Earth

 

Topics:

a) bulk properties

.................Diameter (13,000 km)

.................Mass (6 x 10^27 grams ) --how do we know?

.................Density = Mass/Volume

.............................. = 5.5 g/cm3

.................Surface density lower (1- 4 g/cm3) --what does this mean about the interior?

.................Shape (oblate spheroid)

.................Equator = Rpoles + 20 km ..................due to Earths spin (why?)

 

Earth Structure from Center Outward:

(Geo-morphology)

..................................Inner Core

.................................Outer Core

.................................Mantle

.................................Crust

.................................Hydrosphere

.................................Atmosphere

.................................Magnetosphere

 

Note that Crust and Hydrosphere together are known as the Lithosphere

 

Structural Evolution:

Differentiation:

Why does the Earth's density increase towards the center? Remember that the formation process caused the Primeval Earth to be Molten...

 

after accretion (the combing of larger particles to form planet sized objects), heavy elements like Iron and Nickel floated to core, while lighter silicates floated to crust, eventually cooling.

 

Cratering:

After most of Earth's Mass was accreted four and a half billions years ago, there was still lots of debris left in solar system. The Earth rapidly "swept up" debris in its path. The resulting collisions reshaped the Earth's surface and brought new materials.

 

--In particular, planetesimals that condensed in outer part of solar nebula were full of volitiles such as water ice

 

--at least ten percent, and possibly more, of our water was accumulated in this way.

 

--Cratering collisions to the present, although much reduced in frequency.

 

--the Moon was most likely created in a collisional event.

 

Flooding of cratering basins:

decay of radioactive elements heated interior so that lava welled up through fissures in the crust, flooding deeper basins with basalt. Water also began to fall, filling basins with oceans.

 

Slow surface evolution:

crustal motion (plate tectonics) result in mountain building and sea floor spreading, erosion due to water, volcanic activity, and restructuring by man and animals.

 

Present structure of Interior:

Core--inner 22% is solid Fe-Ni due to high pressure and resulting high melting point. Outer 33% is molten Fe-Ni due to lower melting point. Still hot from radioactive decay.

 

--Outer part in convective motion coupled with Earth's rotation. This process is known as the dynamo effect.

 

--Ni-Fe is an excellent conductor (better than copper) so that currents flow freely there creating the Earth's magnetic field.

 

--Motion is a bit chaotic, so that over time, reversals occur, changing direction of the magnetic field or turning it off all together.

 

Mantle:

--The Next layer out from the Core. 3000 km thick layer comprises 80% of Earth's Volume.

 

--Comprised of dense rock (silicates) and metal oxides. not molten, but not solid.

 

--actually a type of plastic that flows under high pressure (due to weight of crust). Also hot, but not so hot as the core.

 

Crust:

--Lighter silicates (density approx 4 gm/cm^3).

 

--Floats on Mantle due to lower density.

 

--Brittle and breaks easily.

 

Plate tectonics

energy to move plates comes from hot interior.

Convection currents of hot mantle rise through deeper layers and spread out under the crust. Cooler regions sink. Circulation like this drags the crust around at speed of a few cm/year.

 

mid ocean rises

areas where hot mantle material rises out of the thinner crust and spreads out along the sea floor as plates spread apart.

 

crust can sink below a plate

(pacific ocean floor is slipping below northwestern North America), or plates can be locked together (Atlantic plate is locked to Eastern North American plate, pushing it westward--its been doing this for 200 million years since continents were locked together (Pangea)


Earthquakes:

Often due to the slipping of one plate past another. An example is the pacific plate(carrying Southern California) past the the North American plate as the pacific plate moves northward.

 

--Can be triggered by Volcanic activity

 

--Can cause separated plates to resonate with the Earth's vibrations like a giant tuning fork.

 

--Study of seismic waves allows Geoscientists to model the Earth's interior.


Planetary Thermodynamics:

Where does the energy to produce volcanism, drive crustal plates, and maintain the Earths magnetic field come from?

 

I) Heat remaining from the Accretion Process

--without an continuing source of heating, the Earth's Core will still be far warmer than the crust. Like the cheese topping on a pizza keeping the sauce from cooling, the Thick Mantle insulates the heat contained below.

 

II) Fission of Heavy Nuclei (Nuclear Fission)

--Unstable heavy elements (primarily Uranium) release heat as they decay.

--Most Heavy Nuclei are found in the inner core due to the differentiation process.

 

Primary Nuclear Fission Reaction:

Uranium atoms decay to Thorium Atoms with the release of a Helium Nucleus

U238 -->> Th234 + He4

(238,234, & 4 refer to the combined number of protons and neutrons)

 

--the half life of this reaction (T1/2) = 4.5 x 10^9 years. Note that this is approximately the age of the Earth! (quick Question: what fraction of the Earth's original Uranium supply remains?)

 

--energy released = 4.25 Mev/decay. One Mev is about 10^-17 food calories (so 10^17 atoms must decay to produce one calorie!)

 

-- Energy released because mass of products "daughters" is less then mass of "parents" . If you could weight the Thorium and Helium Nuclei, they would weigh less than the original Uranium atom.

 

-- The "m" in Einstein's E = mc^2 refers to the mass that is "lost". This equation describes the amount of energy released when mass is lost, or converted to energy.

 

-- rate of decay and rate of energy release is proportional to amount of parent nuclei. Since Uranium atoms have been decaying over time, these rates were much higher in past.

 

--Consider what it would be like to live on an Earth with over twice as much heat being released....This was the world of the Archean Era which began once the acretion period was completed.

Radioactive Dating methods:

Understanding the decay of nuclei allows scientists to "Date" samples found on the Earth.

 

Uranium Dating allows determination of the age of inorganic material.

 

- Daughter/Parent ratio gives date when an inorganic object solidified. In many cases, the parent is Uranium 238, and the daughter is lead 207 (the rapid decay product of Thorium)

 

i.e. U-238 --> Th-238 --> pb-207

 

From the ratio of pb-207/U-238 and knowledge of the half lifes, we can determine when the rock formed.

This is accurate all the way back in time! For example, if we find a rock sample with the same amount of lead as uranium, we know that sample is 4.5 billion years (one half life) old.

 

Carbon dating:

Carbon dating can be used to determine the amount of time since a living (organic) object died.

--Carbon, like most elements, has several isotopes. Carbon 12 (six protons and six neutrons) is the most common and stable form.

 

--Carbon 14 (six protons and eight neutrons) is continuously produced in the atmosphere from Cosmic rays. It decays with a half life of 6000 years.

 

--While an organism is alive, it has as much Carbon 14 as the atmosphere in general. When it dies, its Carbon 14 decays away.

 

--The C14/C12 ratio gives time since organism died. For Example, a piece of wood with a C14/C12 ratio half that of the atmosphere is approximately 6000 years old.

 

--Currently accurate to 50,000 years in the past. After that, the amount of Carbon 14 becomes too small to detect accurately.

 

--Can be tested against historical records (i.e. a page in a book can be dated, and compared with the date on the text).

 

Formation of Earth's Atmosphere:

Our atmosphere has been evolving continuously since the planet accreted. However, it is useful to consider three stages in its evolution....

 

I) Primeval atmosphere--gasses drawn in from solar nebula (methane, hydrogen, helium, ammonia. Water ice also arrived from space as part of planetsimals (meteorites and comets)

time: 5 billion B.C. to 4.5 Billion B.C.

 

II) Secondary (pre-biotic) atmosphere .

The period of constant change before the onset of life is called the Archean Era

time: 4.5 billion B.C. to 3.2? billion B.C.

 

--CO2 and N2 released in large quantities from volcanic outgassing. Remember that volcanic activity was much greater in earlier epochs.

 

--Planetesimals brought water from all over the solar system.

 

--destruction of methane, ammonia, and water from ultraviolet photons.

No ozone to block u.v., plus a pulsating sun caused ionization to occur all the way down to ground level.

 

--Cooling atmosphere produced rain, so CO2 dissolved in the water and reacted with Silicon, calcium, and magnesium to form limestone.

 

An important process in the development of our atmosphere is the escape of hydrogen and helium into space. Why does this occur?

 

whether or not a particular gas (i.e. Oxygen) will stick around depends on:

--what the temperature is

--how heavy (massive) the molecules/atoms are

--what the surface gravity is.

 

Conditions on Earth are just right for O2, and N2 to stick around--i.e. they are created as rapidly as they are destroyed or leak into space. Lighter molecules and atoms such as H2 and Helium leak away far more quickly than they are replenished, while heavier molecules such as CO2 remain until they are destroyed or transformed.


III) Transition from second to Terciary

(formation of present atmosphere)

Time: 3.2 billion B.C. to present:

 

-- evolution of plant life (phytoplankton or cyanobacteria, or blue-green algae) in oceans and on land produced most our present O2 from photosynthesis.

 

--- Slowly at first since O2 "waste" was a poison gas to early plant life.

 

--Atmospheric oxygen is still increasing at a rate of about 1 percent every 36 million years.


IV) Present Atmospheric Composition

Now we have:

.....................................O2 21%

.....................................N2 78%

.....................................ARGON 1%

.....................................CO2 .03%

.....................................METHANE 1.7PPM

.....................................WATER VAPOR VARIES

 

V) HUMAN ATMOSPHERIC EFFECTS

....................BIG PROBLEMS:

GREENHOUSE EFFECT --CO2 RISING BY APPROXIMATELY 20%/DECADE!

OZONE DEPLETION--ANTARCTIC HOLE, UV RISING.

 

....................RELATED PROBLEMS:

DEFORESTATION

HUMAN OVERPOPULATION

ANIMAL AND PLANT EXTINCTIONS

ICE AGE ISN'T FINISHED? OR IS IT?