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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.
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! 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 small-scale profile. The lower profile is vertically exaggerated by about 100x. (The map at left has a pretty extreme VE too!)
Sea Level
Mid-Atlantic Ridge
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!
What other evidence do we have that the ridges are sites of crustal movement?
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.
Source of base map uncertain. If anyone recognizes it let m
e know so I can credit it properly.
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
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.
Partial Melting
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?
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”. 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.) Furthermore, it was obvious that the times (in MA or M.Y.) when the field reversed could be determined radiometrically.
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.
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.
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.
Holocene Pleistocene Pliocene Miocene
The oceanic crust is youngest at the ridge and gets progressively older with distance from the ridge!
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.
Measured Distance from Ridge
Age
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.
The positive slope of the line indicates the islands get older with distance from the ridge.
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 Plst Pli Mio Oli Eo Pec K J
Hol Plst Pli Mio Oli Eo Pec K J
Hol Plst Pli Mio Oli Eo Pec K
Hol Plst Pli Mio Oli Eo Pec K
Hol Plst Pli Mio Oli Eo Pec
Hol Plst Pli Mio Oli Eo Pec
Hol Plst Pli Mio Oli Eo
Hol Plst Pli Mio Oli Eo
Hol Plst Pli Mio Oli
Hol Plst Pli Mio Oli
Hol Plst Pli Mio
Hol Plst Pli Mio
Hol Plst Pli
Hol Plst
Hol Plst
Hol Plst Pli
Hol Hol
NO
SE
DIM
EN
T!
The chronostratigraphic age of the sediment at the bottom of the core
increases away from the ridge.
Cenozoic Mesozoic
Hol Plst Pli Mio Oli Eo Pec K J
Hol Plst Pli Mio Oli Eo Pec K J
Hol Plst Pli Mio Oli Eo Pec K
Hol Plst Pli Mio Oli Eo Pec K
Hol Plst Pli Mio Oli Eo Pec
Hol Plst Pli Mio Oli Eo Pec
Hol Plst Pli Mio Oli Eo
Hol Plst Pli Mio Oli Eo
Hol Plst Pli Mio Oli
Hol Plst Pli Mio Oli
Hol Plst Pli Mio
Hol Plst Pli Mio
Hol Plst Pli
Hol Plst
Hol Plst
Hol Plst Pli
Hol Hol
NO
SE
DIM
EN
T!
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.
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.
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!
