On the Road to the Plate Tectonic Theory
More Background

 In the 1960s, several new discoveries were made due
to new development in the mapping of the ocean
floor.
 The findings showed the:
 Existence of an underwater mountain chain that
extends around the world
 Presence of deep ocean trenches
 That the ocean floor is a relatively young age.
The Oceanic Mountain
Chains

 These mountains outrival all mountain chains on
land
 These mountains are not like the mountains we find
on most continents.
 The chain extends for some 65,000 km in all oceans
and are 1500 km wide.
 The chains are known as “mid-ocean ridges”, and
new sea floor is being created by volcanic activity
 Lava eruptions from the mid-oceanic ridges takes
place from rifts in the ocean ridge.

 This process creating a new seafloor creates a
spreading as the older lava is pushed to either side of
the rift.

 Oceanic Trenches were later related to areas where
geological destruction is occurring.
 These two ideas known as Seafloor Spreading and
that of Wegener’s Continental Drift Theory were
amalgamation that form what is now known as the
Plate Tectonic Theory
The Evidence

 Paleomagnetism – the study of the Earth’s ancient or
fossil magnetic field.
 Fossil magnetism can be detected in rocks by using an
instrument called a magnetometer
How Does
Paleomagnetism Work

 The Earth is essentially dipolar (two poles), like a bar
magnet with a North and South Pole.
 The magnetic field generated is believed to be
created from the liquid outer core of the Earth.

 The magnetic field create lines of force, which are not
visible to the eye but can be detected by small
magnets
 When rock is molten (lava or magma), magnetic
minerals (containing iron) in the molten material are
randomly oriented

 With solidification of the molten materials, the
magnetic minerals (or fossilized compasses) will
align themselves with the Earth’s lines of force.

 Using a magnetometer on a solid rock can give us
two pieces of information at the time of
crystallization

 The direction of the North Pole
 The angle of magnetic dip.
 This information combined together can tell us what
latitude that rocks were formed.

 This in turn has told us that land masses have
moved.
 An example is that there are rocks in part of Nova
Scotia that indicate they were formed at latitude 0º,
the equator, yet the location is presently sitting at 45º
North
The Other Discovery

 Further paleomagnetic studies of rocks have also
indicated that the North Pole is in a different location
today from where it was in the past
 The moving of the poles is termed
apparent polar wandering
 The study of the ocean floor by paleomagnetism
revealed more information.

 An invisible striped of high and low magnetic
intensity on the ocean floor.
 Rock with patterns of high or positive magnetic
intensity that are aligned with the Earth’s magnetic
field of today are referred to as normal polarity.

 Rocks of low or negative magnetic intensity, which
are opposite direction of present day are termed
reversed polarity.
 This pattern on the ocean floor means that the
Earth’s magnetic poles have revered a number of
times over the Earth’s history.

 The stripped pattern also being symmetrical about
the oceanic ridge supported seafloor spreading
hypothesis

 Later drilling of the ocean
floor confirmed that the
youngest ocean rocks are
centered on the ridge and
get progressively older
as one moves away form
the ridge in either
direction.