Calculating the age of the Earth
The early geologists would come to realize that the processes that have shaped the earth we know today would take a great deal of time, BILLIONS of years.
Archbishop James Usher
deduced that the age of the earth was 6,000 years old based on counting generations in the bible.
James Hutton the "father of geology
Realized that geologic processes take vast amounts of time (originally a farmer in scotland). Introduced uniformitarianism.
Uniformitarianism
The present is the key to the past. Processes at work today like deposition, erosion, tectonism were also at work in the past.
Charles Lyell
Published a series called "Principles of Geology" that explains relative age dating principles. These also include some of Nicholas Stenos principles.
Charles Darwin
Published on the "Origin of Species" describing evolution of species by natural selection. Through certain traits not just survival of the fittest. "Sneakiest
Lord Kelvin
Undertook the colossal taks of calculating the earth's actual age using Thermodynamics. How long the earth would take to cool to its present state. 20-100 million years was his guess. He didn't know about Radioactive decay or heat! Didnt come along for an
Common Radioactive Elements:
K (potassium), TH (thorium), U (uranium).
Arthur Holmes
Used radioactive isotopes to determine the actual age of the earth.
Age of the earth
4.54 billion years old.
Oldest Rock was 3.4 billion years old.
Oldest mineral a little older than 3.4 billion.
*No rocks or minerals that are actually 4.5 billions years old.
*Meteorites tell age of actual solar system 4.6 billion years old.
Relative Age Principles
Using Steno's and Lyell's relative age principles, we can put geological formations (rock layers) and events into a sequence or determine its Relative Age.
Steno's Law: Grand Canyon Stratigraphy
Youngest down to oldest. All exposed.
1st Principle
Superposition: In an undisturbed sequence of sedimentary rocks, the oldest is on the bottom and the youngest is on top. (stenos)
2nd Principle
Original and Horizontality and Lateral Continuity: A sedimentary rock layer will remain at a constant thickness and will continue until it tapers out.
3rd Principle
MOST IMPORTANT. Cross Cutting Relationships: a rock formation must be older than a fault that cuts through it or an igneous intrusion that invades it. Fault is younger than rock it is cutting.
4th Principle
Inclusions: Inclusions of a one rock formation in another indicate that the included rock is older.
5th Principle
Faunal Succession: A rock's relative age can be determined by the fossils types it contains. Ex the presence of a dinosaur trace fossils indicates Mesozoic age rocks (>65 years).
6th Principle
Succession of Landscape Development: Erosional features sucha as canyons and river channels must be younger than the rock formation they erode.
Ex Grand Canyon
Contacts
-Conformable Contact or Conformity
Represents no missing time between formations.
Contacts
-Unconformity
Represents missing time in "rock record". There are 3 types.
Contacts
-Unconformity
1. Disconformity (Erosional Unconformity)
The contact is parallel, but the contact surface shows signs of erosion.
*Caused by Rivers changing course or mountain uplift.
Contacts
-Unconformity
2. Angular Unconformity
Underlying layers are tilted relative to overlying layers, represents missing time due to tectonic event. Missing section of time when uplift occured.
Contacts
-Unconformity
3. Nonconformity (Ig/Met)
Contact between eroded igneous or metamorphic layer and overlying sedimentary rocks.
*Igneous is younger when intruded. Erodes showing some igneous then new rocks put on top making you lose time.
Absolute (Numerical) Age Dating
Determining an actual age (ex. in yrs) for geologic events or formations.
Absolute Ages
Are determined by radiometric dating and determining how much radioactive decay has occurred.
Isotopes
Have the same number of protons, but a different number of neutrons.
Stable Isotope
Retain all their neutrons and protons over time.
Ex: H2 is Deuterium
Unstable Isotope
RADIOACTIVE! "Spontaneously" lose neutrons and protons over time.
Ex: 3H is Tritium
Three Types of Radioactive Decay
1. Alpha Decay
Loss of an "alpha" particle. Equivalent to 2 protons and 2 neutrons.
The atomic number will decrease by 2.
The atomic mass will be decreased by 4.
Three Types of Radioactive Decay
2. Beta Decay
Loss of an "beta" particle. Equivalent of the loss of an electron from the nucleus. Losing a negative charge from the nucleus. Neutrons change into Protons. Atomic number can increase by 1. The atomic mass stays the same.
Three Types of Radioactive Decay
3. Electron Capture
Take something positive and cancel out that charge. Electron "obtained" by nucleus. A proton becomes a neutron. Take in a negative charge. Atomic number decreases by 1. Atomic mass has no change.
Age Dating With Radioactive Decay
Decay happens spontaneously in a single atom, but over many atoms the statistical results are significant and a half-life is determined.
Half Life
The time it takes for half of the radioactive element in a sample to decay from parent to daughter, varies from seconds to billions of years.
Electron Capture Example
Potassium 40 (parent) > Argon (daughter) 40 is equal to 1.3 Billion yrs (Half life)
Alpha Decay Example
Uranium 238 (parent) > Led 206 (daughter) is equal to 4.5 billion years (half life)
Beta Decay Example
Carbon 14 > Nitrogen 14 is equal to 5,730 years (half life)
Radiocarbon Dating CANNOT
be used for determining the age of the earth.
1. Earth Forms
Accretion of meteorites 4.5 billion years ago. Hadean Era.
2. Oceans Form
Accumulation of meteorites 7 percent water form clouds from vapor and comets hitting the earth. Archean.
3. Continents Form
Bowen's Reaction Series. Granite continents. Archean.
4. Oxygen is Abundant
1st organisms able to undergo photosynthesis. Cynobacteria was one. Photosynthesis can occur now. They form from Stramateolights. Proterozoic.
5. Snowball Earth
Largest glaciation event. Rodinia formed near the south pole; ocean currents shut down. Proterozoic.
6. Cambrian Explosion
Starts Cambrian period. Life diversifies in the oceans. Phanerozoic eon and Paleozoic era.
7. Ozone Layer Forms
O2 changes to O3. Life is able to conquer the continents.
Phanerozoic eon and Paleozoic era.
8. Freshwater Swamps
Coal comes from these. Plants conquer the planet. Likopods.
Phanerozoic eon and Paleozoic era.
9. Largest Mass Extinctions
Caused by increase in global temperatures. Volcanic eruptions release Co2 causing global warming. Pangea forming. 95 percent of marine species extinct. End of Paleozoic era.
10. Rise of Dinosaurs
Dinosaurs were in cretaceous not Jurassic period. High temperatures. "lukewarm blooded". Mesozoic Era.
11. Meteorite Impact > Mass Extinction
End Mesozoic mass extinction. 65 million years. Global climate change. Meteorite impact released ash in atmosphere that cooled temperatures.
12. Rise of Mammals
Cenozoic era. Birds, flowering plants become abundant.
13. Current Ice Age Cycle
Cenozoic era. 2.5 million years ice age cycle started. warm and cold period cycles: Panama inlet. Ocean circulation between atlantic and pacific.
14. Human Civilization Began
10,000 years ago. Holocene Epic. Cenozoic era. We've changed climate and what the earth looks like in this little amount of time.