Download Chapter 1 – Plate Tectonics

Name: ________________________________
Date: _________________________________
Flynt - ____ Period
____th Grade Science
Developing a New Theory

Scientists rejected Wegener’s Theory of Continental Drift because he
could not explain HOW or WHY the continents moved, but they did not
forget about it!

In the 1950’s, new evidence began emerging that made geologists take
a second look at Wegener’s theory.
 Four Areas of Discovery (new evidence)
1.) Ocean Floor Mapping/Bathymetry (previously discussed)
2.) Magnetic Striping and Pole Reversals
3.) Seafloor Spreading and Recycling Old Crust
4.) Seismic Evidence/Concentrations and Distribution of
Earthquakes
I. Magnetic Stripes and Pole Reversals

The Earth behaves like a giant magnet,
with a magnetic north pole and a magnetic
south pole
 Throughout Earth’s history, Earth’s
magnetic poles have flip-flopped several
times
- A flip or reversal in the Earth’s magnetic poles occurs on
average every 250,000 years.
- The last reversal occurred 780,000 years ago, so we are long
overdue for the next magnetic pole reversal!

During World War II, ships dragging magnetometers (devices to
measure magnetism) found bands of alternating strong and weak
magnetism in the rocks of the seafloor called magnetic stripe
"anomalies".
- The alternating magnetic bands had different widths and ran
parallel to the mid-ocean ridges like zebra stripes.
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- The bands were also symmetric (mirror image) on either side of
the MOR.
Summary
- The magnetic stripes were caused by the presence of magnetic
minerals in the rocks.
~ Basalt, the rock that makes up most of the oceanic crust,
contains a strongly magnetic mineral called magnetite
(lodestone).
~ When magma (molten rock containing minerals and gases)
cools to form a solid rock, minerals like magnetite will line
up with Earth’s magnetic field.
 The magnetite minerals in some of the bands were lined up with
the current position of Earth’s magnetic north and south poles.
These rocks had normal polarity.
 In the other bands, the magnetite minerals were lined up in the
opposite direction, with the north end of the rock’s “compass
needle” pointing to the south. These rocks had reversed
polarity.

1963 – Two British geologists (Fred Vine and Drummond Matthews)
joined the map of the Mid-Atlantic Ridge with the maps of the
symmetric bands of magnetism on the seafloor.
 The overall pattern created by the alternating bands of
normal and reversed polarity rocks became known as magnetic
striping.
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Summary |
II. Seafloor Spreading and Recycling Oceanic Crust

The discovery of magnetic striping raised new questions:
- How does the magnetic striping pattern form?
- Why are the stripes symmetrical on either side of the MOR?

1961- American geologist Harry Hess developed a new theory (the
Theory of Seafloor Spreading) to explain what was happening at
MOR’s:
 MORs are weak places in the ocean floor where the ocean floor
is being ripped apart.
 New magma from deep within the Earth rises along these weak
zones and erupts along the crest of the ridge, forming new
oceanic crust.

The Theory of Seafloor Spreading was supported by this evidence:
1.) Rocks are youngest near the crest of the ridge and the rocks
gradually become older as you move farther from the ridge.
~ Older rocks are carried away from the ridge like a
conveyer belt.
2.) The youngest rocks located along the ridge crest ALWAYS have
normal (present-day) magnetic polarity.
~ These rocks show normal polarity because they have just
recently erupted from the ridge and cooled to form new
crust.
3.) Each of the magnetic stripes found in the rocks parallel to the
MOR match up with past flip-flops of the Earth’s magnetic poles
(normal and reversed polarity).
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Summary |
 Harry Hess’s theory of Seafloor Spreading is important because it
says that new crust is continually being created along the midocean ridges!!!
- The ocean floor is ripped apart by convection in the mantle (in
the asthenosphere).
- As the sea floor spreads apart, new crust is added in the gap
that forms.
- The magnetite minerals in the new crust line up with Earth’s
current magnetic field.

Question - If new crust is continually being created at MORs, does
that mean that the Earth is getting bigger and bigger?
- Some geologists first thought that maybe the Earth has been
expanding over the past 4.7 billion years. (NOT true!)
- The Earth has actually stayed about the same size!!!

Harry Hess thought that old oceanic crust is recycled back into the
mantle at deep-ocean trenches!
- At a deep-ocean trench, the ocean floor bends downward and
plunges into a deep canyon.
- Old oceanic crust is subducted and sinks back into the mantle.
- In the asthenosphere, old, cold, dense oceanic crust sinks and
melts.
~ This happens in the same place where the convection
currents are sinking!!!
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Summary |

Question: Why does old oceanic crust get subducted?
 BATTLE OF DENSITY!!!
~ New oceanic crust is warm (it just formed from hot
magma)

has LOW density!!!
~ Old oceanic crust is very cold

VERY DENSE!!! (old + cold = high density)
~ Continental crust is made of rocks like granite that are
less dense than the rocks in oceanic crust.

Continental crust is the LEAST dense!!!
 When two plates collide, the one with the highest density
will be subducted!!!

Subduction and Earth’s Oceans
 The ocean floor is renewed every 200 million years or so.
 Seafloor spreading and subduction can change the size and
shape of Earth’s oceans.
~ The Atlantic Ocean has been steadily expanding over the
past 250 million years

New crust is forming at the Mid-Atlantic Ridge.

There are no subduction zones in the Atlantic.
~ The Pacific Ocean is has been slowly shrinking over the
past 250 million years.
Summary:
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Summary |

New crust spreads out quickly from the active East
Pacific Rise.

More crust is destroyed than created in the Pacific
because it is surrounded by active subduction
zones.