record
altered hard parts: The parts that have been replaced by
minerals or by recrystallization.
This process is called permineralization
Index fossils: Fossils that can be used by geologists to
correlate rock layers over large geographic areas, or to date a
particular rock layer. The four criteria are:
1. Easily recognized
2. Abundant
3. Wide spread
4. Lived for a short period of time
Sect 21.4 Remains of Organisms in the
Rock Record
Mold: is formed when the original part of an organism
creates an imprint in the rock after the original part was
weathered away.
Cast: When a cavity left by an organism is filled by
minerals or sediments.
Vocabulary quiz
Define the following terms:
1. Photosynthesis
2. Absolute age
3. The principle of superposition
4. Correlation
1. Which of the following represents 90% of Earth’s
geologic history?
A. Cenozoic
C. Paleozoic
B. Mesozoic
D. Precambrian
2. Which term describes when sedimentary rocks
overlie non-sedimentary rocks suggesting uplift of the
non-sedimentary rocks?
A. Unconformity
C. Angular Disconformity
B. Disconformity
D. Nonconformity
3. In each diagram, which letter represents the oldest
rock?
A. A
B. B
C. C
D. There is no way to tell
4. Which of the following is not a characteristic of an
Index fossils?
A. Lived for a short period of time
C. Wide spread
B. Abundant
D. Rare
5. Substance X has a 100 g and a half life of 20
minutes. How many grams of substance X will be left
after one hour?
Bonus:
How many years will it take to have 25% of the
original carbon left?