The Rock Record
Chapter 32 Earth Science Book
read pages 596-609
restate & answer Topic Questions 1-14
Topic 1:
Page 597 Earth Textbook
Telling Time
James Hutton (1726-1797).
Proposed that most geologic
processes happen very slowly.


Relative Time - Position in that sequence; older or
younger than nearby layers

Absolute Time - Numerical Age can be
determined through analysis of the products of
radioactive decay
Topic 1:
Page 597 Earth Textbook
Stratigraphy
Layered rocks contain
clues about past
environments at/near
surface.
 Sequence and relative
ages provide basis for
reconstructing Earth’s
history.
 The study of strata is
called stratigraphy.

Topic 1:
Page 597 Earth Textbook
Stratographic Classification
The basis of rock stratigraphy is the formation.
- a collection of similar strata that are sufficiently
different from adjacent groups
 - basis of physical properties they constitute a
distinctive, recognizable unit that can be used for
geologic mapping over a wide area.

0072
Topic 1:
Page 597 Earth Textbook
Geologic Column & Geologic Timescale


19th century –
 geologists assembled a
geologic column
 composite column
containing, in
chronological order, the
succession of known
strata, fitted together
 based of fossils or other
evidence of relative
age.
This is the geologic time
scale.
Topic 2:
Page 597 Earth Textbook
Relative Time
Relative ages: younger or older
*Law of Superposition
*Law of Cross-cutting Relationships
*Law of Included Fragments
*Unconformities
Topic 2:
Page 597 Earth Textbook
Law of Superposition
Topic 2:
Law of Superposition:
Grand Canyon Strata
9
Topic 2:
Page 597 Earth Textbook
Law of Superposition: Sediments
 Most
sediment is laid down in the sea, in
shallow waters, or in streams.
 Each new layer is laid down
horizontally over older ones
 Principle
of original horizontality sediments are deposited in strata that are
horizontal or nearly horizontal
Topic 2:
Page 597 Earth Textbook
Law of Superposition: Horizontality
11
Topic 2:
Page 597 Earth Textbook
Topic 2:
Page 597 Earth Textbook
Law of Cross-Cutting Relationships
A
fault is a break or crack in Earth’s crust
where rocks shift their positions.
 Law
of cross-cutting relationships = a fault
or igneous intrusion is always younger than
the rock layers it cuts through
Topic 2:
Page 597 Earth Textbook
Law of Cross-Cutting Relationships
14
Law of
Cross-Cutting
Relationships
Topic 2:
Law of Cross-Cutting Relationships
1) Based on the law of superposition, what is the
oldest layer pictured?
Igneous intrusion
Fault
Topic 2:
Law of Cross-Cutting Relationships
1) Based on the law of superposition, what is the
oldest layer pictured? A is the oldest, D is
youngest
Igneous intrusion
Fault
Topic 2:
Law of Cross-Cutting Relationships
1) Based on the law of superposition, what is the
oldest layer pictured? A is the oldest, D is
youngest
2) Based on the law of crosscutting relationships,
which is older, the fault or the igneous intrusion?
Igneous intrusion
Fault
Topic 2:
Law of Cross-Cutting Relationships
1) Based on the law of superposition, what is the
oldest layer pictured? A is the oldest, D is
youngest
2) Based on the law of crosscutting relationships,
the igneous intrusion is older than the fault ?
Igneous intrusion
Fault
List the labeled formations from youngest to oldest.
List the labeled formations from youngest to oldest.
C B A E D
Topic 2:
Page 597 Earth Textbook
Law of Included Fragments
 Pieces
of one rock found in another rock must
be older than the rock in which they are found.
 “ingredients are older than the rock”
included
fragments
are older
than the
rock
Law of
Included
Fragments
Topic 2:
Page 597 Earth Textbook
Unconformities
 Unconformities
are breaks in the geologic
record that show deposition stopped for a
period of time, rock was removed by erosion,
and deposition resumed.
Angular unconformity
0071
Topic 2:
Page 597 Earth Textbook
The Significance of Unconformities



Evidence of former
seafloors uplifted by
tectonic forces and
exposed to erosion.
Later tectonic forces
depress the surface.
The surface, in turn,
becomes a site of
deposition of
sediment.
Topic 3:
Page 598 Earth Textbook
Geologic Timetable
summary of major events in Earth’s History
5 Eras of Earth’s History
1. Archean: 4.5-2.5 Billion Years Ago
2. Proterozoic: 2.5 BYA-570 MYA
3. Paleozoic: 570-250 MYA
4. Mesozioc: 250-65 MYA
5. Cenozoic: 65 MYA- Today
*Eras broken into Periods, Periods into Epochs
Topic 3:
Page 598 Earth Textbook
Geologic Time and the Rock Record
Chapter 8
Chapter 8
Topic 3:
The
divisions
of the
geologic
time scale.
Figure 11.10
Topic 3:
Topic 3:
Scale of the Geologic Time
Topic 3:
Topic 4:
Page 602 Earth Textbook
Fossil Formation
*Any evidence of earlier life preserved in rock
1.original remains: unchanged organisms
2. replaced remains: become calcite, silica
3. molds, imprints, casts or hollows
4. trace fossils: trails burrows, footprints
Topic 4:
Studying The Past
What is a Fossil?
Definition: The evidence or remains of once-living plants or
animals
Why Do We Study Fossils Found in Rocks?
A. To provide evidence of the past existence of life forms
B. To provide information about past environmental conditions
C. To provide evidence that populations have undergone change
over time due to environmental changes (evolution)
Topic 4:
Fossil Formation
Original Preservation
Description: plant or animal remains that have not undergone
change since death.
a. Uncommon because frozen, extremely dry, or oxygenfree environments are required to form these fossils
b. Examples:
• Mummified humans
• Frozen organisms (Ice Man)
• Mammoths & cats in La Brea Tar Pits
• Fossilized insects in tree sap (amber)
Topic 4:
Fossil Formation
Replaced Hard Parts
Description: all organic material has been removed and the
hard parts of the organism have been changed
a. Minerals seep in slowly and replace the original
organic tissue, forming a rock-like fossil
b. The fossil has the same shape as the original object,
but is chemically more like a rock!
c. Examples:
• Petrified wood
• Recrystallized shells
Topic 4:
Fossil Formation
Molds and Casts
Description: Fossils that do not contain any shell or bone
a. A mold is formed when original shell parts are
weathered and eroded, leaving an impression of the
shell.
b. This cavity might later become filled with minerals or
sediment to create a cast.
c. Examples:
• Common with shellfish
Topic 4:
Fossil Formation
E. Trace Fossils
1. Description: Indirect evidence of plant and animal life
a. Provide information about how an organism lived,
moved or obtained food
b. Examples:
• worm trails
• burrows
• footprints
How
Fossils
Are
Formed
Topic 4:



Freezing (refrigeration)
Best means of preservation of ancient materials.
Rare - continually frozen from death til discovery.
Mammoths and wooly rhinoceros found in ice from
Alaska and Siberia.


Specimens with flesh, skin, and hair intact have been
found.
suggest that they were flash frozen, with food still in the
mouth and stomach.
Topic 4:
Page 602 Earth Textbook
Drying (desiccation)
Mummified bodies
 discovered in arid parts
 Soft tissues preserved if completely dried.

Topic 4:






Asphalt
La Brea Tar Pits - 100 pits filled with sticky asphalt or tar.
formed by crude oil seeping through fissures in the earth.
lighter elements evaporated leaving thick sticky asphalt.
pits are famous for Pleistocene fossils
The fossils date between 10 and 40 thousand years old.
Asphalt is an excellent preservative.
Topic 4:
Page 602 Earth Textbook
Amber (Unaltered preservation)


Insects, spiders, and even small lizards have been found,
nearly perfectly preserved in amber.
Over millions of years, sap with our fly inside is
polymerized and hardened into amber.
Permineralization (Petrification)
most common
 Minerals fill the cellular spaces and crystallize.
 Shape of the original plant or animal is
preserved as rock. Sometimes the original
material is dissolved away leaving the form and
structure but none of the organic material
remains.

Topic 4:
Carbonization
Plant leaves, and some soft body parts of fish,
reptiles, and marine invertebrates decompose
leaving behind only the carbon.
 This carbon creates an impression in the rock
outlining the fossil, sometimes with great
detail.

Topic 4:




Casts & Molds
molds and casts of organisms which have dissolved or rotted
away, leaving only a trace of their existence.
Casts and molds are types of fossils where the physical
characteristics of organisms have been impressed onto rocks.
buried or trapped in mud, clay, or other materials which
hardened around them - leaving molds of the organism.
There are two types of molds: external and internal.
Topic 5:
Page 603 Earth Textbook
Fossils: Evidence for Evolution
The process of gradual change that produces new
life forms over geologic time.
Natural
Selection:
survival of
the fittest.
Topic 5:
Page 603 Earth Textbook
Fossils: Evidence for Evolution
Relative-Age Dating:
1. Definition: Dating rocks and fossils by placing them in
chronological order without exact dates.
2. Geologic Principles (used in this dating process):
a. Original Horizontality
• Sedimentary rocks are deposited in horizontal layers
b. The Law of Superposition
• in an undisturbed sequence the oldest rocks are at the
bottom and each successive layer is younger
c. Principle of Cross-Cutting Relationships:
• an intrusion or a fault is younger than the rock it cuts
across
Topic 6:
Page 604 Earth Textbook
Index Fossils & Key Beds
Description: Remains of unique species that can be used to
correlate rock layers or to date a particular rock layer
a. Must be easily recognized, abundant, and widely
distributed geographically
b. Must have lived during a relatively short time period
c. Examples:
• The mollusk Ecphora has a distinctive shape
Topic 7:
Page 604 Earth Textbook
Rock Correlation
matching of rock layers from one area to another
Topic 7:
Page 604 Earth Textbook
Rock Correlation
Topic 8:
Page 605 Earth Textbook
Other Uses of Fossils
*fossils indicate past climate
*micro fossils used in oil exploration
Topic 9:
Page 606 Earth Textbook
Absolute Time: Tree Rings
Absolute-Age Dating:
Definition: Dating rocks and fossils by using techniques
to determine their actual age. (actual numbered dates)
Methods:
Tree Rings and Seasonal Climatic Changes
• Each tree ring represents 1 year of growth
• Good growth years are thicker
• Although accurate, this method
can be used to date very far
back in time.
Topic 10:
Page 606 Earth Textbook
Absolute Time: Varves
Varves are annual layers found in some sedimentary
rocks that can be counted like tree rings.
Winter: dark fine
sediment
Summer: Thick light
colored layers
Topic 11:
Page 607 Earth Textbook
Absolute Time: Radioactivity
Atoms of the same element have different
numbers of neutrons (isotopes)
Topic 11:
Page 607 Earth Textbook
Kinds of Radioactive Decay
1. Alpha decay: emission from the nucleus of a
heavy atomic particle consisting of two neutrons
and two protons called an α (alpha) particle.
2. Beta decay: emission of an electron from the
nucleus.
3. Gamma ray emission: emission of γ rays (gamma
rays), which are very short-wavelength, highenergy electromagnetic rays.
 Gamma rays have no mass, so gamma ray
emission does not affect either the atomic
number or the mass number of an isotope.
Figure 11.12
Figure 11.14
Topic 11:
Page 607 Earth Textbook
Radioactive Decay

A few isotopes, such as
14C, are radioactive.
 instability within the
nucleus.
 transform
spontaneously to a
nucleus of a more
stable isotope of a
different chemical
element.

The process is called
radioactive decay.
Topic 12:
Page 607 Earth Textbook
Absolute Time: Half-Life
The time it takes for half of a radioactive
element to decay to a stable end product
Page 607 Earth Textbook
Half-Life
After a 2nd
half-life has
passed half of
the remaining atoms
have also changed
After 1
half-life has
passed, half
the atoms
have changed
or emitted
energy.
Topic 12:
Page 607 Earth Textbook
Absolute Time: Half-Life
Suppose a rock sample has 1/64th of the expected
amount of radioactive material.
1  1/2  1/4  1/8  1/16  1/32  1/64
This means that the sample has been through 6 halflives.
If each half-life is 100,000 years then the rock layer
is 600,000 years old
Topic 12:
Page 607 Earth Textbook
Absolute Time: Half-Life
Topic 13:
Page 608 Earth Textbook
Absolute Time: Radiocarbon Dating
 14C
is useful for dating young samples.
 Half-life of 14C is short - 5730 yrs
 14C continuously created in the atmosphere
through bombardment of 14C by neutrons created
by cosmic radiation.
Topic 14:
Page 608 Earth Textbook
Other Radiometric Methods



Radioactivity in a mineral is like a clock.
The length of time this clock has been ticking is the
mineral’s radiometric age.
Many natural radioactive isotopes can be used for
radiometric dating, but six predominate in geologic
studies:
 Two radioactive isotopes of uranium plus
radioactive isotopes of thorium, potassium,
rubidium and carbon are used.
 In practice, an isotope can be used for dating
samples that are no older than about six half-lives
of the isotope.
Topic 14:
Page 608 Earth Textbook
Age of the Earth
“Moon dust” brought
back by astronauts,
is 4.55 billion years
old.
 The Earth was
formed
approximately 4.55
billion years ago.

The Rock Record
Chapter 32 Earth Science Book
read pages 596-609
restate & answer Topic Questions 1-14