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Does the Solar Nebula Theory which describes the formation of the solar system also agree with the detailed knowledge that we have about earth? A number of the more salient features of earth are:
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Does the Solar Nebula Theory which describes the formation of the solar system also agree with the detailed knowledge that we have about earth? A number of the more salient features of earth are:
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The structure is from lightest to heaviest. This suggests that in the very early history of the planet elements migrated according to weight - provided that the earth was molten (liquid). This gives rise to differentiation of the planets composition and also makes sense according to the SNT. As planetesimals collided and fused some of the kinetic energy of the planetesimals would be converted into heat. It would be quite easy to produce the temperatures needed to melt rock. As the era of planet formation ended the planet would begin to cool down - the silicate crust would harden around a hot and still molten interior.
The most useful method of determining these ages uses the technique called radioactive dating . Some elements are unstable and spontaneously split into lighter elements. This gives off energy and leaves behind "daughter" elements that, if they are stable, will slowly accumulate. THe basic idea behind radioactive dating methods is to compare ratios of abundances of mother/daughter elements. From this the amount of time needed to produce the observed abundance ratio can be determined.
Suppose you took a pure lump of Uranium and used it as a paper holder. The French physicist Henri Bequerel did this in 1895 to hold photographic (light sensitive) paper in a drawer. When he used the paper and developed the picture he also had a distinct outline of the rock! He had discovered radioactivity . Had he waited about 700 million years he would have found that his lump of uranium-235 was now half lead . Radioactive elements slowly turn into lighter elements and the time required for half of the "mother" element to turn into the "daughter" element is called the half-life of the element. Half-lives can be measured very precisely in the laboratory by measuring the amount of radiation given off by a radioactive sample each second.
Suppose you find a rock that has 7 times as much lead as uranium - how old? (assume there was no lead originally present in the rock and that it all came from the radioactive decay of the Uranium)
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Fraction
that is Uranium-235
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Fraction that is Lead-207
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1
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1/2
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1/2
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2
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1/4
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3/4
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3
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1/8
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7/8 (7 times as much lead)
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This would take 3 half lives or about 3 x 700 Ma = 2.1 billion years.
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In the Flash movie shown on the right the "mother" element is shown in red, the "daughter" element is shown in green. Blue represents stable element(s). In practice, 2 or more different minerals are used to compare the ratios of daughter/mother species and from this determine the age of the rock sample. The term "age" needs some explaining. This refers to the time at which the chemical composition that you are measuring was set. The time of solidification could mark the beginning of our time measurement, for example. |
| Mother | Daughter | half life | |
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Uranium-235 |
Lead-207 |
700 Ma |
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Uranium-238 |
Lead-206 |
4.8 Ga |
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Potassium |
Argon |
1.3 Ga |
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Rubidium |
Strontium |
47 Ga |
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When a variety of dating techniques are used the results are always similar and lead to the understanding that the earth is about 4.5 Ga old.
NOTE: In 1935 Hubble's constant was thought to be 550 km/s/Mpc. Why would geologists dispute this finding?
The SNT must also be consistent with our theories of star formation and certainly timescales such as the one suggested above are quite consistent with the theory. Had the earth only been 100 Ma old for instance then the SNT would be in trouble.
The answer has to do with the "speediness" of atoms. Both H and He are the lightest of elements and can escape from the earth's gravitational pull. They " out gas" into inter-planetary space. It turns out that the N and O atmosphere of the earth is most probably as result of gas release from vegetation . In a very real sense we must reach beyond the SNT to fully account for the structure of earth's atmosphere.
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Geologists
discovered early in this century that the continents "ride along" on
large subsurface sheets or plates
that move across the earth's mantle with dizzying speeds of a few cm per
year! The earth's crust looks like a child's attempt to glue together her
mother's favourite tea cup! Along the joins in the crust we see a great
deal of tectonic activity.
Earthquakes and volcanoes are prevalent along and indeed help trace out the location of plate boundaries where one plate collides with another. There are probably 11 plates covering the earth's mantle. As plates meet mountains may be thrust upward or one plate may may be pushed under another which can lead to powerful earthquakes. |
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The origin of plates makes sense in the SNT. As the molten earth cooled a skin formed. The surface tension in the skin and churning convective motion of the molten core helped prevent the skin from cooling into a uniform continuous covering. The motion of the plates is powered by the convection that is occurring in the deep interior and mantle of the earth. We see far from plate boundaries a different phenomenon - sea-floor spreading which allows mantle material to cycle upward to replace old crust lost through subduction. As it turns out, this is a vital process in maintaining earth's hospitable atmosphere. |
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The main plates and their boundaries. |
As molten iron-nickel moves in churning convective patterns we see the establishment of a current in the earth's interior. Rotation also helps because any moving charge is a current. But currents produce magnetic fields. The combined action of the convecting iron-nickel core and the earth's rotation conspire to produce the magnetic dynamo - still a very mysterious phenomenon
Seeds:
Chp 20
