Dike A

Fault 1

Fault 2

Dike B

What is the sequence of events that shapedthis geologic setting?

TODAY: Chapter 10 Part 2

Geologic Time 3) Fossils as time-pieces

4) Absolute dating w/ radioactivity

5) Geologic time scale

6)Regional correlation of rock layers 

3. Fossils as timepieces

Fossils

William Smith (late 1700s) noted that sedimentary strata in widely separated area could be identified and correlated by their distinctive fossil content

fossil organisms succeed each other in systematic fashion, always in ordere.g., age of trilobites, age of fishes, age of reptiles,age of mammals

any time period can be determined by fossil content

“principle of fossil succession”

3. Fossils as timepieces

“Index fossils” = Fossils as time-indicators

geographically wide-spread

short span of time

Overlapping rangesof fossils help daterocks more exactly(time-period is ‘skinnier’).

Using fossil assemblages, formations I and II of Outcrop A and II and III of Outcrop B can be put in relative sequence of each other (3).

3. Fossils as timepieces

4. Absolute dating w/ radioactivity

Atommodel

Chemistry review

nucleusele

ctro

ns

protronsneutrons

# protrons = atomic #,defines the element

# neutrons can vary:“isotopes”

See Fig. 10.13 for an example of radioactive decay.

4. Absolute dating w/ radioactivity

Parent DaughterNow 1000 0 Next year 500 500 2 yrs from now 250 750 3 yrs from now 125 875 10 yrs from now 1 999

Example: an isotope with a half life of 1 year

# of atoms

Half-life: The time required for half of the nucleito decay

4. Absolute dating w/ radioactivity

Fig. 10.14: Radioactive decay is exponential.

See Table 10.1 on radioactive elements used for radiometric dating.

What to remember…about radioactivity

 Some isotopes are unstable  Parent nucleus breaks up (decays)   By-product (daughter) will be different element   Rate of decay is constant & can be determined   Half life = Time required for half of parent atoms to decay to daughters   Isotopes with long ½ lives (uranium) are used to date old rocks

Short ½ lives (carbon-14) are used for young rocks

Phanerozoic - last 570 million years

Cenozoic - recent life

Mesozoic - age of middle life

Paleozoic - ancient life

Precambrian - from birth of Earth up to before complex life forms developed

5. Geologic time scale - a calendar of Earth’s history

eons

eras

0-66.4 m.y.a

66.4-245 m.y.a

245-540 m.y.a

(mammals, humans)(mammals, humans)

(dinosaurs, 1st scrawny mammals)(dinosaurs, 1st scrawny mammals)

(first fossils with shells, (first fossils with shells, fish, trilobites, clams, corals, ferns )fish, trilobites, clams, corals, ferns )

(algae, bacteria, some fossils without shells like jellyfish, worms)(algae, bacteria, some fossils without shells like jellyfish, worms)

Massive extinction events

See Fig. 10.12, 10.17, 1.12

5. Geologic time scale

From Tarbuck and Lutgens, Earth, 7th ed.

6)Regional correlation of rock layers

Using relative dating principles and fossils, strata from different localities can be pieced together to form one continuous record of geologic history.

Box. 10.1., pp. 226-227

And now a few sample MC…

The principle of superposition states that __________.

A. a fault is younger than the rocks it cutsB. sediments are deposited as essentially horizontal layersC. the present is the key to the pastD. undisturbed sedimentary layers get progressively younger from bottom to top

And now a few sample MC…

 What is an unconformity?

A. a gap in the geologic recordB. a period of depositionC. a sedimentary layer of variable thicknessD. a sequence of deformed rocks

And now a few sample MC…

The study of faunal succession allows _________.

A. absolute dating of fossil-bearing strataB. correlation of marine fossils with modern mammalsC. reconstruction of paleoclimatesD. rocks to be correlated from different outcrops

And now a few sample MC…

Which of the following radioactive isotopes is the most useful for dating very young (<10,000 years old) wood and charcoal?

A. carbon-14B. potassium-40C. rubidium-87D. uranium-238

And now a few sample MC…

Human beings (homo sapiens) evolved during which geologic era?

A. Cenozoic B. MesozoicC. PaleozoicD. Precambrian