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

• Alfred Wegener• 1910Pangaea

Strange Observations

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 permanentmovement of mass

– Frequency = peaks/s– Velocity = frequency * wavelength

• P-wave = – compressional

• S-wave – shear (secondary)

Body Waves(Waves that pass through the Earth)

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