What’s the oldest thing you

have ever touched?

Evolution of Landforms and

Organisms

Age of Earth?thousands - millions - billions trillions

4.6BillionYears

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 Half-Life 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 radium-226 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 DatingRadioactive

ElementHalf-Life (years)

Dating Range (years)

Carbon –14 5,770 500-50,000 Potassium – 40 1.3 billion 50,000-4.6 billionRubidium –87 48.8 billion 10 mill – 4.6 billThorium – 232 14 billion 10 mill – 4.6 billUranium – 235 713 million 10 mill – 4.6 billUranium – 238 4.5 billion 10 mill – 4.6 bill

– A half-life is the amount of time it takes for half of the isotope to decay.

• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their number of neutrons.

Calculating Half LifeCarbon-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.0.125 gb.0.8 gc.0.1 gd.0.025 g

Calculating Half LifeCarbon-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.0.125 gb.0.8 gc.0.1 gd.0.025 g

1.Divide 5,700 / 17,100 = 32.1g X 0.5 = 0.53.0.5 X 0.5 = 0.254.0.25 x 0.5 = 0.125g

Determining the Absolute Age of Rock LayersRadioactive 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

Which block is the oldest ?Which block is the youngest?

• Stack the blocks • Do not stack according to size

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 are younger & come after parents & grandparents.

• Kids• Parents

• Grandparents

• Great-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

Which block is the oldest ?Which block is thy youngest?

• Tilt the blocks to at an angle.• What happens to the rock layers

when they are tilted?

Blocks

Which block is the oldest ?Which block is thy youngest?

• Continue tilting the blocks until the layers have reversed positions.

• Now….

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.

UnconformityIgneous 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 UnconformityThe 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

PaleontologistsPaleontologists study fossils theyfind embedded in “sedimentary”rocks. They use the informationto determine what the earth and life was like in the past.The fossil record explains aboutlife in the past and how it andthe environment has changed over time.

FossilsThey are evidence of once-living things.They show how species have changedover time and how some species are related to one another.

Fossils can give us evidence of past lifeFossils 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