Download Plate Tectonics II

Plate Tectonics II: The Ocean
Floor and the Seafloor
Spreading Hypothesis
Arthur Holmes
British Geologist (1890-1965)
• Used radiometric dating to
determine an accurate age for
the Earth and to create a
geologic timescale.
• Proponent of the continental
drift hypothesis.
• Proposed that convection
currents in the mantle pushed
continents apart, creating new
ocean basins in between.
• Acknowledged that no hard
evidence existed to support
his hypothesis.
Prior to 1950, humans were
almost completely ignorant of the
structure of 2/3 of the surface
of the Earth - the Ocean Floor
We knew more about
the surface of the
moon than about the
deep sea floor.
Questions for Discussion
Historically, what human L
activity has generated the
most amount of funding for
scientific research?
R
What major global event of
the mid-20th Century led to
the invention of radar, sonar,
and advanced magnetometry?
Technology developed
during World War II
• Sonar - image the deep ocean using
reflected sound.
• Magnetometer - detect faint magnetic
fields.
• Developed to search for German Uboats in the north Atlantic.
World War II
Submarine warfare
Cold War
Nuclear Submarines
SSN Nautilus
•1st nuclear
powered submarine
Military funding of earth
science during the Cold War
(1950’s)
• Detailed mapping of the topographic
features of the deep ocean floor.
• Detailed mapping of the magnetic
signature of the deep ocean floor.
• World Wide Seismic Monitoring network
(detection of atomic tests, monitoring of
above ground testing ban).
Study of the ocean floor overturned
many accepted ideas:
The deep ocean is a flat,
featureless expanse.
False.
The ocean floor is
covered with
seamounts and
guyotes, many of
which form linear
chains called
‘aseismic ridges’.
Emperor Seamounts
Hawai’ian Ridge
Hawai’ian Islands
Study of the ocean floor overturned
many accepted ideas:
The deepest regions of
the ocean are found in
the center of ocean
basins.
False.
The deepest
regions of the
ocean floor are
elongate trenches
found along the
margins of some,
but not all, ocean
basins.
Study of the ocean floor overturned
many accepted ideas:
Mountain ranges are only
found on continents.
Mid-ocean ridges are the largest mountain ranges
on Earth.
Study of the ocean floor overturned
many accepted ideas:
The ocean floor is
covered in a deep pile of
marine sediments,
accumulated over billions
of years.
False: The deep ocean sediment cover is very thin at
the mid-ocean ridge and, although it thickens toward
the continents, is much thinner than would be
expected after billions of years of accumulation.
Thin cover of sediments
Map of the ocean floor - circa 1964 - Marie
Tharp and Bruce Heezen - Columbia University
Making sense of the deep ocean
Harry Hess
• Harry Hess - Geologist at
Princeton U.
• 1962 publishes “geopoetry.”
• Suggested that the ocean
floor itself might be
moving.
• Continents are carried
along, embedded within the
ocean floor.
• Hypothesis is named “Sea
Floor Spreading” by Robert
Dietz at Scripps Institute.
Sea floor spreading
mid-ocean ridge
trench
mantle convection
How would seafloor spreading expain...
The absence of
thick sediments on
the deep sea floor?
LR
The existence and
location of mid-ocean
ridges?
RR
The existence and
location of deep
ocean trenches?
LF
The existence of
extinct, eroded
volcanoes on the deep
ocean floor?
RF
The seafloor spreading hypothesis was
not immediately embraced by geologists
• Evidence was largely
circumstantial
• Definitive proof of seafloor
movement was still lacking
• However, research into the
history of the Earth’s
magnetic field was
convincing many geologists
that the continents had
moved through time.
Robert Dietz
Remnant Magnetism
• 1940’s - geologists discover that the Earth’s
magnetic field is recorded by iron-bearing
minerals during the formation of rocks.
• Remnant magnetism is trapped in the rock.
• Magnetic minerals act like tiny compasses,
pointing to the former position of the
magnetic pole (declination).
• Magnetic minerals also record the polarity of
the Earth’s magnetic field in the past.
Remnant Magnetism
Apparent Polar Wandering
• Observation that progressively older rock records
different positions for the north magnetic pole at
different times in Earth history.
Two possible
explanations:
N
Apparent Polar
Wander: drifting
continents, stable
poles
Apparent Polar
Wander: drifting
continents, stable
poles
N
Apparent Polar
Wander: drifting
continents, stable
poles
N
Apparent Polar
Wander: drifting
continents, stable
poles
N
N
Apparent Polar
Wander: drifting
poles, stable
continents
N
Apparent Polar
Wander: drifting
poles, stable
continents
N
Apparent Polar
Wander: drifting
poles, stable
continents
Apparent Polar
Wander: drifting
poles, stable
continents
N
Question for Discussion
What evidence could geologists use to distinguish
between the two hypotheses proposed to explain
polar wandering?
Apparent polar wander
paths for Europe and
North America They DO NOT
match!
Polar wander assuming no continental drift
Europe
N.Am
Polar wander with continental drift
The continents had to have moved!
Magnetic field reversals
• 1940’s - geologists discover that the
Earth’s magnetic field randomly “flips” so
that the N and S magnetic poles become
reversed.
• Reversals occur randomly and last for
100’s of thousands to millions of years.
• Pattern of reversals recorded in the rock
through time is unique - like a bar code.
Supercomputer
simulation of the
Earth’s magnetic field
N
S
N
S
Magnetic field
becomes weakly
polarized and unstable
Magnetic polarity reversal
N
S
N
R
Reversals of the Earth’s magnetic field are
recorded in rocks as intervals of Normal(N)
and Reversed (R) polarity.
Pleistocene
Pliocene
Miocene
Oligocene
Cretaceous
Paleocene
Tertiary
Eocene
Pattern through
time of polarity
reversals is
random and
forms a unique
‘bar code’ that
can be used to
identify
different
intervals in
Earth’s past.
Seafloor Magnetic Polarity Stripes
• Geophysicists mapping the magnetism of the
ocean floor notice a peculiar pattern of
magnetic stripes in sea floor crust around
mid-ocean ridges.
Raff and Mason, 1961
Results from a magnetic
survey off the west
coast of North America.
What could explain the
peculiar pattern of
magnetic polarity
stripes in the crust of
the ocean floor?
Stripes are centered on
the mid-ocean ridge.
normal
reversed
normal
reversed
Question for Discussion:
How could sea floor
spreading explain the pattern
of magnetic stripes seen in
the ocean floor?
normal
reversed
Magnetic field reversals
• 1963 - Lawrence Morley (Canadian geophysicist)
proposes that seafloor spreading creates
magnetic stripes. His paper is rejected by two
scientific journals and is never published!
• 1963 - Fred Vine and Drummand Matthews
(British geophysicists) propose essentially the
same hypothesis in a paper that is published in
Nature.
• The Vine and Matthews paper does not generate
much interest at first.
Question for Discussion:
How could you prove that
magnetic stripes were
caused by sea floor
spreading over geologic
time?
Think about
the ‘bar code’!
normal
reversed
Continent
Seafloor
1966: Fred Vine publishes
in Science on the
correlation between
magnetic stripes on the
seafloor and the magnetic
timescale developed from
continental rocks.
Seafloor spreading
becomes an accepted
‘fact’, leading the way to
plate tectonic theory.