Transcript Geologic Time

Geologic Time
The geologic time scale is based
upon rock and fossil evidence. It
is broken into the following
divisions from largest to
smallest: eon, era, period, and
epoch. An interactive timeline
shows how long geologic time
really is.
Charles Darwin’s Theory of
Evolution describes the
process of change that
produces new life forms over
time.
Evolution is driven by the
theory of natural selection,
the idea that the organisms
that survive to produce
offspring are those that have
inherited the most favorable
traits for surviving in a
particular environment.
These cactus plants show
convergent evolution. The one
on the left is from Arizona, the
one on the right is from Africa.
A fossil is evidence of
earlier life that has been
preserved in rock.
A fossil can be original
remains (not very
common), replaced
remains, molds/casts,
trace fossils, and
carbonaceous films.
Fossils aid in relative
dating of rocks.
Relative Dating is the
process of placing geologic
events in the process in
which they occurred.
The Grand Canyon is one of
the best places for observing
relative time.
There are certain principles
for dating rock layers.
The principle of superposition
states that the oldest rock layer
will be at the bottom and the
youngest at the top. Which
picture shows the oldest rocks?
The principle of original
horizontality says that
sedimentary rocks are first
deposited horizontally.
Rocks can be folded, tilted, or
faulted after they are deposited
horizontally.
Rocks can also be intruded by
magma which later cools to form
rock.
The Principle of Cross Cutting
Relationships says that an
igneous intrusion is always
younger than the rock it has
intruded or cut across.
Number this diagram from oldest
to youngest. 1 is always oldest
(the first to form).
Observe this animation to
help you visualize geologic
processes. Notice the
unconformity, a gap in the
sedimentary rock record.
An angular unconformity occurs when younger,
flat strata are deposited on top of the older strata
that have been tilted at an angle.
A disconformity occurs when younger, flat strata are
deposited on top of the older flat strata. The older flat
strata is uplifted and eroded. The layers are then resubmerged under water and a second instance of
deposition occurs on top of the unconformity.
A nonconformity occurs when sedimentary layers are
deposited on top of igneous or metamorphic rock.
Correlation is the matching of
rock layers from one area to
another. Correlated strata will
have the same age.
Methods of Correlation
•Walking the outcrop
•Matching Rock Characteristics
•Using Index Fossils
•Climate indicating fossils
•Matching Key Beds
•Stratigraphic Matching
Index Fossils are useful to date layers because
each layer contains fossils unlike those in the
layer above or below.
Where is The Grand Staircase?
What is The Grand Staircase?
Pictures of The Grand Staircase
Use the following images to give you a sense of
what the Grand Staircase and the Colorado
Plateau Look Like
from South to North
Sedona, Arizona
Grand Canyon with San Francisco Peaks in the
Background
Zion National Park
Bryce Canyon National Park
Relative dating helps us to
figure out the relative ages of
rocks, but it does not help us
to figure out the absolute age
of the rocks. So, what do we
do?
We use Absolute Dating to
put numbers on our dates.
Trees can be used to record time. We can
use tree ring history (dendrochronology) to
put an actual date on a historic occurrence
and to gauge past climates.
Varves, annual deposits of
sediments, can be used for
geologic dating purposes.
Sediments deposited in glacial
lakes vary with the seasons.
Thick, light colored, sandy
layers are deposited in spring
and summer when runoff of
water from a glacier is greater.
Thin, dark colored, clay layers
are deposited in winter.
Counting annual glacial varves
can help us to date items up to
15,000 years old.
Absolute dating can also be
based upon the concept of
radioactivity of chemical
isotopes. This is also known
as radiometric dating.
Radioactive decay is based
upon the conversion from
one isotope to another, but
what is an isotope?
Remember from chemistry
that an isotope is any
element with more neutrons
than protons.
Radioactive isotopes are
unstable and want to achieve
stability. To do this, they give
off radiation until they are
stable. This usually involves
changing from one element
to another.
The parent isotope is the
original element and is
unstable. The daughter
isotope is the product of
decay and is stable.
There are three types of radioactive decay
Alpha Decay: Atomic # decreases by 2 and atomic
mass decreases by 4 because 2 protons and 2 neutrons
are expelled from the nucleus of the atom. Remember
that the proton determines the atomic # and protons +
neutrons determines the atomic mass.
There are three types of radioactive decay
Beta Decay: Atomic # increases by 1 and atomic mass
does not change because an electron is expelled from a
neutron, thereby turning the neutron into a proton.
Remember that the proton determines the atomic # of an
atom.
There are three types of radioactive decay
Electron Capture: Atomic # decreases by 1 and atomic
mass does not change because an electron is added to
a proton, causing a neutron to form. Remember that the
proton determines the atomic # of an atom.
Radioactive decay will
continue until the resulting
atom is no longer radioactive.
In other words, a stable
isotope is formed.
Half life is the time it takes for
half of the atoms of unstable
parent isotope to decay to
stable daughter isotope.
Graph of Half Life
Number of Parent Isotope
Remaining
Graph of M&M Half Lives
100
80
60
40
20
0
0
1
2
3
4
5
6
7
Number of Runs (half lives)
8
9
In Radiometric Dating,
scientists use radioactivity
and half-lives of elements to
measure absolute time.
Scientists measure the
amounts of parent and
daughter isotope in a rock to
find its age.
Isotopes Used in Radiometric Dating.
Parent
Isotope
Decay
System
Daughter
Isotope
Half-Life
(years)
Effective
Range
(years)
Possible
Materials for
dating
Carbon-14
Beta
Nitrogen-14
5730
100-70,000
Once-living
matter
(wood,
charcoal,
bone)
Uranium-238 Alpha and
Beta
Lead-206
4.5 Billion
> 10 Million
Uraniumbearing
minerals
(zircon)
Rubidium-87
Beta
Strontium-87 47 Billion
> 10 Million
Micas,
feldspars,
metamorphic
rocks
Potassium40
Beta capture
Argon-40
> 50,000
Micas,
amphiboles,
feldspars,
volc. rocks
1.3 Billion
Example 1: If a scientist finds
100% parent isotope in a rock,
how old is the rock if the half life
of the parent isotope is 2 million
years? Look at the following
graph, find where the line
intersects 100%, then read the
value for the number of half lives
on the x axis.
Graph of Half Life
Number of Parent Isotope
Remaining
Graph of M&M Half Lives
100
80
60
40
20
0
0
1
2
3
4
5
6
7
Number of Runs (half lives)
8
9
Age of Rock = # of half lives
X(times) half life of isotope
Age of Rock = 0 X
2,000,000 = 0 years
Example 2: If a scientist finds
25% parent isotope in a rock,
how old is the rock if the half life
of the parent isotope is 4 million
years? Look at the following
graph, find where the line
intersects 25%, then read the
value for the number of half lives
on the x axis.
Graph of Half Life
Number of Parent Isotope
Remaining
Graph of M&M Half Lives
100
80
60
40
20
0
0
1
2
3
4
5
6
7
Number of Runs (half lives)
8
9
Age of Rock = # of half lives
X(times) half life of isotope
Age of Rock = 2 X
4,000,000 = 8,000,000 years!