Download Plate Tectonics -- Structure and Behavior of Oceanic Ridges

Late 20th Century Tests of the
Continental Drift Hypothesis
4 – Characteristics of the Ocean Ridges
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What to look for:
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Ridges are the centers of spreading in seafloor spreading. The crust “ages” away from
them.
In some cases (Atlantic and Indian) they originally formed under a continent. This rifted the
continent and subsequently separated the pieces.
In these cases the continental edges on either side appear to fit (the earliest ever
observation suggesting drift) and they also fit the ridge!
In other cases (Pacific and Arctic) they originally formed in oceanic crust and nothing
matches anything else. (Except the two sides of the ridge).
Intense earthquake activity in the rift is direct evidence of movement there.
Normal faults in (active) and parallel (inactive) to the rift indicate both tensional stress are
responsible for the earthquakes, and that they have done so for a very long time. (The
faults, like the crust they are in, “age” with distance from the ridge.)
These faults allow for the formation of mafic magmas in the upper mantle and their rapid
transit toward the surface. The resulting mafic rocks replace the crust that the tension
moves away from the ridge.
Though the initial reaction is always to assume that a continent moves away from a ridge, it
is also possible for the ridge to drift away from the continent.
If this science had started with the hypothesis that
certain continents were once joined, the early
scientists should have been able to make a
number of predictions.
For example, they might have
predicted the things that actually were first
observed: things that match, same biota, similar
climates on now distant continents, and so on.
There’s another type of prediction that they might have
made, and, in fact, did make after the fact. These
predictions involve the oceans between them and the
oceans on the opposite sides of them.
For example: if two things were once joined and
now are not, we might expect to find the place they
were originally joined somewhere between them.
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.
The transforms faults that
cross-cut the ridges are
simply how the thin, brittle
plates accommodate the
need to move faster in some
places (near equator of
rotation) than others (toward
poles of rotation).
(We are tempted here to imagine
Africa and North America both
moving away from the M.A.R.,
but actually N. America and the
M.A.R. are both moving away
from Africa, which is nearly
stationary)!
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.
Curious note:
Africa is the only
continent whose
adjacent ridges have
roughly the same
shape as the continent
(red).
Along the northern
border of its plate are a
set of subtle trenches
and transforms (black)
that accommodate very
slow movement of
Africa northward, very
close to its pole of
rotation. (Which is
probably in the Atlantic
near the ridge.
Because the continent is
almost completely
surrounded by ridges it
cannot be moving away
from them. That would
mean it would be moving
eastward from the M.A.R.
and westward from the
M.I.R., which, I hope,
seems absurd to you.
Particularly since there are
rifts within Africa. and no
convergent mountains to
speak of
Instead, the ridges must be
moving away from Africa.
Whenever we say
“continental drift” we have
to remember that the
oceans are drifting too –
the entire plate, not just the
part that Francis Bacon
could see.
This explains several
interesting things about Africa:
1) It sits over warmer mantle
than any other continent, with
several hotspots. Presumably
it has been insulating the
mantle and “storing” heat.
2) It has the narrowest
shelves of any continent
(probably because the warm
mantle buoys it up).
3) Despite having only one
small mountain chain (the
Atlas, on the NW corner) it
has the highest average
elevation of any continent.
The maximum elevation in the
Atlas is 13,663’; there are
numerous peaks in Colorado
alone higher than that. Also,
presumably, because the
warm mantle buoys it up.
So while it is tempting
to think of the continent
moving away from a
ridge, remember that it
can work both ways.
YES!
NO!
Take-home Message:
•
•
•
•
•
•
•
•
Ridges are the centers of spreading in seafloor spreading. The crust “ages” away from
them.
In some cases (Atlantic and Indian) they originally formed under a continent. This rifted the
continent and subsequently separated the pieces.
In these cases the continental edges on either side appear to fit (the earliest ever
observation suggesting drift) and they also fit the ridge!
In other cases (Pacific and Arctic) they originally formed in oceanic crust and nothing
matches anything else. (Except the two sides of the ridge).
Intense earthquake activity in the rift is direct evidence of movement there.
Normal faults in (active) and parallel (inactive) to the rift indicate both tensional stress are
responsible for the earthquakes, and that they have done so for a very long time. (The
faults, like the crust they are in, “age” with distance from the ridge.)
These faults allow for the formation of mafic magmas in the upper mantle and their rapid
transit toward the surface. The resulting mafic rocks replace the crust that the tension
moves away from the ridge.
Though the initial reaction is always to assume that a continent moves away from a ridge, it
is also possible for the ridge to drift away from the continent.