Transcript Geologic Resources: Part 2

The Scientific Method
Goal: Explain How Universe Works
• Scientific method: the process where ideas
are formed, tested, and refined
• Step 1:
– Questions sparked by observation or
experiments
Strange Observations
Pangaea
• Alfred Wegener
• 1910
But what does it all mean?
• Step 2:
– Formulate hypothesis: tentative explanation built
on strong supporting evidence
• Continental drift?
– Continents were once part of giant
supercontinent, Pangaea (“all earth”) and have
since drifted apart to their current locations
Wegener’s
Hypothetical Earth
• Late Carboniferous
– 300 Ma
• Middle Tertiary
– 50 Ma
• Early Quaternary
– 10 ka
But… (the hardest part)
• Step 3:
• All hypotheses must be tested
– Counter-evidence
 you’re wrong, back to beginning
– Support
 confidence, becomes scientific theory
 basis for future tests
Wegener couldn’t test it…
• Mechanism for continental migration?!
• His idea: continents floated like boats on
ocean basin crust pulled by tides of Sun and
Moon
– Unfortunately, forces are much too weak
• Open and shut?
– Wegener was apparently wrong
The Building Blocks of Earth
Science
Eratosthenes (~250 B.C)
• Experiment
– If the Sun is far enough
away, its rays are parallel
– A well in Syene, a pole in
Alexandria 800 km apart
at same time, same day
– No shadow in Syene but
shadow in Alexandria
Disproving the Flat Earth Theory
• Earth must be curved
• Calculated that earth was a sphere, 250,000
stadia in circumference at the equator
– Not sure how big a stadion
• Attic stadion (~185 m): 46,620 km (over by 16.3%)
• Egyptian stadion (~157.5 m): 39,690 km (<1%!)
– Actual circumference 40,075.16 km
Isaac Newton
• Discovered how to measure the Earth’s mass
by the strength of gravity
– Density (D) = Mass (M) / Volume (V)
– With Eratosthenes’ circumference…
– Could estimate average Earth (D) = 5.5 g/cm3
What Was the Earth Made Of?
• Major problem:
– Surface rocks mostly low density = 2.7 g/cm3
– Some iron-rich rocks had density = 3.5 g/cm3
• So where was the dense material?
– Something REALLY DENSE had to account for the
high average density
The Dense Nougat Center
• Emil Wiechert, German Physicist
 Pressure increases with depth, makes sense that deep
rocks, more compact, higher density
 Iron-nickel meteorites densest material identified
• Wiechert’s hypothesis: Earth must be layered and
iron/nickel settled to core under gravity
 Si-rich low-density “mantle” – sandstones, etc
 Iron-rich “core”
How to Go from This to This?
Earthquakes Provide Answers
• What are earthquakes or seismic waves?
– Waves that travel through rock
• What is a wave?
– Movement of energy
with little permanent
movement of mass
– Frequency = peaks/s
– Velocity = frequency * wavelength
Body Waves
(Waves that pass through the Earth)
• P-wave =
– compressional
• S-wave
– shear (secondary)
Surface Waves
(Waves that occur at the Earth’s surface)
• Love =
side to side
• Rayleigh =
up and down
Notes About Earthquakes
• All earthquakes involve all wave types
• Body waves travel fastest
– P-wave (5-6 km/s in crust), S-wave (3.5 km/s)
• Seismic waves travel faster through denser,
more rigid rocks
– P-waves can travel through liquids, solids, gas
– S-waves cannot travel through liquids, gases
How Do We Know?
The Seismograph
• Recording earthquakes
What is an Epicenter?
• Epicenter –location on earth’s surface
• Focus – place where fault first moved
• Recall P and S-wave times