Plate Tectonic Theory
Plate Tectonic Theory
Plate tectonics is a relatively new theory that
has revolutionized the way geologists think
about the Earth.
 Plate: Large slab of solid rock
 Tectonics: from the Greek root “to build”


Plate tectonics is a combination of two earlier
ideas, continental drift and sea-floor
spreading
Tectonic Plates

According to the theory, the surface of the
Earth is broken into large plates.
Earth’s Interior
The lithosphere is the rigid, upper part of
the mantle and all of the crust
It is broken into 30 “plates” which vary
greatly in size and shape.
The lithosphere floats on the asthenosphere
which is the flowing part of the upper
mantle.
Historical Theories
1596Abraham Ortelius
(April 14, 1527 – June 28, 1598)
was a Flemish
cartographer (map maker)
and geographer.
Continental Drift
Ortelius is believed to be the first person to
imagine that the continents were joined
together before drifting to their present
positions.
Ortelius was the first to see that
the shape of the coasts of South
America and Europe-Africa were
similar, and to propose
continental drift as an
explanation
Nicolaus Steno
1636-1686
Nicolaus Steno
observed the changes in
a sequence of rock
layers in the mountains
of Italy.
•Steno's observations
became known as the
Law of Superposition
Danish Anatomist
and Geologist
Law of Superposition
– in
a sequence of
sedimentary rock
layers, each layer of
rock is older than the
layer above it and
younger than the rock
layer below it.
Youngest
rock layer
Oldest
rock layer
James Hutton
1785James Hutton (1726-1797)
was a Scottish
geologist.
In 1785 he presented
his uniformitarian
principle to the Royal
Society of Edinburgh.
Uniformitarian principle
“The present is the key to the past”
suggests that catastrophic processes
were not responsible for the landforms
that existed on the Earth's surface.
 This idea was opposed to the ideas of that
time period which were based on a biblical
interpretation of the history of the Earth.

Unitarianism
means “the same throughout”
 Scientists believed that the rates of all
geologic processes had been the same
throughout time.
The theory of uniformitarianism
suggested that the landscape developed
over long periods of time through a
variety of slow geologic and geomorphic
processes.

The Grand Canyon in Arizona shows how geologic
processes such as weathering and erosion happen slowly
over time.
Alfred Wegener
Alfred Wegener (1880-1930)
• German geophysicist
• Professor of meteorology
and geophysics at the
University of Marburg
• Studied fossils on different
continents
1912- Supercontinent Theory
around 200 million years ago, the
supercontinent Pangaea began to split apart .
 Pangaea comes from the Greek “all the Earth”

Wegener's
theory was
based in part on what
appeared to him to be the
remarkable fit of the
South American and
African continents, first
noted by Abraham
Ortelius three centuries
earlier.
Fossil Evidence
Wegener was also intrigued by the occurrences
of unusual geologic structures and of plant and
animal fossils found on the matching coastlines of
South America and Africa, which are now widely
separated by the Atlantic Ocean.
 He reasoned that it was physically impossible for
most of these organisms to have swum or have
been transported across the vast oceans. To him,
the presence of identical fossil species along the
coastal parts of Africa and South America was the
most compelling evidence that the two continents
were once joined.

Pangaea
Continental
Drift
225 million years
ago- Supercontinent
Pangaea
Over millennia,
continents drifted
apart.
Present Day:
7 continents which
are continuing to
drift (separate) at a
rate of about 10 cm
per year.
A Theory Contended
The theory of continental drift would
become the spark that ignited a new way
of viewing the Earth. But at the time
Wegener introduced his theory, the
scientific community firmly believed the
continents and oceans to be permanent
features on the Earth's surface.
 Wegener’s proposal was not well received,
even though it seemed to agree with the
scientific information available at the
time.

The Fatal Weakness

A fatal weakness in Wegener's theory was
that it could not satisfactorily answer the
most fundamental question raised by
critics of his time:
What kind of forces could be strong
enough to move such large
masses of solid rock over such
great distances?
Arthur Holmes Lived 1890–1965
British Geologist

1929 - Convection
Currents Theory –
about the time Wegener's
ideas began to be
dismissed for lack of a
mechanism of movement,
Holmes elaborated on the
idea that the mantle
undergoes thermal
convection
Thermal Convection in the
Mantle

As magma is heated, it tends to rise, then
cools and sinks again. This repeated
heating and cooling results in a current
which may be enough to cause continents
to move.
Convection Conveyor Belt

Arthur Holmes compared this
thermal convection to a
conveyor belt. The pressure
from the upwelling of magma
could break apart a continent
and then force the broken
continent in opposite directions
carried by the convection
currents. This idea received very
little attention at the time.
What We Know Today
Even though the theory of continental drift
was proposed in 1912 by Alfred Wegener,
the idea of moving continents wasn't
generally accepted until the early 1960s.
 That's when Wegener's theory was
resurrected by Harry Hess, Robert Dietz,
Fred Vine, and Drummond Matthews

Forces that Shape the Earth
Geologists now understand the forces that
cause tectonic plates to move.
 The force that causes plates to move is
called convection.
 This convection force occurs in the
asthenosphere of the mantle

Convection Currents
The lithospheric plates
are thought to be moved
around by circulating
motions.
 This process is similar to
what you see in a lava
lamp.

– The material in the lamp is
heated by the bulb. The
material then rises and is
replaced by the cooler
material that sinks to the
bottom.
– This causes circulating
movement
Convection in the Earth
convection currents
in the mantle move
tectonic plates as
the plastic-like
asthenosphere
circulates due to
the heat present in
the core.
The large scale circulations (motions) in the
asthenosphere move the lithospheric plates on the
surface of the Earth leading to the continental drift
observed today.
Harry Hess
Lived 1906–1969 American Geologist

1962 - Sea Floor Spreading
Theory – idea that the seafloor
itself moves and carries
continents with it, as it expands
from a central point
 caused by convection currents in
the molten, very weak upper
mantle, or asthenosphere.
Sea Floor Spreading
molten magma from
beneath the earth's crust
could ooze up between
the plates in the rift in
the ocean floor.
 As the hot magma cooled
in the ocean water, it
would expand and push
the plates beside it.

Mid Atlantic Ridge
North and South America
would move to the west
and Eurasia and Africa
would move to the east.
 The Atlantic Ocean would
get wider, but the
coastlines of the
landmasses would not
change dramatically.

Dan McKenzie
Lives 1942– English Geophysicist

1968 - Theory of Plate Tectonics – is
a combination of two earlier ideas:
continental drift and sea-floor
spreading.
Plate Tectonics
Earth's outermost layer, the
lithosphere, is broken into 7 large,
rigid pieces called plates.
 Several minor plates also exist.
 The plates are all moving in different
directions and at different speeds,
from 2 cm to 10 cm per year in
relationship to each other.

Main Features of Plate Tectonics
The Earth's surface is covered by a series
of crustal plates.
 The ocean floors are continually moving,
spreading from the center, sinking at the
edges, and being regenerated.
 Convection currents beneath the plates
move the crustal plates in different
directions.
 The source of heat driving the convection
currents is from radioactivity deep in the
Earth’s mantle.

Plate Boundaries

Stay tuned for our next topic:
Plate Boundaries
References
USGS
http://pubs.usgs.gov/publications/text/historical.html
 BBC Schools
http://www.bbc.co.uk/schools/gcsebitesize/science/21c/
earth_and_space/continentaldriftrev1.shtml
 University of Oregon
http://jersey.uoregon.edu/~imamura/121/lecture9/tectonic.html
 PBS
http://www.pbs.org/wgbh/aso/tryit/tectonics/
 University of California, Berkeley
http://www.ucmp.berkeley.edu/history/wegener.html
