1/17/06ge146 plate tectonics1 the scientific method question: what is the ``scientific...

1/17/06 GE146 Plate Tectonics 1

The Scientific method

Question: What is the ``scientific method''?Answer: The best way yet discovered for winnowing the truth from lies and delusion.

The simple version looks something like this:

1. Observe some aspect of the universe.2. Invent a tentative description, called a hypothesis,

that is consistent with what you have observed.3. Use the hypothesis to make predictions.4. Test those predictions by experiments or further observations

and modify the hypothesis in the light of your results.5. Repeat steps 3 and 4 until there are no discrepancies

between theory and experiment and/or observation.

When consistency is obtained the hypothesis becomes a theory.

This is the accepted recipe for scientific conduct


Wegener and the Scientific methodOBSERVATIONS BY WEGENER

• Matching coastlines of Africa and the Americas• Matching geologic formations• Matching fossils• Evidence for past climate changes• …

HYPOTHESIS Continental driftPEDICTIONS Motions of continentsPUBLICATION “The Origin of Continents” (published in 1912)REVIEW BYGEOLOGISTS This is impossible!

Hypothesis rejected ……. for 40 years, until new observations (primarily) inthe oceans led to a new mechanism for continental drift

REVISION Wegener’s hypothesis was revived and it is nowestablished theory: The Theory of Plate Tectonics


Earth facts

Earth (average) radius = 6371 km

Average Distance to moon = 381,555 kmAverage Distance to sun = 149,600,000 kmAverage Distance to Alpha Centauri (closest star) = 41 trillion km

Average Distance to moon = ~1 light-secondAverage Distance to sun = ~8 light-minutesAverage Distance to Alpha Centauri (closest star) = ~4 light-years


Earth in the solar system

Out of 9 planets, Earth is the third planet at a distanceof 8 light-minutes (150 million km) from the sun.


Our solar system in the Milky Way galaxy

100,000 light-years across

SUN

One milky-way rotation takes about 250 My


Our Milky Way galaxy in the Universe

A 100 billion galaxies eachcontaining over 300 billion stars

14 billion light years wide,and growing …


The empty Universe: the space between stars

The diameter of the Sun = 1,391,980 kmThe distance from Sun to Earth = 146,000,000 kmThe distance from Sun to Alpha Centauri= 4 light-years = ……… km

To get a perspective of these distances, let’s scale them down.Suppose we want to make a museum model of the Sun, Earth, and Alpha Centauri.We construct a sun out using a tennis ball that is 10 cm (= 0.1 m = 0.0001 km) large.

(1) How far should we put Earth from our “tennis ball Sun” torealistically represent the distance between them?

(2) How far should we put Alpha Centauri from our “tennis ball Sun”?

(is it likely that stars collide with each other?)


The origin of atoms …

Where do atoms come from?The oxygen (O) in the air ..The silicates (Si) in stone ..The mercury (Mg) in thermometers ..The iron (Fe) in railway lines ..The sodium (Na) in sea-salt ..

About 13 Ga (13 billion years ago), just after the Universewas formed after the Big Bang, all these atoms didn’t exist!


… nucleo-synthesis in stars

Hydrogen (H) atoms “fuse”to produce Helium (He) atoms,and a whole lot of energy(like hydrogen bombs!).

•Big Bang nucleosynthesis occurred within the first three minutes of theUniverse and is responsible for most of the Hydrogen (H) and Helium-4 (He).•Stellar nucleosynthesis creates the lighter elements by fusion of H and He.•Supernovas (steller explosions) produce the elements heavier than oxygen.

We are all made of stardust … !


The Periodic Table


The Nebular synthesis: formation of solar systems IHydrogen (H), He (He) and “star dust”

(heavier elements that were formed in previous stars)


The Nebular synthesis: formation of solar systems II


The Nebular synthesis: formation of solar systems III


The Nebular synthesis: formation of solar systems IV


Sun, Terrestrial, and Juvian planets

Terrestrial (inner) planets•relatively small•contains the heavier elements (Fe, Si, O)•rocky shell over a metallic core•accretion began 4.567 billion years ago•it took about 0.1 billion years(100 million years) for planets to form

Juvian (outer) planets•mostly volatile gases (H,He)•same composition as the sun,but internal pressure is too lowfor nucleo-synthesis to take place

The sun has 99.8 % of the total mass

What’s up with Pluto (and Xena): should it be called a planet?


Pluto: a planet?

•comprised of rock and ice•not in the plane of plane of rotation around the sun•more eccentric (less circular) than other planets

Is Pluto a captured comet?

Pluto

Charon(moon)

sun orbital plane of planets

Pluto’s orbit around the sun


Earth formationNebula forms out of H, He cloudsand fusion products from now dead stars.

Gravity pulls gas and dust inward, and rotationmakes a accretionary disk. The proto-sun is atthe center and fusion begins when it is hot enough.

Heavier elements concentrate in the inner rings. Collisions andgravitational attraction leads to the formation of planetesimals.

Gravity reshapes theproto-Earth into a sphere

Soon after Earth formed, a Mars-size planet collided withEarth, blasting debris into space that will form our Moon.

Volcanic gases form theatmosphere, and as Earth cools, moisture

condenses and rains into oceans. Somegases may have come from passing comets.


Observation of impacts on the planets

Crators on Moon

Crators on comet Wild-2

“Happy-Face Crator” on Mars


Observation of impacts on Earth

“Meteor crater” in Arizona


Impacts on Earth and their effect on life“Meteor crater” in Arizona


StarDust Mission by NASA: 1999-2006

Recovery of capsule in Utah, Jan 15, 2006

http://stardust.jpl.nasa.gov

Entry in the atmosphere

Clean-room work

Capsule

Capsule on ground


Moon formation

~ 100 My after Earth accreted

Spin axis makes 23o angle with orbital plane


Why do we have seasons?

northern summer


Earth’s differentiation I

Early in Earth’s history Today

Differentiation of Early Earth rendered a “layered” planet (much like an onion)comprised of a dense (primarily iron), core, a light crust, and a “residual” mantlein between


Earth’s differentiation II

Early in Earth’s history Today

Differentiation (due to Earth’s hot interior):•early Earth is hot due to violent impacts and•and the abundance of radio-active element (K40, U)

•the outer half of the earth is molten (“magma (molten rock) ocean”)•the interior is hot and very soft: it can easily flow

The energy released when a Mars-sizeobject impacts Earth (this causedMoon formation) is equivalent to100,000,000 megaton nuclear bombs(one megaton bomb destroys a city)

Differentiation: the heavy stuff goes down,the lighter stuff goes to the surface


Earth as a layered planet I


Earth as a layered planet II

Earth’s core:Iron (Fe) and Nickel (Ni), which are muchdenser than other elements, sank to forma central core. It begins at 2900 km depth.The core is liquid. Currents in the core areresponsible for the generation of Earth’smagnetic field

The inner-core is solid. Scientists are stilldebating when it formed in Earth’s history.


Earth as a layered planet III

Earth’s crust:The lighter elements (O, Si, Al), floatedtowards the surface of the “magma ocean”and formed the crust.The oldest crustal rocks are 4.3 By.The crust is about 10 km thick beneaththe oceans and about 30-40 km beneaththe continents. The thickest crust (~70 km) isfound beneath the Andes and the Himalayas.


Earth as a layered planet IV

Earth’s mantle:Between the core and the crust is the mantle.It forms the bulk of the solid Earth, and has“intermediate” density. The mantle rangesfrom 40 to 2900 km depth.


Earth differentiation III

Earth’s composition:From the 100 elements 8 elements make up 99% of Earth’s mass.In fact, only 4 elements make up 90% of Earth’s mass.These are:

(1) Iron (Fe) 35%(2) Oxygen (O) 30%(3) Silicon (Si) 15%(4) Magnesium (Mg)13%

The core is almost entirely made out of Iron. The lighter elements form the crust(Note that crustal rocks on which we are standing are almost 50% oxygen).


Earth differentiation IV: oceans and atmosphere

Two ideas:

(1) Air and water come from volatile-richcomets that impacted the Earth, which(we know now) consists of water-ice andfrozen CO2.

(2) The Wet-Earth hypothesis:Planetisimals that aggragated into Earthcontained ice, water and other gases.They were orinially locked in minerals,but escaped as Earth melted duringheavy bombardements.

Mostly Hydrogen (H), which escaped into outer spaceCarbon-dioxide (CO2)Nitrogen (N)water vapor (H2O)other gases (methane, CH4)


Home work: give an example of the Scientific Method

I want each of you to come up with yourown example of the Scientific Method.

We will discuss these on Tuesday in class.Short power point presentation (two minutes)on a memory stick is prefered, or e-mail meyour presentation at least a few hours before class.Hand me a paper copy of your presentation.

1. Make an observation2. Make a hypothesis3. Test it.4. Draw a conclusion

1/17/06ge146 plate tectonics1 the scientific method question: what is the ``scientific...

Documents