Lecture 2: Key steps to plate tectonics
North America and Europe can be ‘fitted together’: 1600 (Francis Bacon)
An ‘ancestral continent’: 1858 (Snider Pellegrini)
‘Earth’s Plan’: 1910 (Frank Taylor)
‘Continental Drift’ and Pangea: 1915 (Alfred Wegener)
Mantle convection currents: 1928 (Arthur Holmes)
Mid-ocean ridges and trenches: 1957 (Marie Tharp and Bruce Heezen)
-‘Sea floor spreading’: 1962 (Harold Hess)
1965 ‘Wander paths’ for continents (Ted Irving) and ‘magnetic stripes’ on the ocean floors (Fred
Vine)
Computer reconstruction of Pangea: 1965 (Edward Bullard)
Transform faults and hot spots: 1965 (Tuzo Wilson)
Plate tectonics 1968 (Tuzo Wilson)
Supercontinent cycle 1974 (John Dewey)
SIR FRANCIS BACON 1561-1628
ANTONIO SNIDER-PELLEGRINI 1858: AN ‘ANCESTRAL CONTINENT’
Carl Schuchert (1858-1942)
Yale University
‘PERMANENTISM’
Continents are fixed in position
connected by land bridges
which sink to create oceans
BUT
EXPANDING EARTH MODEL
VERSUS
CONTRACTING AND COOLING EARTH MODEL
‘MOBILISM’
Frank Bursley Taylor 1910: ‘Earth’s plan’
where continents drift as ‘sheets’
that are moved by tidal forces
‘
Alfred Wegener: 1915
Die Entstehung der Kontinente und
Ozeane (“The Origin of Continents
and Oceans”)
ALFRED WEGENER
‘CONTINENTAL DRIFT’ 1915
PANGEA (‘ALL THE LANDS’)
DIED IN GREENLAND 1930
Matching rock types of the same age from one continent to
another to reconstruct Pangea
warm
And it made sense of the fossil record
on continents that are now far apart
The response to ‘Continental Drift’ was
extremely negative
‘A German fairy tale’
‘Poppycock’
‘Impossible’
And opposed by geophysicists such as
Sir Harold Jeffreys (1891-1989)
The Earth’s interior was too considered to be
too stiff to allow
continental drift
Arthur Holmes (1928) Convection currents driving the drift of continents
-largely ignored in North America
1939-1945 Second World War
followed by the Cold War beginning in 1948 resulted in anti-submarine warfare
and the development of new sonar devices to find them and map the ocean floor
Types D 1 and D2
1960’s: THE OCEAN FLOOR GETS MAPPED
THE OCEAN FLOOR REVEALED: MID-OCEAN RIDGES, RIFT VALLEYS
FRACTURES AND TRENCHES Marie Tharp and Bruce Heezen: 1957
magma
LATE 1950’S:
APPARENT POLAR WANDER CURVES
1995: TRUE WANDER CURVES
Ted Irving, Trond Torsvik and others
Confirms that continents do drift and at which rates rate
Edward Bullard,
1965:
Computer
reconstruction
of the ‘fit’ of modern
day continents either
side of the
Atlantic Ocean
PANGEA TODAY
THE MID-ATLANTIC RIDGE EXPOSED ON ICELAND
• Harold Hess introduced idea of sea
floor spreading (1962)
• Sea floor created at MOR’s recycled
into mantle at trenches
• Implied continents are moving!
The idea greatly
influenced
Tuzo Wilson who
abandons
Permanentism
MAGNETIC STRIPES (1965)
The Vine-Mathews-Morely hypothesis
• Magnetometer surveys of ocean floor identified
patterns of anomalies on each side of MORs
• But how did they form?
Paleomagnetism
•Episodically the
Earth’s magnetic field
flips
–north magnetic pole
changes to south
Paleomagnetic
changes through time
• Magnetic reversals
occur on average
every 500,000 years
• Takes 10,000 years
or less for a reversal to
occur (so you won’t
notice if it occurs!)
Fred Vine: 1965
Magnetic stripes
on the ocean floor
“Vine-Morley-Mathews
Hypothesis”
ICELAND:
THE ISLAND THAT IS EXPANDING!
Direct measurement of plate movement by GPS
ROCKS
+
BMW
SUMMER IS ON ITS WAY
B.Sc in Geophysics
U of T 1929
John ‘Jack’ Tuzo Wilson
1908-1993
Ph.D Princeton 1935
Professor of Geophysics at U of T after 1946
‘PLATE TECTONICS’
Mount Tuzo
Moraine Lake and the Valley of the Ten Peaks
Hetty Tuzo
First World War
‘trench map’
made from air
photographs of part
of the
Western Front
in Northern France
near Amiens.
Technology was
transferred to
mapping Canada.
Vickers Vedette 1923 with cameraman
Tuzo Wilson’s mapping of the Canadian Shield
1941 using air photographs
Tuzo Wilson was a committed Permanentist
arguing that continents grew in size but were stationary
TYPES OF EARTHQUAKES BASED ON DEPTHS OF FOCUS
SINUOUS RIDGE
SEGMENTED RIDGE
Wilson’s paper model
REAL RIDGE
SIMPLIFIED RIDGE
TRANSFORM FAULTS: ALLOW SPREADING EITHER SIDE OF SINUOUS MID-OCEAN RIDGES
BY DIVIDING RIDGE INTO SEGMENTS
Wilson’s 1965 recognition of ‘tectonic plates’
outlined by trenches, ridges and transform faults
1968: THE UNITED PLATES OF PLANET EARTH:
TUZO WILSON
THE NORTH
AMERICAN
PLATE
DRILLING THE OCEAN FLOORS
AGE OF THE OCEAN FLOORS
Hot spots prove plate movement
Fig. 2.39
Fig. 2.38
HOT SPOTS:
TRACKING PLATE
MOVEMENT
Fig. 2.38
THE WILSON CYCLE
Approximately 500 million years
in duration from beginning to end
-Last supercontinent was Pangea
250 million years ago (Ma: mega
annum)
-Next one will be Pangea II
c. 250 million years in the future
Oldest supercontinent so far
recognised is about 3 billion years
old (3 Ga: Giga annum)
Re assembling past continents
The supercontinent cycle
And a guide to the future…
ONTARIO SCIENCE CENTRE