Category: Orientation

Posts useful if you are new and just getting yourself set up in the lab.

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Blog Orientation Projects

What I’m doing in the Bay of Bengal, Part 1

[Now with illustrations!]

I realized shortly before I left the US that I’d written a few posts on what it is that a paleomagnetist does, but nothing about the purpose of our research cruise. I have a couple of days before Expedition 354 starts (I’m spending a few of those in Japan, filling up on ramen before I go!), so I can start with some of the basics now. While the cruise is going on, I’ll get to some more of the specifics while we’re out there.

The Bengal Fan is a huge, thick, wedge-shaped deposit of mud and other sediments that extends from the edge of Bangladesh out into the ocean near Sri Lanka. It is the thickest such deposit in the world. At the sediments’ thickest part – where the Ganges, Brahmaputra, and Meghna Rivers empty into the Bay of Bengal – the fan is over 20 kilometers thick. For comparison, this is at least twice as thick as Mt. Everest is tall! [1] The fan tapers out to the south and also thins out to the east. Where we will be drilling one of our first cores, on the flank of Ninetyeast Ridge, geologists think that the sediments are less than 2 km thick. [2]

Mt. Everest and the Himalaya are themselves part of the reason that the sediments are as thick as they are. As winds blow toward a mountain range, the air is blocked by the mountains and needs to rise. Rising, moist air produces rain. Rain, in turn, fills rivers and causes soil and rock to weaken and eventually slide. This erodes rock from the rainy side of the mountains. The eroded rock and soil has to go somewhere. In the eastern Himalaya, the the mountains’ detritus is funneled into the river systems – the Ganges, Brahmaputra, and Meghna – that feed the Bengal Fan.

But it hasn’t always been this way. The Himalaya were raised to their current, high elevations as India slammed into Asia , starting about 60 million years ago. Before that collision, India was probably shedding sediment off in all directions. Material eroded from India’s upper crust, mixed with ash and sediment from nearby volcanoes, fed an early apron of sediment around India (we think). As the blocks of crust that formed the Himalaya were uplifted, they were ground down by the action of rivers, rain, and landslides, until all of the upper crust was lost. Sediment from the lower crust began to feed the sedimentary system. My impression is that this material built the fan itself. [2]

IndiaEurasia70Ma
IndiaEurasia55Ma
IndiaEurasia50Ma

This is only part of the story. The fan is a huge, complex system, part of an even bigger and more complicated system (the Himalayan uplift). More explanation will come when I have time.

 

[1] It is also twice as thick as normal ocean crust, and, if I remember my stratigraphy right, over twice as thick as the thickest package of Catskill deltaic sediments in the New York / Pennsylvania area.

[2] This is mostly summarized from Curray, J. 2014. The Bengal Depositional System: From rift to orogeny. Marine Geology 352: 59-69, which is a nice introduction to the area.

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A Geologic Mystery: #ThinSectionThursday and #TBT… on Friday

Geologists like me like to cut and grind thin slices of a rock [1] – thin enough to see through – and look at them in a microscope using polarized light. The polarized light allows us to identify minerals more easily than we can by eye [2]. The thin slice of rock is called a thin section. When made with care, thin section photomicrographs can be beautiful (in this geologist’s opinion). Geologists have been posting them on Twitter under the hashtag #ThinSectionThursday for a while. For example:

Thursday was also #tbt (throwback Thursday), and this #thinsectionthursday photo fills both niches. This is a slide I found in my old desk when I was a grad student at the Scripps Institution of Oceanography. The desk used to belong to Gustaf Arrhenius, I think.

Slide of "Rock 17" thin section, back. Inscription: "Copy Transparency / Fritz Goro / Reproduction Prohibited Without Permission"

Slide of "Rock 17" thin section, front. Inscription: "gas bubble/ 40x / 62 / Rock 17"

“Rock 17” contains gray and white striped crystals, which are plagioclase feldspar (a calcium-sodium-aluminum silicate), as well as blobby blue-orange crystals of clinopyroxene (a calcium-iron-magnesium silicate). There are some black blocky particles of opaque minerals (probably the iron oxide magnetite) and at least one gas bubble or vesicle (the black round thing in the middle). The rock’s composition is therefore similar to that of a basalt, one of the most common rock types on Earth. The crystal sizes are difficult to determine without a some sort of a scale [3]. However, given the presence of the vesicle, I think this was probably erupted on Earth’s surface or on the seafloor. That would mean that the rock was cooled fairly quickly and the crystals are probably relatively small. I’d go with basalt as the name of this rock.

Where did it come from? One clue is the identity of the photographer: Fritz Goro. Goro was a well-known photographer around the middle of the 20th century whose work focused on science and scientists. His photos often illustrated articles in Life magazine. Goro took this photo of Arrhenius (along with Roger Revelle and others) on the Project Mohole cruise, an early attempt at scientific ocean drilling. I suspect this rock was one of those recovered as the 1961 drilling project attempted to drill through Earth’s crust into the mantle. Although Project Mohole failed in this respect, it became the foundation for the Deep Sea Drilling Project, which became the Ocean Drilling Program, and later the Integrated Ocean Drilling Program. The effort continues today as the International Ocean Discovery Program – the program in which I begin participating at the end of this month. [4]

 

[1] See Dave Hirsch’s page on making thin sections. We can do this, with the initial stages here at UW Tacoma and the finishing at University of Puget Sound or Pacific Lutheran University.

[2] We can do this, too: we have a Leica DM 750 petrographic microscope in the lab available for student use. Basic principles of petrography are presented in this document. To learn the details of how to use the scope, take TESC 347: Earth Materials.

[3] The “40x” inscription doesn’t help much – that’s just the microscope’s objective lens magnification. The microscope, camera and reproduction process also introduce some enlargement or reduction.

[4] I’ve been hesitating to post this because of the label on this slide, but I think my photo probably constitutes fair use in that it’s not for the purposes of reproducing the image. I took a photo of the slide itself, as an artifact.

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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.

 

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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.