Tag: paleomagnetism

Categories
Blog Projects

How Old is Earth’s Inner Core?

This post summarizes a publication in the journal Nature (note paywall):

Biggin, A.J., Piispa, E.J., Pesonen, L.J., Holme, R., Paterson, G.A., Veikkolainen, T., and Tauxe, L., 2015, Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation: Nature, v. 526, no. 7572, p. 245–248, doi: 10.1038/nature15523.

 

Earlier this month, an article about the age of Earth’s inner core made a big splash on both traditional and social media. The early history of Earth’s core – and particularly the inner core – aren’t things that scientists understand particularly well. Nonetheless, the question of when the inner core formed is a part of bigger questions about how Earth got here and what makes it unique.  We think that the inner core began to form once Earth had cooled a certain amount after its fiery infancy [1]. The inner core, then, is a sort of a thermometer for the early Earth: knowing exactly when the inner core started to form gives us some insight into just how hot Earth was shortly after its formation [2]. And I’m talking really hot here – magma oceans, not global warming. Other aspects of Earth that we consider more or less unique would have developed once Earth became cool enough. Plate tectonics and life are two examples.

Geochemists have fairly strong evidence from isotopes of the elements Hafnium and Tungsten that Earth’s metallic core must have formed in the first 50 million years of our planet’s history [3]. Earth has an unusually large core for a planet of its size. We think this is at least partially due to a Mars-size rock that plowed into the Earth early in the Solar System’s history. Computer simulations of this planetary collision indicate – and geochemistry confirms – that the event would have spun off some rocky material, becoming the Moon. The denser, metal-rich stuff left behind in the crash would have migrated toward the center of the developing Earth, forming our oversized core.

The origin of the inner core presents a bigger problem. Right now, we know about the inner core from the patterns of seismic waves that travel from one side of the Earth to the other, crossing different parts of the core. But seismic waves are ephemeral, dissipating soon after we record them. There are no seismic records from Earth’s past.

Earth’s magnetic field, generated in the fluid outer core (the geodynamo), might give us some clues about when the solid inner core formed. It takes some energy to generate Earth’s magnetic field. In the modern Earth, we think that the growing inner core powers the geodynamo: as the iron-nickel inner core crystallizes, light elements are spit out into the fluid outer core. The density difference between the sinking iron and nickel and the rising light elements, along with the temperature difference between the hot core and relatively cold mantle, cause the outer core to churn like a boiling pot of soup on the stove. This vigorous churning – thermal and compositional convection – produces a strong magnetic field relative to those of most other terrestrial planets. Without the inner core to drive the compositional part of the convection, the core would need to be very much hotter if we wanted it to produce a magnetic field like the one we have today.

Unlike seismic waves, we do have a record of magnetic fields from very old rocks. Since about the 1980s, geophysicists have been trying to identify patterns in the waxing and waning of Earth’s magnetic field over the billion-year timescales that we would need to look at planetary evolution. If we can identify a time when Earth’s strong and relatively stable magnetic field switched on, the reasoning goes, we might be able to tell when the inner core began powering the geodynamo.

I hedge my bets here about the magnetic field-inner core connection because of two main problems: first, it’s hard to measure the strength of Earth’s magnetic field in the past, and second, Earth’s magnetic field has varied through time due to a number of factors besides the inner core.

As far as the first problem goes, we have to make a lot of assumptions when using old rocks to look at Earth’s magnetic field. We assume that the rocks recorded Earth’s magnetic field reliably in the first place, through processes that we understand well. We assume that rocks are able to hold onto their magnetization for billions of years. We assume that the same rocks have not been remagnetized or demagnetized throughout their long histories. Although we have ways to test some of these assumption, our ability to do the tests depends on geological circumstance – whether the rocks contain certain dateable minerals, whether they have been folded or broken up and re-cemented together, or whether they are crosscut by more recent rocks, for instance. Even for young rocks, reading the strength of Earth’s magnetic field is much harder than looking at the direction of the ancient magnetic field (more on the specifics in a later post). Add to that the fact that rocks that have been around longer are generally more likely to have been re-magnetized, and you have a lot of reasons to doubt the paleomagnetic record of multi-billion-year-old rocks. For this reason, Andy Biggin and his co-authors on the recent Nature paper are choosy about which published data they use for their analysis [4]. Unfortunately, because of the problems mentioned here, and because there just aren’t many old rocks to choose from, reliable data on Earth’s magnetic field from the first four billion years of Earth’s history are few and far between. This is not the fault of the paper’s authors: reliable as the data may be, there are a lot of gaps. There are almost no useful data from the time interval between 300 million and one billion years ago. If you consider only the most reliable ancient field data from very old rocks – as Biggin et al. do – it is still difficult to draw a convincing long-term trend, since the estimates of Earth’s magnetic field strength vary widely for any particular range of ages. Nonetheless, in the set of data discussed in October’s Nature article, the average magnetic field estimate for the time period between  1.4 and 2.4 billion years ago is lower than that of the chunk of time between 0.5 and and 1.3 billion years ago. So perhaps we should be looking at rocks between 0.5 and 2.4 billion years old for other indications that the inner core formed. We also need to work at sorting out long-term trends from short-term variations for these very old rocks.

The second problem with the magnetic field-inner core connection is that Earth’s magnetic field is sensitive to things besides the growth of the inner core. For example, we think that the geographic pattern of hot and cold spots at the boundary between the mantle and the core might influence geodynamo activity [5]. We see this effect in both the intensity and in the reversals of Earth’s magnetic field. The hot and cold spots are related to convective motion in the mantle and perhaps to old, cold chunks of tectonic plates that have sunk over hundreds of millions of years. This would mean that the strength of Earth’s magnetic field may change on the few-hundred-million-year time scales associated with plate tectonics. Biggin et al. average data over long time periods to get rid of these fluctuations – akin to averaging weather variations to look at climate – but they acknowledge that especially long-lived patterns of heating and cooling may influence their averages. The further we look back in time, the fewer assumptions we can make about plate tectonic activity and its consequences. A electrically conductive molten layer of rock that may have blanketed the core early in Earth history (a “deep magma ocean”) could also have affected the early geodynamo [6]. More importantly, although geophysicists have made incredible strides over the past decade simulating core behavior, it’s not entirely clear how the average intensity of the magnetic field is related to the power driving the geodynamo. My impression (as a non-modeler) is that modelers agree that if the power available to move stuff around in the outer core drops below a certain lower limit, the geodynamo would shut off. But does more power necessarily mean a stronger field? The intensity of Earth’s field is not the only indicator of geodynamo activity. We might seek clues from other aspects of Earth’s magnetic field, such as how often the North and South magnetic poles switch, the rate at which the poles move, or the degree to which Earth’s field is similar to that of a bar magnet (Biggin et al. do explore the latter issue to a certain extent).

All of this is to say that, while the Nature article represents the best of our knowledge right now, there’s still a log way to go before we can conclusively say when the inner core formed. We are much closer than we were when I was a graduate student (when I graduated in 2003, only about 36% of the studies used in this paper had been published [7]). This is an exciting time to be a paleomagnetist!

 

[1] Rubie, D.C., Nimmo, F., and Melosh, H.J., 2007, Formation of Earth’s Core, in Treatise on Geophysics, Elsevier, p. 51–90.

[2] The Earth may have got rid of some of its initial thermal energy before the core formed.

Stevenson, D.J., 2007, Earth Formation and Evolution, in Treatise on Geophysics, Elsevier, p. 1–11.

[3] Basically, radioactive Hafnium-182 preferentially goes into molten metals, whereas its decay product, Tungsten-182, stays in rocks. Comparison between Tungsten-182 concentrations in meteorites and the Earth indicates that the radioactive Hafnium-182 must have separated from the mantle about 50 million years after Earth initially formed.

Lee, D.-C., and Halliday, A.N., 1995, Hafnium–tungsten chronometry and the timing of terrestrial core formation: Nature, v. 378, no. 6559, p. 771–774, doi: 10.1038/378771a0.

Rubie, D.C., Nimmo, F., and Melosh, H.J., 2007, Formation of Earth’s Core, in Treatise on Geophysics, Elsevier, p. 51–90.

[4] The Nature paper that I’m discussing here is a meta-anlaysis, meaning that it analyzes data collected in many studies to draw a broad conclusion. The authors rate estimates of magnetic field strength from 53 studies on a scale (QPI) according to how many assumptions have been verified in the experiment and field work of the original study. Most studies published these days get about a 3 out of 6 on the QPI scale. More about the types of experiments – called paleointensity experiments – in a future post.

[5] Although there has been more recent work on this topic, here’s the original reference:

Glatzmaier, G.A., Coe, R.S., Hongre, L., and Roberts, P.H., 1999, The role of the Earth’s mantle in controlling the frequency of geomagnetic reversals: Nature, v. 401, no. 6756, p. 885–890, doi: 10.1038/44776.
[6] Ziegler, L.B., and Stegman, D.R., 2013, Implications of a long-lived basal magma ocean in generating Earth’s ancient magnetic field: Geochemistry, Geophysics, Geosystems, v. 14, no. 11, p. 4735–4742, doi: 10.1002/2013GC005001.
[7] Just for my own interest, I plotted up the number of studies and the number of magnetic field intensity estimates from the dataset that Biggin and co-authors use. The graph shows how many of these studies (and field estimates) of different quality indices have been published each year. There’s a definite shift from publishing papers with few assumptions justified (QPI 1 or 2) to those with more checks (QPI 3 or above)! I do wonder how many more rock units are available for study – sounds like a useful GIS project!
Composition of Biggin et al. dataset by QPI and publication year. The graph on the left counts the fraction of studies included in the Biggin et al. dataset; the graph on the right counts the fraction of paleointensity estimates. The year of my Ph.D thesis (2003) is indicated in red for comparison with today (2015). Note the large fraction of the QPI 5 and 6 studies and estimates published since 2003. Note that Biggin et al. average paleointensity estimates to counteract the over-representation of certain studies with large numbers of paleointensity estimates in the database.
Composition of Biggin et al. dataset by QPI and publication year. The graph on the left counts the fraction of studies included in the Biggin et al. dataset; the graph on the right counts the fraction of paleointensity estimates. The year of my Ph.D thesis (2003) is indicated in red for comparison with today (2015). Note the large fraction of the QPI 5 and 6 studies and estimates published since 2003. Note that Biggin et al. average paleointensity estimates to counteract the over-representation of certain studies with large numbers of paleointensity estimates in the database.

 

Categories
Blog Orientation

Reversals, Part 3

Two lava flows and their magnetic directions

Two lava flows and their magnetic directions

Categories
Blog Orientation

Reversals, Part 2

Why do we call them magnetic reversals? Because the way some lavas are magnetized, Earth's magnetic NORTH pole would have had to be where the SOUTH pole currently is.

I’m finally getting back to the blog after about a week of frantic magnetometry (we discovered a bug in our magnetometer software, because of which we had to measure lots of stuff all over again!) and report-writing. Here is another in my reverse-color series on magnetic reversals.

Why do we call them magnetic reversals? Because the way some lavas are magnetized, Earth's magnetic NORTH pole would have had to be where the SOUTH pole currently is.

Categories
Blog Orientation

Basics of Magnetism 4: Reversals Part 1

A picture of a volcano with a Roman temple on top, and a story about building stones and magnetism

Earth has a magnetic field, which is what keeps your compass lined up with the North Pole [1]. The Earth’s outer core generates that magnetic field. You may have heard before that Earth’s magnetic field has, in the past, switched its North and South Poles. This is true, and kind of amazing and mysterious, but useful at the same time. This is the first in a series of picture-posts – not quite comics – that discusses magnetic reversals, and why and how we use them. I owe Maxwell Brown for this one.

A picture of a volcano with a Roman temple on top, and a story about building stones and magnetism

 

[1] Previous relevant posts are under the paleomagnetism tag.

 

Categories
Blog Orientation Projects

Lab Equipment on the Drill Ship

I spend about 12 hours in the lab most of the days I’m at sea [1]. So do most of the other scientists on board. Sometimes we get a little silly talking about our lab equipment after (or during) our shifts. Right now the lab is kind of quiet, waiting for cores to come up from our next site, so I have a chance to take pictures of the equipment without getting in anyone’s way.

Photo of lab instrument on track
The Section Half Multi-Sensor Logger

This is an instrument that records the color and magnetic susceptibility of split cores (“section halves”) [2]. It’s actually a robot that slides along a track taking measurements. We call it the Section Half Multi Sensor Logger, or SHMSL (pronounced “schmizzel”). The Germans on board have started calling it the Schnitzel.

The SHMSL isn’t really in our lab, but we use data from it all the time. In fact, there’s a back-and-forth between all of the labs on the ship. Paleontologists use the ages when plankton species appear and disappear from the fossil record to help us narrow down which magnetic reversals we’re measuring. We talk to the sedimentologists about sedimentation rate and what kinds of (magnetic) minerals might be in the sediments. The physical properties scientists help us decipher seismic reflection diagrams – more on those later – and collect most of the magnetic susceptibility data (three of the phys props scientists are paleomagnetists as well!). We collect samples for each other, too – I’ve even collected samples for organic geochemistry!

Silver bullet magnetometer
The 2-G superconducting rock magnetometer rocks on

This is the superconducting rock magnetometer, or SRM. We use it to measure the record of Earth’s past magnetic field in split cores (“section halves”) [3]. Everybody likes to say “superconducting rock magnetometer” because it makes you sound cool. But it is a mouthful. We sometimes call it the silver bullet. But usually we just call it the SRM (“ess-are-emm”). We used to have one like it in grad school. We named her Flo.

Boxes with flashing lights, connected to SQUIDs
A selection of SQUIDs

At the heart of the SRM are three rings made of superconducting wire. These are part of very precise magnetic field sensors called superconducting quantum interference devices, or SQUIDs. We have the other kind of squid out here, too. They are good on the barbecue.

DTECH D-2000
Alternating field demagnetizer. Don’t put your credit cards in here.

While this looks like the SRM’s little brother, it’s actually a different kind of device. This is the Dtech D-2000 alternating field demagnetizer. Samples that have had their magnetic records partially obscured by big magnetic fields from the drilling process (or by years of growing iron minerals at the bottom of the ocean) need to have those layers of extra magnetic grime scrubbed off by this machine. It works kind of like those old VHS tape erasers, but it’s a lot more precise. It also beeps VERY LOUD.

Box of plastic wrap watching you
Plastic wrap is a hot commodity in the core lab

We love plastic wrap in the core lab. We use it to make a nice flat surface for the SHMSL measurements, and to keep the sand and mud from cores out of our magnetometer. We wrap cores in plastic after we’re done analyzing or describing them. Hendrik, a sedimentologist, loves the boxes, too. He was very disappointed that other people kept throwing them away. Some people here think that you can wrap a core faster without the box. Hendrik disagrees. So there was a wrap-off between Hendrik and another sedimentologist. I don’t know who won. I’m agnostic about the boxes. But I do like to keep my magnetometer clean.

[1] In case you are just starting to read this blog, this post is part of my series of posts from the JOIDES Resolution, where I am participating in IODP Expedition 354 to study turbidites on the Bengal Fan.

[2] The optical sensor on the SHMSL is very similar to one that we have in the physics teaching lab at UW Tacoma. You will use it if you take Physics 3. It measures the visible and near-infrared spectrum of light. Magnetic susceptibility – “mag sus” around here – is a measurement of how much magnetic material is in a sediment core. The susceptibility meter applies a very weak magnetic field to the core, and measures the change in the sediment’s magnetization. We have one like it (and a track system for cores) in the Environmental Geology lab at UW Tacoma. Sorry, no SHMSL, though.

[3] Previous posts about the basics of Earth’s magnetic field are here, here, and here. Watch this blog for more about how we use the geomagnetic polarity time scale, or GPTS, to figure out the age of rocks – coming soon!

 

Categories
Blog Orientation Projects

How sediments get magnetized

We are currently drilling through a big pile of mud and sand on the seafloor (the biggest such pile of mud and sand in the world), and I’m spending most of my day sitting next to the “silver bullet” in this photo:

paleomag_labIf you can’t see the sign in the photo, this is the superconducting rock magnetometer (SRM) on the JOIDES Resolution. We use it to measure the record of Earth’s ancient magnetic field in rocks and sediments. Right now, we’re running sections of marine sediment cores through the machine. The SRM tells us what direction your compass would have pointed if you were standing here hundreds of thousands – or even millions – of years ago.

Muck in the oceans builds up, layer upon layer, so that older mud eventually gets covered with younger stuff. If you look closely at the muck, you’d see it was composed of lots of tiny particles. These are pieces of clay, silt, and sand formed from the detritus of eroded mountain ranges, the decaying bodies and shells of tiny fossil creatures, dust from the air, tiny crystals that form in the oceans, and even microscopic meteorites. Some of those particles are magnetic. For the most part, those contain the magnetic iron oxide magnetite [1], which can be part of the dregs of continental erosion, or it can be made by bacteria in the ocean, or by a number of other things. As the tiny magnetic particles fall through the water, they turn so that they are magnetized in line with Earth’s magnetic field – just like little compasses [2]. After they fall into the sediment accumulating on the seafloor, the magnetic particles get buried, “locked” in position by the other particles surrounding them. If Earth’s magnetic field switches polarity, the “tiny compasses” in new sediment being deposited will align with Earth’s new magnetic field, but the ones already locked in the sediment will stay as they were.

At least, that’s how the typical story goes about how sediment records the direction of Earth’s magnetic field. In reality, it’s not so simple. For one thing, all kinds of creatures live in the sediment – like whoever lived in this burrow:

Burrow in sediment core from Bengal Fan

This sediment core is actually full of fossil burrows. But sediments full of burrows can record Earth’s magnetic field just fine. We think it might be because the creatures burrowing in the sediment stir up the muck just enough that it settles back in line with Earth’s magnetic field again. It’s just that the sediment “locks in” the record of the magnetic field after the burrows themselves get buried. That seems reasonable until you realize that this burrow and others like it did not record a magnetic field in the same direction as the sediment around it [3]. This burrow is filled with pyrite, which, though iron-bearing, is not itself magnetic in the same way as magnetite [4]. Some geologists think that something happened to make new magnetic materials form or old ones dissolve around burrows like this one.

To make things even more complex, the area we are looking at on the Bengal Fan was not formed by sediments settling out in quiet water. Instead, much of the sand and mud deposited here was dumped very quickly from places close to land [5]. Do the magnetic particles in these tremendous currents full of churning sand and mud even have time to be pulled by Earth’s magnetic field, or are the forces in the currents too great? It looks like, at least in the muddy parts of deposits like the ones we’re studying, the sediment does keep a mostly faithful record of Earth’s magnetic field.

In the end, the story we tell about how sediments become magnetized is probably fundamentally OK, but there are parts of it we still don’t fully understand. Those parts of the story we’re still curious about are what keep us doing science!

[1] Magnetite is Fe3O4. To a certain extent, hematite (Fe2O3) and goethite (FeOOH) can also be incorporated into marine sediments, along with other magnetic minerals that can grow there.

[2] Unlike in igneous rocks, where the magnetic minerals “lock in” a record of Earth’s magnetic mineral as they cool. The minerals in igneous rocks DO NOT move.

[3] See Abrajevitch, A., Van Der Voo, R., and Rea, D., 2009. Variations in abundances of goethite and hematite in Bengal Fan sediments: Climatic vs. diagenetic signals, Marine Geology 267:191-206.

[4] Pyrite is paramagnetic, meaning that it can be magnetized only in the presence of a magnetic field, not after the field is gone; magnetite is ferromagnetic, meaning that it can be permanently magnetized.

[5]This is called a turbidity current, and the sand and mud deposits it leaves behind are called turbidites.

 

 

Categories
Blog Orientation Projects

How we get cores

The JOIDES Resolution is a ship made to recover hundreds of meters of rock or mud cores from miles below the ocean. This amazing feat is accomplished by a huge crew and one big drill. To understand what we’re trying to do out here, it helps to know how the drilling works.

The JR, on the right, is dwarfed by the Southern Ocean. I don't know what the Southern Ocean does, but it's not a drillship.
The JR, on the right, is dwarfed by the Southern Ocean. I don’t know what the Southern Ocean does, but it’s not a drillship.

The tower-like structure on the JR is where a lot of the drilling apparatus sits. The drill parts are lowered to the seafloor through a hole in the bottom of the ship – the Moon Pool (no photos of that yet: I can’t go down there).

The drill itself consists of a drill bit, inside of which sits a core barrel. The core barrel sits in the middle of the drill bit. Inside the core barrel is a device that lets sediment in, but not out (the core catcher) and a device to measure temperature. The whole apparatus is lowered down at the end of a drill pipe (an actual pipe), inside of which is a plastic tube (the core liner) that will hold the rock or sediment when we bring it up . Some weights are fitted to the pipe to get it to sit still on the seafloor.

If we are coring sediment, as we will be doing for much of this expedition, we use a device called an Advanced Piston Corer (APC) that punches 27 meters at a time into the sediment, pushed by both gravity and pressurized seawater (or sometimes mud). The APC and the core liner are pulled up out of the drill pipe, the core and liner removed, and the whole thing reloaded for the next 27 meters of coring. Meanwhile, the drill bit spins around, grinding down 27 meters until it gets to the bottom of the hole that the APC made. Then we repeat the process until we get to a couple of hundred meters below the seafloor, where the sediment is too hard for the APC.

A rotary drill bit (center), two APC barrels (far left), an XCB (extended core barrel, just to the left of the bit), and several core catchers (front).
A rotary drill bit (center), two APC barrels (far left), an XCB (extended core barrel, just to the left of the bit), and several core catchers (front).

There are two reasons we want to use an APC on the sediment here. First, APCs tend to recover a lot of sediment (other kinds of core barrels can break up the sediment, and tend to lose about half of it on the way down). Second, and just as important for us paleomagnetists, we can find the cores’ orientation using a compass-like device attached to the APC drill pipe. This is crucial if we need to know the direction in which the sediment of the Bengal Fan got magnetized: if the core turned around as it came out of the seafloor, we would never know if parts of it were magnetized in a different direction than Earth’s present magnetic field, or if they were just turned around during coring.

We will also be using the XCB on this expedition. The XCB is the Extended Core Barrel, a rotating core device that can cut more solid sediment. The XCB gets less recovery, and the core it takes can’t be oriented.

Categories
Blog Orientation

Basics of Magnetism 2: The Geodynamo

Here’s the second in a series that explains the basic ideas in paleo-, geo-, and rock magnetism. I’m hoping to separate the real-life mysteries and wonder from the jargon that sometimes makes magnets seem like magic tricks.  Have a question about any of these posts? Or about any aspect of paleomagnetism? I’d love to hear it. Please comment!

If you’ve taken an intro-level geology class, or if you’ve read much about magnetism, you have probably heard that Earth acts like a giant magnet because of something in its core. Earth’s core is a giant lump of metal at our planet’s center. We’ve never been there and have no samples of it, even though, as the crow flies, it’s just a little further from here than Chicago. We do know three important things about the core:

  1. It is dense, probably because it’s mostly made of iron and nickel.
  2. It has a molten outer shell surrounding a solid inner nugget.
  3. It is hot.

More on all of those later. We also think that Earth’s core the giant magnet responsible for Earth’s magnetic field. But here’s the weird thing about Earth’s core. When I say that the core is a “giant magnet,” I don’t mean it in the sense of the things that stick to your fridge. Although iron-nickel alloys like the core would probably stick to your fridge if they were suitably magnetized, they would lose their magnetic stickiness at the high temperatures deep in the Earth (more on that later, too). So how could the core be at such a high temperature and still be a magnet, producing Earth’s magnetic field?

The answer has to do with those giant electromagnets you might have seen at auto wrecking yards. These have an enormous coil of wire through which runs an electrical current. The electric current produces a strong magnetic field, allowing the coil to hold up big iron things like cars.  In the Earth, though, the electric current isn’t passing through wires – it’s caused by the swirling around of molten iron in the outer core.

Earth’s core is more complicated than a coil of wire. In the wrecking yard, the coil of wire becomes a magnet when it’s hooked up to an electrical generator – forcing a current through it. In the Earth, the outer core is both generator and electromagnet [1]. Electrical generators work by moving conductive wires through magnetic fields. In Earth’s core, the flow of molten iron and nickel in the outer core moves the conductive material through Earth’s magnetic field instead. That is the same magnetic field produced by the electrical currents in the molten metal. This confusing process is an example of a feedback loop.

Physicists love feedback loops (as do other scientists and mathematicians). Physical systems with feedback behave in interesting ways. You could imagine starting the molten outer core flowing in a certain way under a very weak magnetic field, maybe from the Sun or something else. Suppose this situation is not strong enough to cause a big electric current in the core. Earth’s core, then, might not be able to sustain its electric generating activity for very long. Alternatively, you might be able to imagine a pattern in which the outer core fluid flows in a way that causes big electrical currents. such a pattern could make Earth’s magnetic field stronger as time goes on.

The flow of molten metal in Earth’s outer core is controlled by a bunch of other factors besides the magnetic field. For example, the outer core loses more heat where the mantle above it is cold [2]. The formation of the inner core, heat due to radioactive elements, and the rotation of the Earth, all make the behavior of the outer core very difficult to predict. The unpredictable behavior of the core can make Earth’s magnetic field strengthen, decay, wander, and even reverse itself.  Nonetheless, over the past ten or so years, observations of Earth’s magnetic field through geological time have become numerous enough [3], and models of core behavior [4] have become precise enough, that we can draw some conclusions about some features of our planet’s core, which will be a topic for later. [5]

[1] Dynamo is another term for electrical generator. Earth’s outer core is sometimes referred to as the geodynamo. For more information on this topic, see Glatzmaier, G.A., and Olson, P., 2005, Probing the Geodynamo: Scientific American, v. 292, no. 4, p. 50–57, doi: 10.1038/scientificamerican0405-50.
[2] For example, we think that the lowermost mantle is cold where old slabs of subducted lithosphere have piled up. We can actually image this through a technique called seismic tomography (a topic for another day).
[3] The state of the art in crunching together high-resolution records of past magnetic fields is described in Korte, M., Constable, C., Donadini, F., and Holme, R., 2011, Reconstructing the Holocene geomagnetic field: Earth and Planetary Science Letters, v. 312, no. 3-4, p. 497–505, doi: 10.1016/j.epsl.2011.10.031. Definitely not beginner material.
[4] For information about a geodynamo simulation that includes reversals, see Gary Glatzmaier’s website.
[5] Want additional information? See David Stern’s The Great Magnet, The Earth.

 

Categories
Blog Orientation

Basics of Magnetism 1: Compasses

When I tell people that I study the history of Earth’s magnetic field, I get a bit self-conscious – as if I just told someone I specialize in Santa Claus. Geologists call us “paleomagicians” for a reason. You can’t see magnetic fields. You can’t touch them. Unlike most geological stuff, nothing obvious happens if you hit a magnetic field with a hammer. Once you understand a few things about Earth’s magnetic field, though, it becomes a bit less mystical. In the next few articles, I’ll try to bring Earth’s magnetic field … um … down to Earth.

Number 1: Compasses line up with magnetic fields. Although you can’t see a magnetic field, you can see its effects. In the pre-GPS days, when we still used maps and compasses, we used those effects all the time. Compass needles (which are themselves magnets) line up with magnetic fields. One end of the compass needle is the “north seeking” end, which points toward Earth’s North Magnetic Pole [1]. But wait: Earth’s North Magnetic Pole is not its North Pole! And the North Magnetic Pole moves from year to year. Here is a movie showing the angle your compass would point (relative to True North… as in North Star North) at different places on Earth, over the past 400 years more or less. Scientists made this animation in part by looking through old navigation logs, matching ships’ compass readings with the same ships’ positions based on speed estimates (dead reckoning) and star sightings [2]. Keep an eye on the North Magnetic Pole – where the lines converge in the Northern Hemisphere – as it drifts aimlessly around the Arctic. How random is this drift?

We want to how Magnetic North changes through time because it helps us navigate. But that’s really not the main issue now that we have GPS. We want to know how Magnetic North wanders because it’s a puzzle, and because it brings up some even more fundamental puzzles about the Earth. Why does Magnetic North wander? Where has it wandered in the past? If we were to watch a compass for, say, a million years, would it point at the true North Pole on average? And what, if anything, does that wandering tell us about the Earth?

[1] Physicists (and geophysicists) represent magnetic field lines in a few different ways: as arrows that line up the way compasses would (field vectors), as lines that connect those arrows (field lines), or, confusingly, as lines that illustrate the strength of the magnetic field (contour lines). You can play around with some of these representations here.
[2] If you want to see the original work, it’s by Finlay and Jackson (2003) and Jackson et al. (2000). These are not meant to be entry-level papers.

Categories
Blog Projects

How to go to sea as a paleomagnetist, part 2

What? You really wanted to know how I got picked to go to sea?

You might say it was Facebook.

Last spring, the Facebook page for the drillship JOIDES Resolution (“the JR”) posted a call for a paleomagnetist from the US to sail on an expedition to the Bengal Fan. Since that description fits me (and not that many other people), I sent in an application soon after. A few months later I heard that I got the position.

But that would be unfair: one does not simply apply for a position on the JR. I had to read up on what the chief scientists were planning to study, consider what my own contribution might be to the project, write a proposal to convince The Powers that Be that I was the real deal, and back that up with a list of all of my relevant academic work (my CV). To even get to that point, it took me all of college and several years of grad school to know what the JR was, and to be able to call myself a paleomagnetist. Honestly, though, I’m still sometimes not even sure I’m qualified to go on this cruise.

On the other hand, there is a lot you can learn about the JR, our cruise, and what it means to be a paleomagnetist with very little investment. The JR’s blog and website are an excellent way to start. Right now, the ship is not too far from where we’ll be drilling, on a different part of the Bengal Fan. There will be posts about our cruise when we are at sea. I’ll be posting some background here about Earth’s magnetic field and why it’s important for this particular cruise. I’m also planning to write a few posts about the rise of the Himalaya, the collision of India and Eurasia, and the Asian Monsoon.

And then, when the cruise is over, if you’re a UW student and would like to kick your involvement up a notch, you can work with me on samples that we bring home from the drill sites. You don’t need much training – most of the students who work in my lab start just after taking their first geology class. But being involved in research as a college student is a great way to work toward going to sea on a research vessel like the JR!