Geology: The Study of the Earth

Physical geology Study of the materials and processes of the Earth.

Historical geology Study of the origin and evolution of Earth through time.

Early Geological Theories

• Catastrophism (~1600s)– Archbishop James Ussher

– Earth is was created on October 14th, 4004 B.C.

– Subsequent changes in Earth’s landforms happened over very short periods of time during great catastrophies

Early Geological Theories

• Uniformitarianism (late 1700s)– James Hutton

– The same processes that are at work on earth today were at work in the past. (“The present is the key to the past.”)

– Natural forces shape our planet over vast amounts of time

BLAMMO!

Although catastrophism was abandoned, there is certainly evidence

that catastrophies do occur.

Understanding Geological Time

Two main tools• Relative Dating

– Putting geologic events order from oldest to youngest

– Uses various principles and laws

• Absolute Dating– Determining exact ages of substances

– Uses radioactive decay

Understanding Geological Time

Laws and Principles of Relative Dating• Law of Original Horizontality (Law of Spilled Milk)

– Sedimentary layers and lava are deposited horizontally

• Law of Superposition (Laundry Hamper Law)

– The layer below is older than the layer above.

• Law of Cross-Cutting Relationships (Road Stripe Law)

– A rock formation is younger than any rock it cuts across

• Principle of Fossil Succession (Bell Bottom Principle)

– Life forms succeed one another in a definite sequence, so a time period can be determined by its fossils

B

A

E

C

D

F

Geological timeline based on index fossils.

Radiometric Dating• Determining the age of a rock by

measuring the ratios of certain radioactive isotopes

• Radioactive isotopes decay at a constant rate (half-life)– Length of time it takes for half of an

original radioactive isotope (parent isotope) to decay into a stable product (daughter isotope)

Radiometric Dating• Usually only performed on

igneous rocks

• Not used on sedimentary and metamorphic rocks . . . why not?– These types form from

preexisting rocks – Date would be when original

rock formed, not the new rock

• But how do we find the date?

12Radiation and Half Life:

A Brief Foray Into Nuclear Chemistry

• Radiation is used for many purposes

• Only harmful in large amounts

• Many objects naturally give off radiation– Can be used to help determine

age of rocks

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Types of Radiation• Alpha (ά) – a positively charged helium

• Beta (β) – an electron

• Gamma (γ) – pure energy

• called a ray rather than a particle

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End Result of Radiation

• As radiation is emitted, the original radioactive isotope gradually ‘decays’ into a different isotope

• For example: Uranium-238 decays into . . . Thorium-234 Protactinium-234 Uranium-234

Thorium-230 Radium-226 Radon-222 Polonium-218

Lead-214 Bismuth-214 Polonium-214 Lead-210

Bismuth-210 Polonium-210 Lead-206 (stable)

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End Result of Radiation

• By measuring the ratio of parent isotope to daughter isotope, you can determine a rock’s age

• How do we determine the ratio?

• Half-life!

• Animation

• Video

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Kinetics of Radioactive Decay

• For each duration (half-life), one half of the

substance decomposes.

• Example: Start with 50 grams of Ra-234

(half-life = 3.6 days)

After 3.6 days: 25 grams

After 7.2 days: 12.5 grams

After 10.8 days: 6.25 grams

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Half-Life

What remains after 3 half-lives? After 5 half-lives?

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Learning Check!

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

20Radiocarbon Dating: A Specific Type of Radiometric Dating

• Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation (14N + 1

on 14C + 1H)

• The C-14 reacts with oxygen to form CO2, which circulates through the biosphere and is absorbed by plants, then animals who eat the plants

• When an organism dies, the C-14 in its body stops being replenished, but the C-14 already there continues to decay with a half-life of 5730 years

So now, we could use relative dating and absolute dating techniques to figure out the ages of all the layers seen below

The Earth is composed of several integrated parts (spheres) that interact with one another:

• atmosphere• hydrosphere• solid earth

(lithosphere)• biosphere• (cryosphere)

The Earth System

Hydrosphere: the global ocean is the most prominent feature of our (blue) planet. The oceans cover ~71% of our planet and represent 97% of all the water on our planet.

For now, though, we care more about:Atmosphere: the swirling clouds of the atmosphere represent the very thin blanket of air that covers our planet. It is not only the air we breathe, but protects us from harmful radiation from the sun.

A bit more about Earth’s Atmosphere• The creation of the Earth through collisions

initially left the surface of the planet molten• Earth slowly cooled and formed a solid crust• The gasses that were originally dissolved in the

molten rocks were released• Earth’s first atmosphere resembled the gasses from

volcanic eruptions.• The first atmosphere

was composed of Water

vapor, carbon dioxide,

nitrogen and several

trace gasses. No oxygen!

And then what happened?• As the planet continued to cool the water vapor in

the atmosphere fell as rain• Rain evaporated before or upon reaching the

surface• Evaporation increased the rate of cooling• Eventually water accumulated in the low areas

creating oceans and seas• Most of the Carbon dioxide was dissolved in the

oceans• The remaining atmosphere was Nitrogen rich

Continued Atmospheric EvolutionEventually primitive organisms started to use

photosynthesis to generate energyThis produced a byproduct of oxygen gasInitially the oxygen gas would attach to metals

(iron oxide, aluminum oxide etc…)Eventually the oxygen collected in the

atmosphere (2.5 billion years ago)

Or Graphically….

Modern Composition79% N2 Nitrogen gas

20% O2 Oxygen gas.93% Argon an inert

(Noble) gas.039% Carbon Dioxide

(CO2)Although it only

makes a small percent it has large impacts globally

.031% other gasses

Radiometric Dating

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Other Nuclear Particles

e01

n10• Neutron

• Positron – a positive electron

• Proton – usually referred to as hydrogen-1

• Element isotopes (example)

H11

Np23793

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Balancing Nuclear Reactions• In the reactants (starting materials –

on the left side of an equation) and products (final products – on the right side of an equation)

Atomic numbers must balanceand

Mass numbers must balance• Use a particle or isotope to fill in the

missing protons and neutrons

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Nuclear Reactions• Alpha emission

Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.

Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

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Nuclear Reactions• Beta emission

Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

36Other Types of Nuclear Reactions You Don’t need to

KnowPositron (0+1b): a positive electron

Electron capture: the capture of an electron

207 207

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Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He ? + 1n

5 2 0

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Write Nuclear Equations!

Write the nuclear equation for the beta emitter Co-60.

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Transuranium Elements

Elements beyond 92 (transuranium) made

starting with an g reaction

23892U + 1

0n ---> 23992U + g

23992U ---> 239

93Np + 0-1b

23993Np ---> 239

94Pu + 0-1b