Download Late 20th Century Tests of the Continental Drift Hypothesis

Late 20th Century Tests of the
Continental Drift Hypothesis
1 – The Age of the Ocean Basins
Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter.
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Other copyrighted material is used under the fair use clause of the copyright law of the United States.
What to look for:
• By around 1960 a number of new observations about the seafloor
made a few people re-think the idea of drift.
• Characteristics of the ridges (which we’ll examine in detail after
we’ve finished paleomag) and trenches (after that) and the
emerging idea of magnetic field reversals recorded in oceanic
basalts suggested the possibility that the oceans are widening.
• Correct interpretation of the initially enigmatic zebra-stripe map
indicated that the oceanic crust is youngest beside the
volcanically active ridges and gets older with distance across the
inactive (but still basaltic) abyssal plain crust.
• The same is true for all ridges in all oceans.
• Verification is possible by determining radiometric ages of
volcanic islands in the oceans and, independently, by determining
the chronostratigraphic ages of sediment in seafloor cores.
The observation(s) discussed in this show was not made in support or
opposition to Wegner’s hypothesis of continental drift.
All this work was done for different reasons, some of them with commercial
or strategic motivations, others just for the hell of doing it. “There’s a rabbit.
Let’s chase it down a hole” sort of thing.
The idea of continental drift has paid off handsomely in allowing more
efficient exploration for natural resources, but it originated largely as pure,
not applied, science.
From soon after Wegner published his book until the early 1960’s the idea of
continental drift essentially languished.
Most geologists “knew” the continents couldn’t move (because that was what
was predicted by the theories of the time), and so the observations Wegner
thought he had explained must simply be coincidences.
Other geologists “knew” the continents did move because, whereas a little
coincidence is okay, the various things Wegner had explained went
ridiculously far beyond “a little” coincidence.
There was, as with any healthy scientific hypothesis, on-going argument back
and forth, but it was clear that without any new observations any mutually
satisfactory resolution of the issue was not going to happen.
In the language of scientific theory, what was needed was some observation
that the drift hypothesis predicted and the fixed Earth hypothesis did not, or
vice versa -- a test, in other words.
When it can, it opened the door for a lot of other tests as well.
So, as late as the early 1960’s there were few geologist who bought into the idea of
moving continents. Their objections were of two main types:
1. What could possibly generate enough force to move such huge masses?
2. How do the oceans fit into the scheme?
A. Do you expect us to believe the continents move through the oceanic crust
like an icebreaker through sea ice?
B. Do you expect us to believe the continents grind them to rubble and
bulldoze them forward?
C. In either case, what happens on the trailing side of a moving continent?
Leaving the force issue for later, the ocean piece was ripe for solving by 1960!
Ocean Ridges, Magnetic Reversals,
and the Age of the Oceanic Crust
The New Hypothesis of Sea-Floor Spreading
In 1960 Harry Hess proposed a new hypothesis about what happens in the ocean. It was based on
three emerging observations about the oceanic crust. At the time it was not clear whether these
were really observable facts or just some geologists reading too much into their data instead.
So Hess’s hypothesis didn’t, initially, change any minds about continental drift.
The three things Hess’s hypothesis was intended to explain were:
1)The existence and form of the oceanic ridges,
2)The surprisingly young age of the oceanic crust, and
3)Hints that were beginning to come out that something was screwy with the magnetic field of the
Earth, and that this was recorded in the oceanic crust as well as the continental.
Hess proposed that the oceans moved with the continents. This obviously solved one of the
naysayers’ issues (what happens to the oceans), but compounded the other (more mass to shove
around).
He envisioned the crust moving like a conveyor belt away from the ridges (in both directions) with
new crust formed at the ridges to replace what was being moved away.
Over the next five years opinion rapidly switched from “the continents can’t move” to “they must be
moving” as people worked to test his hypothesis.
This hypothesis was called sea-floor spreading.
Piece 1 – The Ridge.
By the late 1950’s it was known that the oceanic ridges exist, that they are
continuous around the globe, that they stand 1000 or more meters higher than the
adjacent abyssal plains, that they are zones of high heat flow and strong magnetism,
frequent
earthquakes
and so on.
The map we all
know and love
did not yet
exist, but much
of the data on
which it is
based was in
hand, and many
people had
access to it.
Base Map © National Geographic Society. Used under fair use clause of copyright law.
Piece 2 – The Age.
By the late 1950’s sampling of the seafloor, including cores of sediment down to
the basaltic crust, were accumulating at oceanographic institutes.
One curious fact about them caught geologist’s attention: no sediment older than
Cretaceous had ever been found in the oceans. (Some late Jurassic sediment
is now known, as you will see, but that’s as old as it gets. We’ve looked
everywhere there is to look now.)
Whereas the continents have a record that goes back far into the Precambrian
Eons (nearly 4,000 million years), the ocean is missing most of the earlier part of
that record (all but the last 100-200 my).
The oceanic crust, in other words, is evidently only about 1/5 as old as the
continents.
Because much of the older rock is actually marine sediment, this doesn’t make
sense. There must have been oceans all the way back.
WHERE IS THE CRUST OF THE OLDER OCEAN BASINS?
Piece 3 – Paleomagnetic
Reversals.
(Paleomagnetism, Part 1)
REMNANT MAGNETISM
Igneous rocks form their crystals above their Curie Point, a temperature where the magnetic
field around a magnetic object cannot exist.
As iron-bearing minerals form and cool, each iron atom eventually forms a magnetic field
around itself when it reaches the Curie point temperature. (Minerals with many metal atoms
other than Fe do so as well).
As this field forms, its polarity is aligned with the polarity of any pre-existing magnetic field
nearby. For most Fe atoms this is the Earth’s field. Once the field forms pointing in this
direction it locks in the direction to north at the time and place it formed.
Metamorphic processes often involve heating rocks above their Curie points, thereby
destroying the internal magnetic fields. As metamorphism ends, the rocks cool back below the
Curie point and reform fields “pointing north” at the time of metamorphism (or soon after).
As sedimentary particles settle through water the fields of any iron-bearing minerals (clays,
hematite, etc.) can align their fields with Earth’s field like little compasses. If the particles are
fine enough, and therefore sink slowly enough, this can be done before final deposition. This
means that the internal magnetism of the sediment will “point north” at the time of deposition.
Magnetic
Stage
Individual
Events
Holocene
Pleistocene
Pliocene
By the late 1950’s it was suspected that
Earth’s magnetic field has frequently and
sporadically changed its polarity many, many
times in the past. This was one of the things
that Hess had incorporated into his
hypothesis, as a test. If the poles reverse,
and if the crust is forming at the ridges and
moving away, the crust is like a taperecorder “remembering” the history of the
Earth’s field!
The pole reversal hypothesis is simple. At
times the field has “pointed north” as it does
now, but just as often it has “pointed south”.
It doesn’t migrate willy-nilly all over the planet,
just north or south.
Miocene
This was confirmed in detail in the 1960’s in
places with thick stacks of basaltic lava flows
like Iceland or Hawaii. (Basalts have lots of
iron-rich minerals in them, each of which has
a tiny magnetic field like a compass does.
These fields form and are locked in when the
rock first cools.)
Furthermore, it was obvious that the times
when the field reversed could be determined.
In 1961, Raff and Mason published the
map at right. (© Geological Society of America. Used under
fair use clause of copyright law.)
It summarized what they had found about
Earth’s magnetic field of the northwestern
coast of the US and Canada. For
obvious reasons it came to be known as
the “zebra-stripe map”.
The dark lines represent places where
the field is particularly strong (“positive
anomaly”) and the light lines where it is
particularly weak (“negative anomaly”).
Raff and Mason offered no explanation
for why the field was like this, and left it
as a puzzling observation needing an
explanation.
At least two different sets of people
figured out the problem. Morley spent
many months trying to publish his idea,
and never managed to do so. Vine and
Matthews did manage to get published,
but initially nobody paid much attention.
Still, they get credit for it.
Vine and Matthews’s 1963 paper
included the figure at left.
They reasoned that the positive
anomalies of the field were
measured where it was “working
with” the remnant magnetism in
the crust below, and the negative
anomalies where it was “working
against” that remnant magnetism.
To put it differently, where the
rocks below had formed with
normal polarity, their field
reinforces Earth’s field, where
they formed with reversed polarity
they partially cancel it.
The next few slides make their
idea more concrete.
(© Geological Society of America. Used under fair use clause of copyright law.)
E+R
E-R
E+R
E-R
E+R
E-R
E+R
E-R
E-R
E+R
E-R
E+R
E+R
E-R
E+R
E+R
E-R
E = strength of Earth’s Field
R = strength of remnant magnetism
from seafloor rocks.
(© Geological Society of America. Used under fair use clause of copyright law.)
Now for the rub: How many differences can you spot?
(© Geological Society of America. Used
under fair use clause of copyright law.)
Though the transform faults
in the region chop the
pattern up and de-prettify it,
the known magnetic
reversals and polarity stages
match the zebra stripes quite
well.
Furthermore, the pattern is
symmetrically repeated
across the ridge, on both
sides.
Brunhes
Matuyama
Gauss
Gilbert
(© Geological Society of America. Used under fair use clause of copyright law.)
Holocene
Pleistocene
Pliocene
Remember that the ages
of these reversals are
known and let’s make a
geologic map of the area
that Raff, Mason, Morley,
Vine, and Matthews all
studied.
We’ll use the colors at
left to represent the rocks
of various ages.
Miocene
Holocene
Pleistocene
Pliocene
Miocene
Progressively
older
The oceanic crust is
youngest at the ridge
and gets progressively
older with distance
from the ridge!
Progressively older
(© Geological Society of America. Used
under fair use clause of copyright law.)
Though the previous (and original) example is very small in scale, the idea has been
shown to apply on a much grander scale and across every oceanic ridge.
Recall that in an
earlier slide (in the
first presentation)
we hypothesized
from fossil evidence
that the South
Atlantic opened
beginning in the
Cretaceous.
Then notice that the
oldest rock that the
oldest continuous
crust between the
two continents is …
(drum roll) …
Cretaceous.
All the ocean basins show the same distribution of ages: younger crust toward the ridge.
Notice that several aseismic ridges are apparent on the map.
Jur.
Rec
© National Oceanic and Atmospheric Administration (NOAA)
The reversals indicate pretty convincingly that the oceanic crust gets older with
distance from the ridge. If we still want to treat this as hypothetical there are two
independent ways of testing it.
The first involves radiometric (numerical) dates of igneous rocks. The sea-floor
basalts are difficult to date themselves because they are both difficult to access
and highly weathered by long contact with salt water. Basaltic islands are a lot
easier to deal with and are not uncommon in the oceans. There are two ways
they have been shown to support the age distribution suggested by the zebra
stripes.
The other way involves dating the sediments above the basalt. In this case we
get a chronostratigraphic (“time-scale) age rather than a numerical one. “Late
Cretaceous” rather than “70 my”.
We talk about and critique these two dating methods in a different part of the
class. For now, take it for granted that they work.
The first way to use radiometric dates of islands is simply to go to as many as possible, find the
youngest flow, and date it. After you have these data you then graph those ages as a function of the
distance of the island from the ridge in that ocean. Though there is always some scatter, the data points
can, in general, be fit to a best-fit line like the one shown, with a high correlation coefficient.
Age
The positive slope of the line indicates the islands get older with distance from the ridge.
Measured Distance from Ridge
Even more interesting are chains
of related islands called “aseismic
ridges”. Unlike the ocean ridges,
which have both basaltic
volcanoes and earthquakes (they
are seismic, in other words),
these ridges have only volcanoes.
Hawaii is the classic example, but
there are others as we’ll see.
The only active volcanoes in
Hawaii are on the “Big Island” at
the southeast end. There is an
even more active volcano some
miles off its southeast coast, but it
is completely submerged. It is
expected eventually to grow large
enough to be an island, but has
not yet gotten that far along.
Mauna Loa, Mauna Kea, and
Kilauea are the active
volcanoes on the Big Island
Nevertheless, all the islands in the
chain are made of basalt.
An active submarine volcano
lies off the Big Island to the
southeast.
Base Map © National Geographic Society. Used under fair
use clause of copyright law.
The islands in the Hawaiian
chain are progressively older
northwestward from the Big
Island.
~40 my
Midway Island is the
northwestern-most major
island in the chain, but
beyond there are submerged
volcanic plateaus called
“seamounts”. These are
subsided volcanic islands that
have been planed off to
wavebase level by waves.
The Milwaukee Seamounts
mark a zig (or zag) in the
chain which continues as the
Emperor Seamounts. The
age of the Milwaukees is
about 40 my (late middle
Eocene).
Several other
island/seamount systems in
the Pacific have similar shape
and age characteristics, as
the next slide shows.
Base Map © National Geographic Society. Used
under fair use clause of copyright law.
“ZEBRA-STRIPE”
MAP
“HOTSPOTS”
Base Map © National Geographic Society. Used
under fair use clause of copyright law.
What happened
~40 my ago???
Direction of
motion prior to
40my ago
Present
direction of motion
To original ridge location
(.40 my ago)
Base Map © National Geographic Society. Used under fair use clause of copyright law.
Hotspot in mantle
below ocean plate
To East
Pacific Rise
The other way of verifying the age pattern suggested by the zebra stripes involves drilling through
the overlying sediment and determining the ages of those sediments and sedimentary rocks. The
core sections that result from an east/west transect of the Atlantic typically look like this:
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Pli
Pli
Pli
Pli
Pli
Pli
Pli
Mio
Mio
Mio
Mio
Mio
Mio
Oli
Oli
Oli
Oli
Oli
Eo
Eo
Eo
Eo
Pec
Pec
Pec
K
K
J
Hol
NO SEDIMENT!
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Pli
Pli
Pli
Pli
Pli
Pli
Pli
Mio
Mio
Mio
Mio
Mio
Mio
Oli
Oli
Oli
Oli
Oli
Eo
Eo
Eo
Eo
Pec
Pec
Pec
K
K
The chronostratigraphic
age of the sediment at
the bottom of the core
increases away from the
ridge.
J
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Pli
Pli
Pli
Pli
Pli
Pli
Pli
Mio
Mio
Mio
Mio
Mio
Mio
Oli
Oli
Oli
Oli
Oli
Eo
Eo
Eo
Eo
Pec
Pec
Pec
K
K
J
Hol
NO SEDIMENT!
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Hol
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Plst
Pli
Pli
Pli
Pli
Pli
Pli
Pli
Mio
Mio
Mio
Mio
Mio
Mio
Oli
Oli
Oli
Oli
Oli
Eo
Eo
Eo
Eo
Pec
Pec
Pec
K
K
J
If we could take an oceanographic vessel back in time to the Eocene, a transect of cores would give
us these sedimentary sequences in the younger, narrower ocean. No deposits younger than Eocene
would yet exist, but the cores below level that would be the same – older away from the ridge.
Eo
Eo
Eo
Pec
Pec
Pec
K
K
J
Eo
NO SEDIMENT!
Eo
Eo
Eo
Eo
Pec
Pec
Pec
K
K
J
Take-home Message
• By around 1960 a number of new observations about the seafloor
made a few people re-think the idea of drift.
• Characteristics of the ridges (which we’ll examine in detail after
we’ve finished paleomag) and trenches (after that) and the
emerging idea of magnetic field reversals recorded in oceanic
basalts suggested the possibility that the oceans are widening.
• Correct interpretation of the initially enigmatic zebra-stripe map
indicated that the oceanic crust is youngest beside the
volcanically active ridges and gets older with distance across the
inactive (but still basaltic) abyssal plain crust.
• The same is true for all ridges in all oceans.
• Verification is possible by determining radiometric ages of
volcanic islands in the oceans and, independently, by determining
the chronostratigraphic ages of sediment in seafloor cores.