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Magnetic Reversals
After molten lava emerges from a volcano, it solidifies to a rock. In most cases it is a black rock
known as basalt, which is faintly magnetic. Its magnetism is aligned with magnetic north and is
frozen in place at the time when the basalt cools.
Instruments can measure the magnetization of basalt. Therefore, if a volcano has produced many
lava flows over a past period, scientists can analyze the magnetizations of the various flows and
from them get an idea on how the direction of Earth's magnetic poles varied in the past. Surprisingly,
scientists using this procedure found that time periods had existed when the magnetization had the
opposite direction from today’s. All sorts of explanations were proposed, but in the end the only one
which passed all tests was that in the distant past, indeed, the magnetic polarity of the Earth was
sometimes reversed.
Ocean Floor Magnetism
In the 1950s electronic magnetometers were developed. Unlike the
older instruments, based on the compass needle, these could be towed
behind an airplane or a ship. Oil companies were soon using them aboard
airplanes, mapping the weak magnetism of rocks to help locate oil
deposits. On land, the patterns of this magnetism seemed jumbled, with
no meaningful order.
Extending those measurements to the oceans, around 1960, revealed a
surprising difference. In the ocean floor the magnetization was orderly,
Mid-Atlantic Ridge
arranged in long strips. The strips on the Atlantic ocean floor, in
particular, all seemed parallel to the "mid-Atlantic ridge." That is a volcanic ridge running roughly
north-to-south (with some zigs and zags), halfway between Europe-Africa and America. It is marked
by the focus-points of earthquakes and by some volcanic islands, and more recently it was explored
by research submarines, which have at times observed lava oozing out at its crest.
Ocean floor magnetization
(USGS figure)
Not only were the magnetic strips lined-up with the central ridge, but their structure and distribution
seemed remarkably symmetric on both sides: if (say) a narrow-wide pair of strips was observed at a
certain distance east of the ridge, its mirror image was also found at about the same distance to the
west.
Sea-Floor Spreading
This puzzling picture was explained in 1962 by Lawrence Morley (whose article was rejected by
the journals as too speculative) and by Drummond Matthews and Fred Vine. They all proposed that
the sea floor was in constant motion, pulling away from the central ridge at a rate of about one inch
(2.5 cm) per year.
As the "plates" on each side are pulled away, lava emerges from the middle, solidifies and
"records" the prevailing magnetic field. The newly formed basalt sticks to the plates and is also
pulled away--some of it towards Europe and Africa, some towards America. Every half million
years, on the average, the Earth's magnetic polarity reverses, and so does the magnetization of the
ocean floor. Each strip therefore represents an epoch of one or the other magnetic polarity, and the
symmetry is also explained. It is as if the sea-floor was a giant tape recorder, with twin tapes
emerging from the mid-Atlantic ridge, recording the Earth's magnetism at the time they emerge and
then traveling in opposite directions. Similar magnetic strips were also observed in all other oceans.
Sea-floor spreading
(USGS figure)
If the sea-floor was moving, then continents
adjoining them might share that motion, just as
Wegener had guessed. The main difference now
seems to be that rather than pushing their way
through a semi-fluid on which they float, the
continents (or some of them) ride on top of
"conveyer belts" in that fluid. These are the "plates"
which emerge at mid-ocean and go down again (at
least in some cases) at the deep oceanic trenches, like
the ones found near Japan or in the Caribbean Sea.
The science of the shaping of the Earth's crust goes
by the name "tectonics," and the process described here is the essence of "plate tectonics" by the
Earth's crust consists of distinct plates which are continually rearranged, sometimes carrying along
continents or parts of continents. The entire motion is indeed driven by convection currents caused
by the Earth's internal heat.
The Pacific plate bordering California, for instance, is slowly rotating, moving northwards. The
edge of California is attached to that plate and also moves northwards, but the bulk of the continent
does not. The juncture between the two, where one slips by the other, follows in part the famous San
Andreas fault.
Magnetic Reversals Questions
Name___________________
Block_________
Date______________
1) What does the magnetism in basaltic rocks do when it solidifies?
2) What did scientists find out about the poles when they looked at the magnetizations of
various past lava flows?
3) What were the patterns of earth’s magnetism like on land?
4) What were the patterns of earth’s magnetism like on the ocean floor?
5) What is the Mid-Atlantic Ridge located?
6) What types of seismic activity are a common occurrence along the Mid-Atlantic Ridge?
7) What did Lawrence Morley propose in his paper in 1962 to explain this ocean floor activity?
8) Briefly describe the process that is happening to the ocean floor to create this magnetic
pattern by the Mid-Atlantic Ridge.
9) How often, on average, does the magnetic polarity of the earth change?
10) Are these magnetic reversals found in any other location? If so where?
11) What is the difference between the theory proposed by Wegener and the theory that
developed from the discovery of these magnetic reversals?
12) What is the driving force behind plate tectonics?
13) What is happening at the San Andreas Fault?