Re: how does magnetic polarity on the ocean floor support plate techtonics?

Dear Marina,

Magnetic stripe anomalies were a key piece of evidence that proved 
decisive in winning widespread acceptance for the theory of plate 
tectonics in the early 1960s.  It will help when you read my answer to 
your question if you have some diagrams of magnetic anomalies and mid-
ocean ridges to hand.  Some very good diagrams and explanation of magnetic 
stripe anomalies and sea-floor spreading can be found at the following 
webpage:   http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part9.html

Another good website you might look at is: http://www.platetectonics.com/oceanfloors/index.asp

A good book that you might find helpful is Simon Lamb & David Sington, 
“Earth Story, The Shaping of our World” (London; BBC Books, 1998), ISBN 0-
563-38799-8.  This book was published to go with an excellent BBC 
television series of the same name.  It covers plate tectonics but also 
much more.  It might still be available through bookshops.

In order to understand how magnetic stripe anomalies support plate 
tectonics you need to understand (1) the basics of plate tectonic theory, 
especially the part about sea-floor spreading; (2) how the Earth’s 
magnetic field behaves, and (3) how magnetic stripe anomalies are formed 
and interpreted.  So, here goes…

Sea-floor spreading.  I am going to assume that you already know about how 
new oceanic crust is created at constructive plate margins (mid-ocean 
ridges, like the Mid-Atlantic Ridge), and how the creation of new crust 
drives plate tectonics.  An important additional point that you need to 
know is that oceanic crust is composed of high density rocks that are rich 
in iron (mainly a coarse-grained igneous rock called gabbro, but also 
dolerite and basalt, which have the same composition as gabbro but are 
finer grained).  Gabbro, dolerite and basalt are formed by melting at mid-
ocean ridges of the rock, called peridotite, which forms the underlying 
mantle.  Peridotite is rich in minerals that contain iron, so the magma 
and ultimately the gabbro, dolerite and basalt derived from it are also 
rich in iron-bearing minerals.  The presence of iron in these rocks and in 
the magma from which they cool is an important point to which we shall 
return.

The Earth’s magnetic field.  Although it is not completely understood, the 
Earth’s magnetic field is thought to be created by convection of the 
liquid iron/nickle alloy that forms the Earth’s outer core.  The outer 
core is extremely hot (its temperature is thought to range from about 3000 
degrees C in the upper regions to about 6000 degrees C in the lower region 
near where it meets the solid inner core) and convects like hot water in a 
tightly enclosed pan, under great pressure.  The convection of liquid iron 
acts like a dynamo, creating the Earth’s magnetic field.  Like an ordinary 
bar magnet, the Earth’s magnetic field is bipolar, with a North Magnetic 
Pole and a South Magnetic Pole.

For reasons that are not yet well understood, but are probably to do with 
instabilities in the convection currents in the outer core, the Earth’s 
magnetic field flips back and forth from time to time, when the magnetic 
poles reverse their position.  Thus the magnetic field has two states:  
either normal polarity like that of the present, or reversed polarity.  In 
times of normal polarity the North Magnetic Pole is located in the Arctic 
and the South Magnetic Pole in the Antarctic.  During times of reversed 
polarity the North Magnetic Pole is located in the Antarctic, and the 
South Magnetic Pole in the Arctic. Today a compass is magnetized so that 
it points north towards the North Magnetic Pole, but when the magnetic 
poles reverse position the same compass would point south toward the North 
Magnetic Pole!

How magnetic stripe anomalies form.  Now we are ready to explain magnetic 
stripe anomalies.  I’m sure you can already see where this argument is 
going!  The iron contained in the rocks of the oceanic crust - gabbro, 
dolerite  and basalt - can be magnetized.  When the magma upwelling along 
mid-ocean ridges cools, the iron will take on the magnetic polarity of the 
Earth’s magnetic field when the temperature of the cooling rock drops 
below about 730 degrees C (known as the Curie temperature for iron).  The 
magnetic polarity imprinted on the iron will be frozen in place in the new 
rock of the oceanic crust at the mid-ocean ridge.  Because the Earth’s 
magnetic field has two states, normal and reversed polarity, the rocks of 
the oceanic crust will be magnetized with either normal or reversed 
polarity, depending on which state happened to prevail when the magma 
cooled to rock.

At mid-ocean ridges new oceanic crust is being formed continually by 
upwelling of magma along the rift.  The magma cools to form rock, taking 
on the magnetic polarity prevailing at the time.  The new crust eventually 
splits apart, with the two sides moving away to either side of the rift to 
make room for fresh magma to rise up.  Thus parallel bands of crust on 
either side of the mid-ocean ridge have the same magnetic polarity.  When 
the Earth’s magnetic field reverses its polarity, the rock being formed 
will take on the new polarity, and so as it splits and moves away from the 
ridge it will create new bands of crust on each side with the opposite 
polarity to their predecessors.

In this way, the ocean floor comes to consist of alternating bands (i.e. 
stripes!) of crust running parallel to the mid-ocean ridges, with every 
band having the opposite polarity to the bands on either side of it.  
Furthermore, the pattern of bands on one side of the mid-ocean ridge is 
exactly matched by the pattern on the other side.  Thus the same pattern 
can be traced on either side of the ridge, often to a distance of several 
thousand kilometers!

Reading and interpreting magnetic stripe anomalies.  We can “read” the 
pattern of magnetic anomalies on the ocean floor.  When the anomalies have 
been mapped we can compare the pattern of stripes on one side of a mid-
ocean ridge to the pattern on the other side.  Invariably we find that we 
can match up each stripe on one side with its twin on the other.  Because 
the patterns match up to great distances away from the mid-ocean ridge, it 
is obvious that the ocean floor must have moved away from the ridge on 
either side.  That is how magnetic stripe anomalies demonstrate the 
reality of sea-floor spreading.  Sea-floor spreading, in turn, is a key 
element of plate tectonic theory.