Late 20 th Century Tests of the Continental Drift Hypothesis

Transcription

1 Late 20 th 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. 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.

2 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.

3 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.

4 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.

5 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!

6 Ocean Ridges, Magnetic Reversals, and the Age of the Oceanic Crust The New Hypothesis of Sea-Floor Spreading

7 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.

8 Piece 1 The Ridge.

9 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.

10 Piece 2 The Age.

11 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 ns (nearly 4,000 million years), the ocean is missing most of the earlier part of that record (all but the last 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?

12 Piece 3 Paleomagnetic Reversals. (Paleomagnetism, Part 1)

13 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.

14 Magnetic Stage Individual Events ocene 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. 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! ocene 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. cene 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.

15 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.

16 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.)

17 E = strength of Earth s Field R = strength of remnant magnetism from seafloor rocks. 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 ( Geological Society of America. Used under fair use clause of copyright law.)

18 Now for the rub: How many differences can you spot? ( Geological Society of America. Used under fair use clause of copyright law.)

19 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.)

20 ocene Pleistocene ocene 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. cene

21 ocene Pleistocene ocene cene 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.)

22 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.

23

24 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)

25 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.

26 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. The positive slope of the line indicates the islands get older with distance from the ridge. Age Measured Distance from Ridge

27 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. Base Map National Geographic Society. Used under fair use clause of copyright law. An active submarine volcano lies off the Big Island to the southeast.

28 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 cene). 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.

29 ZEBRA-STRIPE MAP HOTSPOTS Base Map National Geographic Society. Used under fair use clause of copyright law.

30 What happened ~40 my ago??? Direction of motion prior to 40my ago Present direction of motion Hotspot in mantle below ocean plate To original ridge location (.40 my ago) To East Pacific Rise Base Map National Geographic Society. Used under fair use clause of copyright law.

31 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: NO SEDIMENT! K K The chronostratigraphic age of the sediment at the bottom of the core increases away from the ridge. K K J J

32 K J K J K K NO SEDIMENT!

33 If we could take an oceanographic vessel back in time to the cene, a transect of cores would give us these sedimentary sequences in the younger, narrower ocean. No deposits younger than cene would yet exist, but the cores below level that would be the same older away from the ridge. NO SEDIMENT! K K K K J J

34 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.