Plate tectonics theory was not accepted for
centuries because no one could adequately
answer the question, “What is the mechanism
that drives the plates?”
In the late sixteenth century. Abraham Ortelius,
in compiling New World explorer maps noted
that by carefully considering the coasts lines
of the Atlantic Ocean it appeared that the
Americas had been torn away from Europe and
Africa by, he theorized, earthquakes and floods".
Over the next three centuries other proponents
of an original single continent included:
Francois Paget who invoked the sinking of
the land between the continents to create
the Atlantic Ocean;
Georges-Louis Leclerk who resurrected Ortelius’s
theory of a great earthquake and floods pushing
the land apart, and finally
Antonio Snider Pellegrini who proposed that
the shape of the continents, supported by
fossil evidence argued for the origin of a
single continent which not only joined the
continents across the Atlantic Ocean but also
included Australia.
No one, however, could explain how the continents
moved.
Bathymetric surveys in the following decades
unveiled an extensive submarine mountain ridge
the length of the Atlantic Ocean, between
the continents.
Meanwhile, seismic data began to reveal Earth’s
layers: a core, a crust-mantle boundary, and
a thick outer layer called the lithosphere
that lay above a less-dense asthenosphere.
In 1912 German meteorologist Alfred Wegener,
also intrigued by the fit of the edges of
the continents, championed the concept of
“Continental Drift” based on his study
of similar rock type, geological structures
and fossils on both sides of the Atlantic.
To make it work, he hypothesized that the
mechanisms causing the drift might be the
centrifugal force of the Earth's rotation
or the change in its axis of rotation.
Wegener also speculated that “the Mid-Atlantic Ridge,
is continuously tearing open and making space
for fresh, relatively fluid and hot material
from depth.”
His views were considered preposterous and
improbable, and rejected by most Earth scientists.
Following the discovery of radioactivity in
1896, it became clear that Earth's interior
was heated by radioactive decay, and the insides
would be largely molten.  Indeed, by 1926
a liquid core was determined.
In 1927 geologist Arthur Holmes, studying
radioactive decay, proclaimed that mantle
convection, was the answer to Wegener’s
missing power source to drive Continental
Drift.  He based it on the fact that as a
substance is heated, its density decreases
and rises to the surface until it cools, then
sinks again.
But convection of the solid mantle alone was
still unpalatable to most geoscientists.
Nuclear bomb testing in the 50’s motivated
the establishment of the Worldwide Standardized
Seismograph Network to monitor explosions,
prompting a greater concentration of seismograph
stations. The increased data allowed seismologists
to precisely locate far more earthquakes revealing
that most occur in discrete areas: near trenches
and along mid-ocean ridges.
Decades after Wegener’s death, geologist
Marie Tharpe, left in the lab to examine data
from ocean-floor field surveys, theorized
that the mid-ocean ridges appeared to be extensional
rift valleys formed by plate motion paving
the way for Wegener’s continental drift
theory. Her colleague Bruce Heezen, initially
skeptical, published her work in 1956 under
his own name, but ascribed the extension to
an expanding Earth theory. It wouldn’t be
until the mid-sixties that he would accept
Tharp’s interpretation of plate motion. 
Curiously, it was Harry Hess who, in 1962
was credited for recognizing that oceans did
grow from spreading ridges where new seafloor
was created, and then moved away in both directions. He
also defined ocean trenches as locations
where ocean floor was destroyed and recycled.
But he, too, lacked geophysical evidence to
confirm this theory. 
Just one year later, this concept was supported
by ocean-floor magnetic surveys that revealed
symmetrical patterns of magnetic striping on
either side of the mid ocean ridges that were
found to be the same age at similar distances
away from the ridge on each side.
Just one year later, this concept was supported
by ocean-floor magnetic surveys that revealed
symmetrical patterns of magnetic striping
on either side of the mid ocean ridges that
were found to be the same age at similar distances
away from the ridge on each side.
While pondering the mystery of how volcanoes,
such as the Hawaiian chain, could be so far
from spreading ridges or subduction zones,
Tuzo-Wilson proposed the plates moved over
hotspots, and followed this breakthrough with
the discovery that ocean ridges were connected
with transform faults. The science rapidly
blossomed with the definition of the three
main plate boundary types: DIVERGENT margins
where plates move apart, CONVERGENT margins
where plates push together at subduction zones
or mountain ranges, and TRANSFORM margins
where plates move horizontally past each other).
As geophysical evidence supporting plate tectonics
accumulated during the 1960's, scientists revived
Holmes’s theory of mantle convection as
a driving force for moving the plates.
Mantle convection, assuredly plays a role,
but doesn’t explain how some plates creep along faster
than the convective currents
beneath them. This led scientists to a fundamental force:
Gravity.  Gravity acts on the tectonic
plates resulting in what are now referred
to as "ridge push" at the spreading ridges
and "slab pull" beneath subduction zones.
This is an evolving science that not only
includes these three forces,
but involves friction and much more, 
leaving scientists to ponder:
What will be the next tool that helps reveal 
new facets of plate tectonics?
 
