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Summary CH3 earth portrait of a planet
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Samenvatting chapter 3 Drifting continents and spreading seas
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Solutions Manual for Earth Portrait of a Planet, 5th edition by Stephen Marshak All Chapters 1 to 23 complete Verified editon ISBN: 9780393937503
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Chapter 3 – Drifting continents and spreading seas3.2 Wegener’s Evidence for Continental Drift
Alfred Wegener was the first one who viewed the continents and oceans as mobile. He formulated the
theory of continental drift. Arguments for his hypothesis:
1. Fit of continents
2.Locations of past glaciations; glaciers are rivers or sheets of ice that flow across the land surface.
When ice melts, it leaves the sediment in a deposit called, till. The occurrence of till and striations 1
serve as evidence that the locations was covered by glacier in the past. They found these tills at weird
places (not in high latitudes), and by looking where those glaciated areas where, he found that it would
be one big ice sheet if South America, Africa, India, Australia and Antarctica would be together.
3. The distribution2 of climatic belts
If the hypothesis about Pangea would be correct, North America, Europe and Africa would have
straddled the equator and would have tropical or subtropical climates. So Wegener searched for clues.
In sedimentary rock, for example, is it possible to see what kind of nature and animals were growing
so also which climate there was at the time the sediment formed. And he found it; late Paleozoic
sedimentary rock layers included abundant coal and the relicts of reefs.
4. The distribution of fossils
Fossils were found of species at divergent continents, unable to swim.
5. Rocks on different sides of the ocean match.
1 Striation=streep
2 Distribution=verdeling
,3.3 Paleomagnetism- Proving continents Move
Rocks preserve paleomagnetism, a record3 of Earth’s
magnetic field in the past. Circulation of liquid
iron alloy in the outer core of the Earth
generates a magnetic field. The north seeking end points
to the north magnetic pole. (The north pole is southern
polarity!!) Earth’s magnetic poles do move
constantly. The poles are at present, hundreds
of kilometers away from the geographic pole.
So the magnetic dipole tilts at about 10
degrees relative to the Earth’s spin axis. The
angle between the direction that a compass
needle points and a line of longitude at a
given location is the magnetic declination.
The
magnetic field lines
lie parallel to the
surface of the Earth. The angle between a
magnetic field line and the surface of the
Earth, is called
the magnetic
inclination. The inclination is 0 degree at
the equator and 90 degrees at the poles.
Paleomagnetism
When you research old rocks, you will see differences in declination an inclination. These differences
arise because rock is preserving a record of the orientation of the location of the magnetic pole,
relative to the rock, at the time the rock formed.
Igneous rocks, for example, can preserve a good record of paleomagnetism. When lava cools and
solidify into rock, tiny magnetite crystals begin to grow. At first, thermal energy causes the magnetic
dipole to wobble and tumble chaotically but when the rock cools, the dipoles align with the Earth’s
magnetic field. The rock cools down so much that the dipoles can no longer move and lock into
permanent parallelism.
3 Record=opname
, Sedimentary rocks can also preserve a good
record of paleomagnetism. The record forms
when magnetic minerals grown in spaces between
grains after the sediment has accumulated. These
minerals form from ions that had been dissolved
4
in groundwater passing through the sediment.
A proof that continents move
At first, geologists still thought the continents did
not move. So geologists introduced the term
paleopole to refer to the supposed position of the
Earth’s magnetic pole in the past. They measured
paleomagnetism of rocks from the same location but different ages. The positions of the paleopoles
traced out a curving line that came to be known as an apparent -polar-wander path. They did it at
more continents, and they saw that each continent had a different polar-wander path. So they
concluded that it’s not the pole that moves relative to fixed continents but rather the continents that
move relative to a fixed pole.
3.4 The discovery of seafloor spreading
Bathymetry is the shape of the seafloor surface. The invention of sonar
permitted information about the bathymetry to be gathered 5quickly. To
make a sounding 6using sonar, a ship emits sound pulse that
travels down through the water, bounces off the seafloor, and
returns up as an echo through the water to a receiver.
Bathymetry features
-Abyssal plains
Flat regions of the ocean that lie at a depth of 4 to 5 km below sea level
-Mid ocean ridges
Submarine mountain ranges whose peaks lie about 2 to 2,4 km below sea level. The crest 7is called the
ridge axis. Mid-ocean ridges are roughly symmetrical.
4 Dissolve=oplossen
5 Gathered=verzameld
6 Sounding=peiling
7 Crest=top
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