2/19 notes

Quantifying time

Absolute vs. relative ages

• The named eras, periods, epochs, etc. of the geological time scale are based on the occurrences of fossils in sedimentary rocks

• They provide relative ages or ages without numbers

  • E.g. the Jurassic is older than the tertiary but younger than the Permian

• Absolute ages are ages that are numerical

  • E.g. the start of the Cambrian period was at ~542 million years ago (=My), and the end of the Cambrian period was at ~488My

  • Comes from a study of radioactive decay

Radioactive decay 1

• Alpha decay

→ two protons and two neutrons (an alpha particle) leave the nucleus

-> Decrease atomic number by 2 and atomic weight by 4

radioactive decay 2

• Beta decay

→ a neutron in the nucleus “splits into a proton (+1) and an electron (-)

→ the electron (called a beta) leaves the nucleus

→ atomic number increases by 1, atomic weight remains the same

radioactive decay 3

• Electron capture

→ = reverse of beta decay

→ an electron joins a proton to make a neutron

→ reduces atomic number by 1; atomic weight remains same

-a given radioactive atom (= an unstable parent isotope) may undergo many decay ‘events’ in a series until a stable daughter is finally produced

E.g. uranium 238 (unstable) decays lead 206 (stable) in 14 steps

Half-life

• The time it takes for one-half of the unstable parents to decay to stable daughters

• A CONSTANT >> The rate at which this occurs can never change

• Half-lives can be measured under laboratory conditions

• In geology, we want HLs that are very long

• U238>Pb206 - 4.5 Billion years

• U235>Pb207 - 704 Million years

• K40>Ar40 - 1.3 Billion years

• C14>N14 - 5700 Years

• A given rock contains crystals that contain unstable isotopes

• The “clock” starts at the moment a crystal is formed

• At that time, the crystal will contain 100% parent

• After one half-life, it will contain 50% parents and 50% daughter

• As the crystal sits there and time goes by, the % parent decreases while the % daughter increases

• Knowing half-life AND measuring the % parent can give us an absolute age for the rock

The big problem

• Most absolute age dates come from igneous rocks

  • Dating time of crystallization

• But, we usually want to date living things like dinosaurs, human ancestors, the first evidence of life, etc

• All of the above are far too old for carbon14 dating

  • Remember, all c14 is gone after about 70,000 years

Resolution

• Age date igneous rocks anywhere they are in contact with sedimentary rocks

• sedimentary rocks containing the fossils we are interested in

Example

• If the dike in the image is absolute dated at 185 million years old, and…

• The sedimentary rocks that the dike is cross-cutting contain primitive dinosaurs, then…

• The dinosaurs are older than 185 million years old

-a given rock has 25% parent remaining; the isotope we are measuring has a HL of 250 million years. How old is the rock?

→ 2HL * 250MY = 500MY

-a given rock is 1By and the % parent remaining is 25%. What is the HL

→ A 25% parent = 2HL, so

→ 1BY/2 = 0.5 By or 500My

Carbon14 dating

• C14 or carbon dating is probably the most commonly known type of absolute dating

• But it does not use rocks at all, instead, it uses biological material

• Clock doesn't start at crystallization, it starts up the death of the organism

• Half-life is so short that it cant be used for anything older than about 70,000 years

  • So, for example, dinosaur bones cant be dated with C14

2/24 notes

Carbon dating

• The unstable isotope is carbon 14

• Half-life is 5730 years

• Used for geologic/biologic materials less than 70,000 years old

  • By that time, ‘none’ is left

• Carbon 14 is created in the upper atmosphere

• N14 (7p+7n) becomes C14 (6p+8n)

• This is constantly created so all living things ingest a steady amount of C14

• WHILE THEY ARE ALIVE!

• Once an organism dies, the C14 is no longer replenished

• C14 is no longer at 100%

• Begins to decay

• **Other isotopes begin at the moment of crystallization

• FOR C14, IT IS THE MOMENT OF DEATH

• After death, C14 undergoes a simple beta decay to become N14 once again

• The N14 floats back into the atmosphere to start cycle again

C12= stable

        6p+6n

C14=unstable

        6p+8n

Continental drift

• Continents once together then separated and drifted to present positions

• Alred Wegener

PANGEA

• Northern part: Laurasia

• Southern part: Gondwanaland

• Ocean: Panthalassa

CD evidence 1

• Similar rock types in all southern continents

• Simpler hypothesis: if all of these rocks were formed in one place and then separated then the exact same rocks formed - in exact same sequences (!) in many places

CD evidence 2

• Fit of the continents

  • Especially southern continents

• Shoreline edges fit together

• Reconnecting at fit aligns the similar rock types

  • E.g. glacial rocks, rock cratons (most ancient parts of continents)

CD evidence 3

• Similar fossils on all southern continents

• animals/plants that could NOT cross ocean basins

  • Glossopteris: seed fern

  • Mesosaurus: freshwater reptile

  • Lystrosaurus and cynognathus: slow land-dwellers

CD becomes plate tectonics (PT)

• For many reasons, CD was ignored until the 1960s, when it “morphed” into plate tectonics

• First – recognition of seafloor spreading (SFS)

Seafloor spreading (SFS)

• Sonar reveals bathymetric variations

• Vast highlands in ocean centers (mid-ocean ridges)

• These MORs have extensive volcanic activity and earthquakes

• Heat rises here at plate boundaries spreading the sea floor apart

• Pushes continents along

• Several lines of evidence support SFS by the 1960s

  • Heat flow

  • Earthquake locations

  • Volcano locations

  • paleomagnetism

Earth’s interior

• Seismology reveals that

1. Earth is not a homogenous ball; it is composed of several layers

2. These layers have distinct chemical compositions and physical properties

3. The layers are consistent around the globe

Continental crust

• Primarily granitic composition

• Density: 2.7 g/cm^3

• Thickness: 20-60 km

• mostly brittle deformation

Mantle

• Primarily peridotite composition

• Density: 4.5 g/cm^3

• Thickness: 2900km

• Layer immediately below both crusts

• Primarily ductile deformation

Outer core

• Primarily iron and nickel composition

• Density: 10-11 g/cm^3

• thickness: 2160 km

• So heated, it is in a permanent molten state

• P waves pass through, S waves do NOT

Inner core

• Primarily iron and nickel composition

• Density: 13-14 g/cm^3

• Thickness: 1320 km

• Again, very hot but the pressure is so high that it is solid, NOT liquid

• Both P and S waves pass through

Lithosphere

• Both crusts and uppermost mantle

• Thickness: 75-125 km

• Brittle reformation

• =the hard, outer shell of the earth

• Broken or cracked throughout (quakes)

2/26 notes

Plate boundary 1 - Divergent

• Caused by tensions < > stress

• Generates sea floor spreading; creates new oceanic crusts and basins

• **extensive basalt volcanism; many shallow focus earthquakes

• Creates mid-ocean ridge (MOR) system

Plate boundary 2 - convergent 1

• Continental crust>> <<oceanic crust

• Created by compression stress > <

• Extensive andesite volcanism; extensive shallow, moderate, and deepest focus depth earthquakes

• Creates subduction zone with deep ocean trench

Plate boundary 2 - convergent 2

• Continental crust><oceanic crust

• Created by compression stress

• No volcanism; extensive shallow to moderate depth focus quakes

• Huge batholith formation

• Subduction ends - both plates forced 

Paleomagnetism: proof of SFS and PT

• Rath had a magnetic field - compass points North

• Basalt erupted at MOR contains magnetic minerals that align themselves with this field

• I.E. lava has minerals that point in the same direction as the magnetic field

Effects of reversals at MOR

• Erupting basalt containing lots of iron (magnetic)

• When cooling to solid basalt, iron atoms align with magnetic fields

Reverals and divergent boundaries

• Given: basalt lava containing magnetic minerals is constantly erupting at MOR

• Given: the earth's magnetic field sometimes points north, sometimes south

• Then: the cooled lavas will preserve this pattern of reverals (IF plate tectonics is true)

• Then: 

Paleomagnetism as proof of PT

• The pattern of magnetic reverals of seafloor basalt lavas (=oceanic crust) is now found in ALL ocean basins on Earth

Other symmetries across MOR

• Ages of oceanic crust increase away from MOR

• Ages of sediments increase away from MOR

• Thickness of sediments increases away from the MOR

Oceanic plateaus

• =thickened masses of basalt, oceanic crust; may be large enough to be emergent islands 

• Originate at MOR, pushed towards subduction zones

• TOO thick to be subducted, instead they get scraped off onto the continent

• =obduction, or now… accretion

3/3 notes

Organic evolution

Adaptations

• Specialized features that allow animals/plants to perform one or more functions useful to them

  • E.g.: cats with sharp cheek teeth to cut metal, horses with broad, flat molars to chew grass

• Evolution is “remodeling”, not “new construction”

• **common grounds plans suggest common origins

  • E.g.” > cheek teeth in ALL animals, MANY more

wallace/darwin observations

• Both travelled widely around the world while young

• Observed many adaptations in tropical animals and plants

• E.g., Darwin and finches

  • Many different beak types for different food types; segregated by environments

Artificial selection

• Both wallace/darwin studied artificial selection

• A breeder chooses which individuals will mate to make next generation

• E.g., cows for milk production, sheep for wool, corn/wheat for high yields

wallace/darwin natural selection

• Variation occurs in natural populations

  • Size, speed, metabolism, etc.

• Many more organisms are born in each population that can survive

• Some individuals have features/characters that make them “better” than others in hte population

  • more/less body fat, camoflauge coloration, muscle fibers, etc

• These organisms will have more “success”

  • Will live longer to produce more offspring

• Over geologic time, organisms with the naturally selected features will come to dominae the population

Taxonomy

• Classification scheme for living/fossil organisms - a crude measure of evolutionary relatedness

• Hierarchical

  • Kingdom, phylum, class, order, family, genus, species

Kingdom: animalia

Phylum: chordata

Class: Mammalia

Order: primates (humans, apes, monkeys)

Family: hominidae (humans, extinct ancestors)

Genus: homo (humans, homoerectus)

Species: homo sapiens (us)

• Population: a group of interbreeding individuals

• Species: populations whose members can interbreed IF they come in contact with one another

• Species (like giraffes) can change (evolve) over time AND can change so much that they give rise to a new species

Speciation

• If natural selection operates long enough, and if different populations have different natural selection pressures, then populations diverge to the point where they can no longer interbreed

• Speciation (origination) has occured!