Transcript of Paleomagnetism

[music] The other tool we use to study   plate motion and direction is paleomagnetism – or  the record of ancient magnetism recorded in the   rocks. To understand how, we first have to review  how Earth’s magnetic field is produced. We first   mentioned it in the lecture on Earth formation.  It has existed on Earth since earth’s layers   formed – specifically the liquid iron outer core.  What do you remember that layer is doing? It’s hot   and liquid. So it convects. Because it’s a metal,  that convection acts very much like a current of   electrons moving in a loop, which you’ll learn  in a basic physics class is one way to produce   a magnetic field. Anyone that’s ever seen or  worked with solenoids can visualize this type of   magnetic field. Because of earth’s rotation, the  convection currents tend to stack up on each other   and combine to strengthen the field. The direction  of the field is dictated by the right-hand rule.   Hold up your right hand. Curl your fingers  in the direction of a current of coiled wire,   and your thumb will point toward the north pole  of the magnet. Hopefully you’ve played enough   with magnets to remember they have two opposite  poles, which we call north and south. Try to put   the north end of two magnets together and they’ll  repel. Let one move freely, and it will rotate so   its south end is pointed towards the north end  of the other. Earth’s magnetic field is oriented   pretty closely with its rotational north pole,  but not perfectly. And that pole wanders around.   Why? The currents change. They speed up. They slow  down. They change direction. If you sum up all the   currents, subtracting those going in the opposite  direction, the net sum gives you the strength and   direction of Earth’s magnetic field. Does the  movement of the North Pole happen fast enough   to matter? We can see that the movement has been  considerable and the direction and speed have   varied from one year to the next. If you grew  up in these islands and depended on a compass   to help you navigate, you’d need to reset it each  year to make sure you knew where it was pointing!   As mentioned, the north pole migrates, but so,  too, do the poles switch or reverse. How often?   Every couple hundred thousand years or so. Can  we predict it? Is there a pattern? This image   shows the changes in Earth’s magnetic field over  the past 5 million years. Do you see a pattern?   Can we predict a change? Maybe. As these graphs  of intensity show, the strength of the magnetic   field decreases and then hits zero before either  starting up again and increasing in the same   direction as before or increasing but in the  opposite direction. What happens during a switch   in magnetic polarity? We have no evidence from  the fossil or rock world that a switch has ever   caused an extinction. There are many organisms  that seem to use earth’s magnetic field to help   them navigate. But presumably they also use other  clues, including visual clues. And since the field   decreases and then disappears before flipping,  those organisms must rely on their other senses   and clues more and more before they lose the field  entirely. After the flip, they can easily connect   the new field back to their clues and use it  again, even though it’s in the opposite direction.   They would simply adjust. As would we. The only  major concern would be the increase in solar   winds that are normally deflected. The ionized  particles would collide more with the atmosphere,   as they do now over the poles creating spectacular  auroras. We might see auroras across the planet   at all latitudes, and for a while we might see  more intense storms. But evidence also suggests   that the absence of a magnetic field wouldn’t  last long enough to produce any major changes.   So how do we create these lovely graphs that  tell us what earth’s magnetic field direction   has been in the past? And its strength? We get  them by studying fossil evidence left behind in   volcanic rocks that erupted on land. As this  image of a volcano shows, when lavas erupt,   they are molten. As crystals begin to form  during cooling, they are free to move. Any   magnetic minerals that form will rotate to align  themselves with earth’s magnetic field – and   there’s one magnetic mineral that is found in all  volcanic rocks – it is called, strangely enough,   magnetite. So we can go to the piles of lavas  that make up volcanoes and starting at the   oldest on the bottom, take samples, date them,  so we know how old they are, and then measure   the direction of their magnetite crystals to see  what direction the magnetic field was when they   formed. We call that ancient or fossil evidence  of earth’s past magnetic field, paleomagnetism.   This paleomagnetic record becomes a powerful tool  for studying plate tectonics when we apply it to   seafloor spreading. During World War II, in the  Pacific Ocean, oceanographers conscripted into   the navy were combining wartime activities with  science and gathering data on seafloor magnetic   anomalies. A sensitive magnet towed behind the  boat as it criss-crossed the oceans multiple   times, was able to pick up the magnetism of the  pillow basalts under the sediments at the bottom   of the ocean. Where the magnet picked up positive  anomalies – they registered magnetic fields that   were aligned with the present magnetic field,  which we call normal polarity, and thus boosted   that field making it stronger. Where the magnet  picked up negative anomalies, the pillow basalt   magnetites were aligned in the opposite direction  – reversed polarity – and were subtracting from   the strength of the current field. By recording  these data multiple times as they cross the   mid-ocean ridges, they were able to develop  maps such as these, which not only show the   dramatic mirror symmetry of seafloor spreading,  confirming this essential spreading process, but   also can be used to measure spreading speed and  direction, as we did with hotspots. How? We have   to first find the center of symmetry. This is the  center of the ridge. Now we start at the center,   where we know the rocks to be the youngest, newest  rocks, and as we move outwards from the center,   we can correlate each anomaly boundary with the  known dates for when earth’s magnetic field has   switched. For example, in this image we see that  the most recent flip happened about 700,000 years   ago. Before that there was one 900,000 years  ago, and another 1.2 million years ago. So   back to our anomalies, we can say that rocks  along this line must be 700,000 years old,   and this one, 900,000 years old, and this one  1.2 million years old, and so on. In fact,   we can continue this all the way out to the edges  of the oceans and ultimately create an image like   this one that shows the age of every rock on the  bottom of the seafloor even though only 1% of the   ocean floor has ever actually been sampled.  What an incredibly powerful tool for making   indirect measurements and gathering extremely  useful information in a most economical way!   So back to measuring speed and direction. Let’s  use this normal polarity reading that represents   7 million-year-old rock. Where was this rock  when it formed? Here at the spreading center.   So what’s the direction of plate motion? Away.  And how fast on average has it been spreading?   If 560 kilometers separates the rock from its  origin, then it has travelled 560 kilometers   in 7 million years. That’s 80 km/my or 8 cm/yr.  We call that a half-spreading rate, by the way,   because it represents how fast one half of the  rift is spreading or how fast the western side   is moving away from the center. If we want to  know the full spreading rate and how fast both   sides are moving away from each other, the  distance is doubled, and we get 16 cm/year.   Pause now. [music] For more information and more detail,   continue on to the next video in the series. [music]