Historical Geology/Sea floor spreading
In this article we shall explain what sea-floor spreading is, and the role it plays in plate tectonics; we shall conclude, as usual, with an explanation of how we know that sea-floor spreading is taking and has taken place. The reader will find it useful to be familiar with the article on geomagnetic reversals and the article on marine sediments before reading further.
The nature and role of sea-floor spreading
The sea floor is divided by a system of mountain ranges (mid-ocean ridges) each with a deep valley running down the center (mid-ocean rifts); on the bathymetric map to the right you can clearly see the mid-Atlantic ridge.
According to the theory of plate tectonics, plates move apart at the rifts. As the lithospheric plates move apart, this makes a gap into which magma intrudes; it also reduces the pressure on the athenosphere below, causing partial melting of the mantle material. The intrusion of this material ensures that the rift is always being filled up by a fresh supply of oceanic crust. This whole process is known as sea-floor spreading.
One common misconception is that the intrusion of the magma at the rifts causes the motion of the lithospheric plates. In fact, geologists are well-agreed that this does little or nothing to cause the motion, rather, as explained in the previous paragraph, it is actually the parting of the plates at the ridges which causes the intrusion of the magma.
Nonetheless, sea-floor spreading plays a crucial role in plate tectonics: if the plates were unable to move apart at rifts, they would be unable to move at all.
In the remainder of this article we shall survey the evidence for sea-floor spreading.
Sea floor spreading: how do we know?
The proposition that the sea floor spreads out from the mid-ocean rifts, and has been doing so for millions of years, implies a diverse assortment of testable predictions, all of which turn out to be true.
As we discussed in a previous article, the Earth's magnetic poles keep swapping their positions. This leads us to a prediction. If igneous rocks have been formed at and spreading out from the mid-ocean rifts, then when we look at the paleomagnetic record in the igneous rocks that form the oceanic crust, what we ought to see is a pattern of stripes of alternating normal and reverse magnetism parallel to the mid-ocean rift and symmetrical around it: and this is in fact what we see. It was this discovery that almost overnight turned the concept of continental drift from a minority view among geologists to a widely-accepted idea.
Heat is only conducted very slowly through large bodies of rock. Consequently, if hot rock is produced at the ridges and spreads out from them (cooling, of course, as it does so) we expect the flow of heat from the sea floor to be greatest at the ridges and to gradually decline as we look at the sea floor further away from them. And this is in fact the case (see Pollack et al, 1993, Heat flow from the earth's interior: analysis of the global data set, Reviews of Geophysics 31(3), 267-280, 1993).
It is this cooling process that explains why the rifts are flanked by ridges on either side, which gradually slope down as we get further from the rift: the newly produced rock is hotter, and therefore has greater volume than the older rock; the older rock, having moved further from the rift, has had more time to cool down and so to contract.
This, by the way, explains why islands capped with coral so often subside into the sea, as mentioned in the article on reefs: as the basalt of the islands is carried further geographically from the rift, and further in time from the heat in which it was formed, it and the rest of the oceanic crust below it will cool, contract, and therefore subside.
For the same reason, if the sea floor is spreading out from the rifts, another obvious prediction of the theory is that if geologists apply their dating methods to the basalt sea-floor on either side of a rift, the rocks will be found to be older the further out they are from the ridge system; as is the case, as shown in the map to the right.
Accumulation of sediment
Similarly, sediment will have been accumulating on the older parts of the sea-floor for longer than on the newer parts around the mid-ocean rifts, resulting in a deeper sedimentary layer further out from the rifts: this is also the case.
Likewise, if the theory of sea-floor spreading is correct, then at any point on the sea floor the fossils found by drilling down to the bottom of the sea-floor sediment will be those deposited when that bit of the sea-floor was freshly produced at the rift.
This means that if we look at these deepest-buried fossils, we will see older and older fossils as we look further and further from the ridge; as a result we will see a greater proportion of extinct species. And this is in fact what we see.
The layer-cake effect
Different sediments tend to accumulate on different parts of the ocean floor. If the ocean floor stayed still, then, other things being equal, we would expect a sample of the sediment from any particular place on the sea floor to be pretty much the same all the way down.
But according to the theory of sea-floor spreading, the sea-floor has been continuously moving outward from the ridge systems like a conveyor belt, which implies that different sediments will have settled over the same portion of sea-floor as it moved.
So, for example, marine carbonates settle on the mid-Atlantic rift and rise, because in those shallow waters are above the carbonate compensation depth. Further out, where the waters are deeper, only pelagic clay will settle. So if the sea-floor really has been acting like a conveyor-belt, then when we take a sediment sample from areas of the Atlantic where pelagic mud settles, we should find this clay overlies a layer of limestone that settled when that portion of sea-floor was nearer the ridge; which is what we find.
And in general we can state the rule that for any particular spot on the ocean bed, the layers of sediment from bottom to top should be consistent with the journey of the sea-floor from the ridge outwards; which is what we find.
So, for example, the conveyor belt moving northwest from the east Pacific Rise west of South America towards Japan crosses the equator and the region where siliceous ooze is deposited. So near Japan we should and do find (from bottom to top) carbonate sediments deposited in the shallow waters at the Pacific Rise; pelagic clay from deeper and non-equatorial waters; calcareous/siliceous ooze as the conveyor belt crosses the equator; and more pelagic clay that accumulates north of the equator.
The mid-ocean ridges do not run in a continuous line on the ocean floor: rather, they are discontinuous, being displaced laterally along their length at faults, as can be seen in the map near the top of this article. This leads to an interesting prediction.
The top picture in the diagram to the right shows an ordinary strike-slip fault such as the San Andreas fault, with a road cutting across it displaced by motion along the fault. From either side of the fault, one sees the road as being displaced to the left, making it a left fault. Clearly, if you were standing on one side of the fault during an earthquake, you would see the land on the other side of the fault moving to the left relative to you.
Now consider the lower picture in the diagram. Standing on either side of the fault and looking at the ocean floor on the other side of the fault, you would see the mid-ocean ridge as having been displaced to the left on the far side of the fault.
But if geologists are right about the sea floor spreading out from the mid-ocean ridges, then if you stood on one side of the fault and looked across it during an earthquake, you would see the sea floor on the opposite side moving to the right relative to you. And this is what we do in fact observe.
Rate of continental drift
The measured rate of sea-floor spreading at the ridges agrees with the measured rate of continental drift, and the inferred rate at which geologists calculate it must have taken place in the past.
Structure of oceanic crust
From drilling through the oceanic crust, and by looking at the objects known as ophiolites, we can find out the structure of the crust, which is consistent with the theory of sea floor spreading and completely inexplicable without it. I shall not go into details here, as this topic will be covered in the main article on ophiolites.
All these disparate lines of evidence add up to a convincing demonstration that the sea floor is currently spreading from the mid-ocean rifts, and has done so in the past.