Geologic Time(History as told by Rocks)

The Rock Record

Geological Time Scale• Dividing the

Earth’s history into sections based on the fossils and rock evidence.

How do we know?• Principle of UNIFORMITARIANISM means that

the natural processes in place today have been occurring since Earth formed.

• Processes in the past happened according to the same laws as processes do today.

Dividing It Up• Eon– Longest time unit measured in BILLIONS of years.

• Era– next longest measured in HUNDREDS of MILLIONS

of years• Period – measured in TENS of MILLIONS of years

• Epoch – measured in MILLIONS of years

Historical EventsA. Bacteria Form

B. Trilobites are abundant, First fish appear, Appalachians begin to form

C. First Land Plants and Insects appear

D. Reptiles appear

E. Appalachians finish forming, glaciers retreat

Important EventsF. Pangaea breaks up, Mammals appear

G. Dinosaurs dominate, mountain building in Western U.S.

H. Flowering plants appear, Rockies form, Dinosaurs become extinct

I. Alps and Himalayan Mountains form, mammals dominate

0.01 million years ago – Homo Sapiens evolves.

Land Plants

Trilobite

Fish

Insects

Reptiles

Geological Principles• Original horizontality– sedimentary rocks were formed in HORIZONTAL

LAYERS

• Superposition– oldest rock layers are on the BOTTOM

• Cross-cutting relationships – Intrusions or Faults that cut across a layer are

YOUNGER than the layer they cut through.

• Unconformity– missing layer of eroded rock

Original HorizontalitySuperposition

Cross-cutting Relationships Unconformity

Bending and Breaking Layers• Rock layers are disturbed by geological

processes– FOLDING: Compression forces BEND the layers– TILTING: Lifting forces lift one end of a layer of

rock– FAULTING: Shear forces shift some layers of the

rock

Identifying the RELATIVE AGE of Rock

• We can use the patterns in the rock layers to determine RELATIVE AGE of the layers

Place the layers in order fromOldest YoungestBottom Top

Absolute-Age Dating• In addition to learning about the relative age,

scientists also have methods of determining the absolute-age.

– Absolute age = The rock is 2 million years old– Relative age = The rock is older than the other

rock.

Radiometric Dating• Scientists can use UNSTABLE atoms to determine the

age of some rocks and fossils.

– Unstable atoms decay (release energy) at a predictable rate over time.

– The atoms change as they decay.

– Comparing the number of unchanged atoms (parent) and changed atoms (daughter), the age can be determined

• The rate these atoms decay is called the half-life.

Radioactive Atoms and Their Use• Carbon-14 is used for organic material less than

75,000 years old.

– The half-life of Carbon-14 is 5,730 years

– Carbon-14 has 6 protons and 8 neutrons and it decays to Nitrogen-14 with 7 protons and 7 neutrons

– Organisms continually take in new carbon while they are alive but stop when they die, so the carbon-14 “clock” doesn’t start until after death.

Other Radiometric Dating• Uranium-235 – half-life of 0.7 billion years– Decays to Lead-207

• Uranium-238 – half-life of 4.6 billion years– Decays to Lead-208

• Potassium-40 – half-life of 1.3 billion years– Decays to Argon-40

• These decay processes also have intermediate steps so we can use them to date rocks as young as 50,000 years to the oldest rocks in our solar system.

Dendrochronology• We can examine the annual growth of trees

(tree rings) to determine an absolute age of tree.

• Using tree rings to determine age is called dendrochronology.

• The width of the rings also can tell us if the climate was wet or dry and if any major forest fires occurred during the life of the tree.

Other Age Indicators• We can also determine age by looking for

indicators of climate change

– Varves are bands of deposited sediments in sedimentary rocks. Thick bands mean more sediment was carried – so more water.

– Key beds are layers of distinct sediments formed by unique events – such as asteroid impacts

Fossil Record• Fossil – evidence or remains of once living

organisms

• If you know the type of organism, it can tell you about the environment– If you find a fossil shell in the desert, it was probably a

marine environment when the fossil formed.

• If you have actual genetic material, you can use the DNA record and carbon dating to determine age.

How do fossils form?• Fossils usually only form in sedimentary rock.

– Sediments have to accumulate over the organism in order to preserve it.

– volcanic eruption, mudslide, tree sap or slow sediment accumulation on the bottom of a lake

• Most fossils are marine in origin, or are land animals that fell into a water environment.

– This is because sediments easily accumulate in water environments (lakes, streams, oceans).

Fossil Record• Trace fossils – fossils left by animal or plant ACTIVITY

(footprints, worm burrows)

• Mold – Rock formed around an organism that later decays out, leaving a empty space in the shape of the organism

• Cast – Molds that get filled up with later rock

• Permineralization – minerals from ground water seep into the organism and slowly fill in the pores in the animal

• Original Preservation – remains that have not changed since the organisms death (often frozen!)

PetrifiedFossils

CarbonFilms

TraceFossils

PreservedRemains

MOLD FOSSILThis mold, or imprint, is of an extinct mollusk called

an ammonite.

CAST FOSSILThis ammonite cast was discovered in the United

Kingdom.

Index Fossil

• Index fossils are remains of organisms that can be used to relate rock layers that spread over a large geographic area.

• Index fossils must be – Abundant– Spread out geographically– Lived only for a limited time frame

• If you know when that organism was alive, you can determine when the rock formed.

Which two rock layers formed at the same time?