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Geologic Time
Chapter 9
Relative Dating

Determining the age of a rock in
relation to one another.
 Does not give exact age
 Use
a collection of strategies to
determine relative age of a rock
Relative Ages of Events
 Original
Horizontality - Most
sediments are deposited in
horizontal layers.
 If
they are not horizontal some
event must have affected the layers
after they formed.
Law of Superposition

When a layer is deposited, any rock
unit that it rests on must be older
than that layer.
 Ex- Stack of papers, the oldest
paper is the one on the bottom.
Younger Units Deposited on
Older
Units
Tan sediment deposited
over older rock
Red layers deposited
over tan
Third layer is
youngest and
is on top
09.01.a
Law of Embedded Fragments
 As
a new rock forms, older
fragments of rock can be
embedded into it.
 So…Embedded fragments are
OLDER than the rock in which
they are embedded.
 Ex: Chocolate chip cookie
Younger
Sediment or
Rock Can
Contain
Pieces
Determine which
rock is
younger in each image
of Older Rock
Conglomerate
Dark basalt
Breccia
Gray layers
Gray
granite
Dark metamorphic rocks
09.01.a
Igneous Intrusions
 There
are several types of
igneous intrusions.
 Sill- horizontal intrusion
 Dike- vertical intrusion
 Batholith-large scale intrusions
Law of Crosscutting
 When
igneous intrusions or
faults cut through a rock layer.
 The intrusion is younger than the
rock that it cuts through.
 Ex. Toothpick in a sandwich
Younger Rock
or Feature
Can Crosscut
Determine
which
rock or
an
Older
Rock
feature is younger in
each
orimage
Feature
Red
layers
Dark dike
Limestone
Fractures
Tan
dikes
Dark
igneous
rock
09.01.a
Unconformity
A
buried erosion surface over a
layered surface.
 You cannot account for the
missing layers.
 General term for missing layers.
What Does an Unconformity
Represent?
Limestone folded and
eroded
Gray limestone
deposited under
water
Conglomerate
deposited on
top of eroded
surface forming an
unconformity
Unconformity
09.04.a
Nonconformity
 1st
type of an unconformity.
 Occur over rocks that are not
layered. Sedimentary over ig. or
meta
Disconformity


2nd type of unconformity
When an unconformity occurs over
horizontal sedimentary layers and new
layers form on top.
Absolute Dating
 Relative
dating does not tell you
the actual age of the rock.
 Absolute dating uses radioactive
decay to determine the actual age
of the rock.
Historical Methods

Tree rings- each ring usually
represents a single year.
 Size and color depend on
environmental Conditions.
 Different trees have similar rings for
same years.
 Used to date back to 2000 B.C.
Varves
 Varves-
sediment that is
deposited on a yearly cycle.
 Each annual varve is distinctive.
 Varves in one lake can be
correlated to varves in other
lakes.
Radioactive decay
 Some
particles are unstable and
break down emitting subatomic
particles.
 Every radioactive element has a
specific rate at which it breaks
down.
 That rate can determine the age
of the substance.
Types of Decay
 Radioactive
isotopes emit or
capture tiny particles.
 Three types of decay
Alpha- positive particle
Beta- negative e- emitted
Electron capture- p+ captures
e- and becomes a neutron.
Isotopes
 Isotope’s
atomic number changes
and becomes a new element.
 Parent isotope- original element
 Daughter isotope- product of
decay.
Ex- uranium-238 lead 206
Half-life
 Half-life
– The amount of time it
takes for ½ of a sample of a
radioactive isotope to decay. (1/2
of the radioactive atoms)
How Does Radioactive Decay
Occur?
Before decay,
unstable
parent atoms
Example
for 1000
atoms
Half the parent atoms
decayed to daughter
atoms (time = half life)
09.02.a
After a second half
life, only ¼ parent
atoms remain
Before Any
Decay
After One
Half-Life
After Two
Half-Lives
Atoms of
Parent
1,000
500
250
Atoms of
Daughter
0
500
750
Radiocarbon Dating
uses carbon-14
 carbon-14 is radioactive
 half-life is 5370 yrs
 Produced naturally from reaction
between N-14 and cosmic rays


Only works on organic material and only
on sample that are under 70,000 years
old.
Uranium-lead dating
 Used
to date very old rocks
because uranium has a half-life of
4.5 billion years.
 Does not work on rocks younger
than 10 million years old.
Rubidium-Strontium
 Half-life
of rubidium is 47 billion
years.
 Only works with extremely old
rocks.
 Used mainly on igneous rocks.
Potassium-Argon Dating
 Used
to date metamorphic and
sedimentary rocks as well as
igneous.
 Can date rocks as young as
50,000 years old.
Fossils
 Remains,
traces, or imprints of a
plant or animal that are preserved
in a rock.
 Occurs several ways.
Original Remains




The original unchanged remains of
plants or animals.
Rare!
Example: Woolly mammoths found
frozen in permafrost
Example: prehistoric insects trapped in
resin (a sticky sap that oozes from
trees).
 Resin hardens into amber
Replaced Remains




Remains are slowly replaced, molecule
by molecule, by rock-forming minerals.
Occurs in bones, teeth and shells.
Circulating groundwater removes the
original organic material and replaces
them with minerals from the water.
Petrified wood is an example
Molds and Casts



After an organism (leaf, insect) gets
buried in the mud or sediments, its hard
body parts become a fossil and the
mud/sediment becomes rock.
When the body part dissolves out of the
rock, a hollow depression results called a
mold.
Minerals may seep into the mold and fill
it forming a cast of the original fossil.
Trace Fossils

Any impression left in the rock
by an animal such as trails,
footprints, tracks, burrows and
bite marks.
Carbonaceous Films


A trace of a fossil that is a thin
carbon film resembling a
silhouette.
The remains are affected by high
temperature and pressure
causing the tissues of the
animals and plants to undergo
chemical changes.
How Are Fossils Preserved?
Shells/hard parts
Bones
Cast or mold
Thin carbon film
Impressions
Constructed feature
Amber
Replacement
09.05.a
Using Index Fossils


An index fossil is the remains of an
organism that lived and died within a
particular time segment of Earth’s
history.
4 characteristics of an index fossil.
 Unique so they are easily identified
 Abundant
 Found over a great distance
 Organism can only have existed for
a brief period.
Fossils as Environmental Indicators

Fossils tell you the climate. Since
animals can only exist at certain
temperatures, fossils tell you the
climate.
Matching Key Beds


Key Bed – a single rock layer
that is unique, easily
recognizable and widespread
Ex. volcanic eruptions
Stratigraphic Matching


Matching layers of rock.
Ex. A layer of limestone
sandwiched between
conglomerate rocks.