This is Planet Earth /
Earth Story
Lecture 5
PLATE TECTONICS
Drifting Continents, And
Oceans That Open And Close
Earth Structure
The Lithosphere:
• Forms the outermost layer of the Earth.
• Consists of the crust and top part of
the mantle
• Ranges in thickness from about 7 km
(oceanic) to 125 km (continental)
• Overlies the asthenosphere which is
partially melted, whereas the lithosphere
is rigid.
Plate Tectonics
Evidence for moving continents
(e.g. Francis Bacon 1620)…
In 1858, geographer Antonio Snider-Pellegrini made these two maps
showing his version of how the American and African continents may
once have fit together, then later separated.
Alfred Lothar Wegener
(1880-1930)
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Published „The Origin
of the Continents and
Oceans‟ in 1915
Challenged the „Rigid
Earth Theory‟
Provided evidence
that continents are in
relative motion with
respect to time
(Continental Drift)
Wegener‟s evidence for
Continental Drift
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Continental fit
Correlation of rocks
and climate
Crustal structure
Distribution of fossil
plants and animals
Direct measurements
of changing distance
between the
continents
Best to use continental margins rather
than coastlines
Evidence for
Continental Drift –
Past climates and the
distribution of
sedimentary rocks
According to the
continental drift theory, the
supercontinent Pangaea
began to break up about
225-200 million years ago,
eventually fragmenting
into the continents as we
know them today.
Evidence for Continental Drift –
Structure of the Crust
Isostasy:
Areas underlain by continental
crust (acidic, low density) stand
considerably higher than areas
underlain by oceanic crust
(basic; high density)
Light and thick blocks float high (equivalent
to continental crust), dense thin blocks float
low (equivalent to oceanic crust),
Evidence for Continental Drift –
Fossil Plants and Animals
The locations of
certain fossil plants
and animals on
present-day, widely
separated continents
would form definite
patterns (shown by
the bands of colors),
if the continents are
rejoined.
Continental Drift –
Additional Supporting Evidence
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Arthur Holmes (1931)
suggested that the
continents and oceans
move together, driven by
convection currents in the
mantle
Technology developed
during WW2 was put to
use in surveying the
ocean floor
Mapping of the ocean floor (East
Pacific)
Exploration of the Ocean Floor –
Oceanic Ridges
Oceanic ridges (shown in red) wind their way between the continents
Exploration of the Ocean Floor –
Ocean Trenches (Subduction Zones)
The deepest waters are found in oceanic trenches, which plunge as deep as 11 km
below the ocean surface. These trenches are usually long and narrow, and run parallel t
and near the oceans margins. They are the surface expression of subduction zones
Earthquake
Distribution
As early as the 1920s, scientists noted that earthquakes are concentrated in very
specific narrow zones. In 1954, French seismologist J.P. Rothé published this map
showing the concentration of earthquakes along the zones indicated by dots and
cross-hatched areas.
Earthquake Distribution at
Subduction Zones – Benioff Zone
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Image shows earthquakes
that occurred beneath the
Tonga Trench, Pacific
Ocean during 1965
Each red circle represents
a single earthquake
The earthquake distribution
defines a Benioff zone,
representing the downward
movement of a relatively
cool slab of lithosphere.
Harry Hess
(1906-1969)
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Naval Officer and Professor of
geology at Princeton University,
Very influential in setting the
stage for the emerging plate
tectonics theory in the early
1960s.
He believed in many of the
observations Wegener used in
defending his theory of
continental drift, but he had very
different views about large-scale
movements of the Earth.
Credited with the concept of Sea
Floor Spreading
Plate Movement - Convection
Trench
Oceanic ridge
Hess – “oceanic
rocks are replaced
every 300 – 400 Ma”
due to SEA FLOOR
SPREADING
The oldest oceanic
lithosphere is about
180 million years old,
which is relatively
young
Support for Sea Floor Spreading The Earth‟s Magnetic Field
Magnetic stripes on
the sea floor - Iceland
Magnetic Reversals
(Vine and Matthews Hypothesis)
Drilling and Sampling of the Sea
Floor
The Glomar Challenger was the first research vessel specifically designed in the late
1960s for the purpose of drilling into and taking core samples from the deep ocean floor
Recommended Reading
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Press, F. and Siever, R. 2000.
Understanding Earth (3rd Edition).
Freeman (ISBN 0-71674-117-2).
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Chapter 20
Skinner, B.J., Porter, S.C. and Park, J.
2004. Dynamic Earth: An Introduction
to Physical Geology (5 th Edition). Wiley
(ISBN 0-471-45157-6).
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Chapter 2