Download sea-floor spreading -- the creation of oceanic crust

Plate Tectonics 2
Sea-Floor Spreading
Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter.
Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin.
Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities.
Other copyrighted material is used under the fair use clause of the copyright law of the United States.
Observation 1
Structure and Shape of the
Mid-Ocean Ridges.
The shelf edge of a continent directly
across the Atlantic is not the only thing
that the edge of a continent fits.
It also fits the ridge, which is half the
distance between them.
There is a reason this is called the
“Mid-Atlantic Ridge”. (Except north of
this large transform fault, where it is
called the Reykjanes Ridge. It is still
the same spreading center.)
This works also in the Indian Ocean,
where there is a “Mid-Indian Ridge”,
but not in the Pacific (East Pacific
Rise) or Arctic (Nansen Ridge) where
the ridges are not in the middle.
The continents across those oceans
do not appear to fit because they were
never joined.
Base map © by National Geographic Society.
If we drew a cross-section of the North Atlantic along
the line A-A’ to show the topography or relief of the
seafloor, and if we drew it so that the vertical and
horizontal scales were the same, we wouldn’t see
much. Our profile would look like the upper line
below. The distance along the line from about
Raleigh, NC to Dakar, Senegal is more than 6500 km
(4000 miles) and the maximum depth is only about 5
km (3 mi). In fact, the line is fatter than the ocean is
deep, to scale!
Base map © by National Geographic Society.
A
A’
To represent the relief we have to introduce vertical
exaggeration – i.e., make the vertical scale far
greater than the horizontal. This makes things look
both much taller and much steeper than they actually
are, but we can see the pattern accurately on a smallscale profile.
The lower profile is vertically exaggerated by about
100x. (The map at left has a pretty extreme VE too!)
A’
A
Mid-Atlantic Ridge
Sea Level
The ocean ridges all have a similar structure. They stand high above the adjacent abyssal plains, and the central
part of the ridge has a deep, steep-sided canyon in it called the rift or central rift. For some distance on either
side there are additional steep scarps parallel to the rift, and there are numerous strike-slip or transform faults
that cut across the entire ridge system and offset the segments.
The central rift is a graben – a topographic low formed between paired normal faults that dip toward each other.
The parallel scarps are also normal fault scarps as the diagram shows. Remember how normal faults form.
The ridges are clearly points of divergence of two pieces of crust. The divergence creates tension and causes the
normal faults. The parallel fault scarps are old rift-bounding faults that have drifted away.
If the continents across the Atlantic have diverged then between them we should expect to find a relic of where
they were originally joined. Here it is. It still “looks like” the continents, and it is still diverging!
Base map © by National Geographic Society.
What other evidence do we have that the ridges are sites of crustal movement?
Source of base map uncertain. If anyone recognizes it let me know so I can credit it properly.
Check and see if the circled zones of intense earthquake activity are not ridges and transform faults.
Each earthquake epicenter is direct evidence for the edges of the plates moving.
ALL faulting takes place within the crust
and within the rift. Therefore the band of
epicenters is very narrow and the depths
to foci very shallow
Partial Melting
The ridges are also sites of intense igneous activity as well as earthquakes.
The mantle is made primarily of peridotite, which is mostly olivine. There is also a small
fraction of the rock that is not olivine, but lower temperature minerals like amphibole
and Ca-feldspar. These minerals are hot enough in the upper mantle to melt, but
the extreme pressure keeps them from doing so.
Whenever an earthquake occurs in the ridge it is because something has pulled the two
plates on either side in opposite directions, opening tilted cracks down which the hanging
wall blocks can slip, creating the normal faults of the rift graben.
This opening of cracks has the side effect of dropping the pressure in the upper mantle.
Suddenly the low temperature minerals find two new conditions: 1) the pressure is now low
enough for them to melt, and 2) there is an open conduit up which they can intrude,
possibly even reaching the surface and extruding.
The result is a set of mafic dikes: gabbro deep in the oceanic crust and basalt near the surface.
Voila: new oceanic crust. Older oceanic crust has been moved laterally away from the ridge.
Make sure you see how this creates the age pattern we spent so much time and effort to understand.
Observation 2
The “Zebra-Strip” Map
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?
Magnetic
Stage
Individual
Events
Holocene
Pleistocene
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.
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”.
Pliocene
It doesn’t migrate willy-nilly all over the planet,
just north or south.
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.)
Miocene
Furthermore, it was obvious that the times (in
MA or M.Y.) when the field reversed could be
determined radiometrically.
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.
Now for the rub: How many differences can you spot?
(Figures from Vine and Matthews)
(© 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.
(Image from Monroe and Wicander, The Changing Earth)
Observation 3
We Can Test the Hypothesis That the
Oceanic Crust is Older with Distance
form the Ridge in Two Ways.
Both Ways Verify the Hypothesis.
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 on each one, 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
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
J
Cenozoic
Mesozoic
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
Cenozoic
Mesozoic
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
Cenozoic
Mesozoic
Eo
NO SEDIMENT!
Eo
Eo
Eo
Eo
Pec
Pec
Pec
K
K
J
All the ocean basins show the same distribution of ages: younger crust toward the ridge.
This map was not based on paleomagnetism, but rather on the fossils in the sediment above the basaltic crust.
Jur.
Rec
© National Oceanic and Atmospheric Administration (NOAA)
The observations that contributed to the hypothesis of sea-floor spreading were therefore these (among
others we haven’t examined):
1)
Characteristics of the ridges:
A) If continental edges “fit” across an ocean then the ridge between them “fits” too.
B) The central rift is the site of intense earthquake activity as normal (tensional) faults move.
C) The central rift is the site of all basaltic volcanism in the ocean. The basalts
farther away must presumably have moved from the ridge to their present location.
2) The age of the oceanic crust:
A) Remnant magnetism in the oceanic basalts shows an odd pattern that indicates the
youngest crust is at the ridges and the age increases with distance from the ridge.
B) The ages of the youngest flows on oceanic islands, particularly those in aseismic ridges
agree with this age distribution.
C) The ages of the oldest sediments overlying the basalts are also progressively older farther
from the ridges. The oldest basalts and sediments in the oceans are middle
Mesozoic (Jurassic).
Once it became clear that the ocean basins were widening, with new crust formed in their centers (the
ridges) and moved away from that zone of origin with time it became easier to see that the continents on
either side of the ocean must be drifting away from the ridge as well. The ridge is the thing that rifts
continents and sends their rocks and fossils off in different directions. They look like they once fit
together, and to the ridge between them, because at the start both edges and the rift were the same thing!