Transcript FOSSILS AND RADIOMETRIC DATING

What’s the
oldest thing you
have ever
touched?
Evolution of
Landforms and
Organisms
Age of Earth?
thousands - millions - billions trillions
4.6
Billion
Years
old
Absolute Age and Rocks
• Absolute age tells the actual age of a rock.
• Radioactive Decay or Radiometric Dating is one
method that gives the age of a rock by comparing
the amount of radioactive material in the rock with
the amount that has decayed
Parent Isotope
Stable Daughter Product
Currently Accepted HalfLife Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
704 million years
Thorium-232
Lead-208
14.0 billion years
Rubidium-87
Strontium-87
48.8 billion years
Potassium-40
Argon-40
1.25 billion years
Samarium-147
Neodymium-143
106 billion years
Radioactive Dating - Carbon Dating
• The half-life of a radioactive
element is the time it takes for
half of its atoms to decay into
something else.
• For example, the half-life of
radium-226 is 1600
• Therefore, in 1600 years, one
gram of radium-226 will turn
into half a gram of radium-226
and half a gram of something
else
• After another 1600 years have
elapsed, only a quarter of a
gram of the original radium226 will remain.
• Finding the ratio of parent to
daughter elements
• Carbon-14 is an isotope that
has a half life of 5,700 year
old.
• Half-life – The time it takes
for half of the atoms in an
isotope to decay
• Radiometric Decay – Process
that uses properties of atoms
in rocks and other objects to
determine their ages.
• Radioactive Dating –
calculating the absolute age
of a rock by measuring the
amounts of parent and
daughter materials in a rock
and by knowing the half-life
of the parent material
Every living thing contains
Carbon -14
• It has been used to date
fossils such as frozen
mammoths, pre-historic
humans, plants and
animals that lived up to
about 50,000 years ago.
• It’s half-life is only 5,700
years so it can’t be used
to date ancient fossils or
rocks.
Carbon dating tells
when this mammoth
died
Elements Used in Radioactive Dating
Radioactive
Element
Carbon –14
Potassium – 40
Rubidium –87
Thorium – 232
Uranium – 235
Uranium – 238
Half-Life
(years)
5,770
1.3 billion
48.8 billion
14 billion
713 million
4.5 billion
Dating Range
(years)
500-50,000
50,000-4.6 billion
10 mill – 4.6 bill
10 mill – 4.6 bill
10 mill – 4.6 bill
10 mill – 4.6 bill
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
– A half-life is the amount of time it takes for half of the
isotope to decay.
Calculating Half Life
Carbon-14 has a half-life of about 5,700 years.
A sample of bone originally had 1 gram of Carbon14.
How much carbon-14 will there be in 17,100
years?
a.
b.
c.
d.
0.125 g
0.8 g
0.1 g
0.025 g
Calculating Half Life
Carbon-14 has a half-life of about 5,700 years.
A sample of bone originally had 1 gram of
Carbon-14.
How much carbon-14 will there be in 17,100
years?
a.
b.
c.
d.
0.125 g
0.8 g
0.1 g
0.025 g
1. Divide 5,700 / 17,100 = 3
2. 1g X 0.5 = 0.5
3. 0.5 X 0.5 = 0.25
4. 0.25 x 0.5 = 0.125g
Determining the Absolute Age of Rock Layers
Radioactive Dating
A technique for measuring the age of an object or sample of
material by determining the ratio of the concentration of a
radioisotope to that of a stable isotope in it; for example, the
ratio of carbon-14 to carbon-12 reveals the approximate age of
bones, pieces of wood, and other archeological specimens.
Blocks
• Stack the blocks
• Do not stack according
to size
Which block is the oldest ?
Which block is the youngest?
Law of Superposition
• In undisturbed
sedimentary ROCK, the
oldest layers are deeper
down, at the bottom
and the youngest layers
are closer to the top.
• Kids
• Parents
• Kids are younger &
come after parents &
grandparents.
• Great-grandparents
• Grandparents
Law of Superposition – Rock Layers
• This law states that if a rock
layer has not been
disturbed then;
– Older layers of rock lie
beneath younger rock
layers
– This should make sense
• The oldest sediments
must be laid down
before the younger
ones can pile up on
top.
Rock Layers
Blocks
• Tilt the blocks to at an
angle.
• What happens to the rock
layers when they are
tilted?
Which block is the oldest ?
Which block is thy youngest?
Blocks
• Continue tilting the blocks
until the layers have
reversed positions.
• Now….
Which block is the oldest ?
Which block is thy youngest?
Unconformity
• Plate movements can fold, tilt or turn rock layers
• An unconformity is a “missing” rock layer
• This sometimes makes it difficult to age rock layers
• An unconformity is a buried erosion surface separating
two rock masses or strata of different ages, indicating that
sediment deposition was not continuous. In general, the
older layer was exposed to erosion for an interval of time
before deposition of the younger, but the term is used to
describe any break in the sedimentary geologic record.
Unconformity
Igneous or metamorphic rock is a nonconformity. The boundary
represents a nonconformity. Igneous or metamorphic rock may be
uplifted to Earth’s surface by crustal movements. Once the rock is
exposed, it erodes. Sediments are deposited on the eroded surface.
Angular Unconformity
The most obvious kind is the angular
unconformity.
Rocks below the unconformity are
tilted and sheared off, and rocks
above it are level. The angular
unconformity tells a clear story:
• First a set of rocks was laid down.
• Then these rocks were tilted, then
eroded down to a level surface.
• Then a younger set of rocks was laid
down on top.
Weathering Created the Grand Canyon
Paleontologists
Paleontologists study fossils they
find embedded in “sedimentary”
rocks.
They use the information
to determine what the earth and
life was like in the past.
The fossil record explains about
life in the past and how it and
the environment has changed
over time.
Fossils
They are evidence of once-living things.
They show how species have changed
over time and how some species are
related to one another.
Fossils can give us evidence of past life
Fossils suggest that birds
evolved from dinosaurs.
A fossil is a rock!
• Most fossils are formed
of sedimentary rock.
• They are formed by
compacting and
cementing together
layered sediments.
KEY CONCEPT
Specific environmental conditions are necessary
in order for fossils to form.
• Amber-preserved fossils are organisms that
become trapped in tree resin that hardens after
the tree is buried.
Preserved remains form when an entire organism
becomes encased in material such as ice.
Most Fossils form in
What Kind of Rock ?
• Layer upon layer of
sand, mud, dead
plants and animals
and other small
pieces build up
and their weight
compacts and
cements the layers
together.
Fossils form Slowly
• It takes
about 1
million
years to
form a
sedimentary
fossil.
Preserved Tracks
• Tracks give us evidence of the size, weight
and stride of the animal. If several tracks are
found that can be evidence of lifestyle: social
grouping an interactions among species.
Index Fossils
• Some species inhabited Earth for
long periods of time without
changing.
• INDEX FOSSILS existed for short
periods of time, were abundant
and were found in lots of different
places on Earth.
• Index Fossils have been found in
many places throughout the Earth
and geologists use them to date the
age of rock layers.
Sea Urchin
Ammonite
Ammonites are excellent index fossils, and it is
often possible to link the rock layer in which they
are found to specific geologic time periods
Geologic Time Scale