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Earth Magnetism and Rock Physics

GeoTalk: The anomaly in the Earth’s magnetic field which has geophysicists abuzz

GeoTalk: The anomaly in the Earth’s magnetic field which has geophysicists abuzz

Geotalk is a regular feature highlighting early career researchers and their work. In this interview we speak to Jay Shah, a PhD student at Imperial College London, who is investigating the South Atlantic Anomaly, a patch over the South Atlantic where the Earth’s magnetic field is weaker than elsewhere on the globe. He presented some of his recent findings at the 2017 General Assembly.

First, could you introduce yourself and tell us a little more about your career path so far?

I’m currently coming to the end of my PhD at Imperial College London. For my PhD, I’ve been working with the Natural Magnetism Group at Imperial and the Meteorites group at the Natural History Museum, London to study the origin of magnetism in meteorites, and how meteoritic magnetism can help us understand early Solar System conditions and formation processes.

Before my PhD I studied geology and geophysics, also at Imperial, which is when I studied the rocks that I spoke about at the 2017 EGU General Assembly.

What attracted you to the Earth’s magnetic field?

Jay operates the Vibrating Sample Magnetometer at the lab at Imperial. Credit: Christopher Dean/Jay Shah

My initial interest in magnetism, the ‘initial spark’ if you like, was during my undergraduate, when the topic was introduced in standard courses during my degree.

The field seemed quite magical: palaeomagnetists [scientists who study the Earth’s magnetic field history] are often known as palaeomagicians. But it’s through rigorous application of physics to geology that palaeomagicians can look back at the history of the Earth’s magnetic field recorded by rocks around the world. I was attracted to the important role palaeomagnetism has played in major geological discoveries such as plate tectonics and sea-floor spreading.

Then, during my undergraduate I had the opportunity to do some research alongside my degree, via the ‘Undergraduate Research Opportunities Programme’ at Imperial. It was certainly one of the bonuses of studying at a world-class research university where professors are always looking for keen students to help move projects forward.

I was involved in a project which focused on glacial tillites [a type of rock formed from glacial deposits] from Greenland to look into inclination shallowing; which is a feature of the way magnetism is recorded in rocks that can lead to inaccurate calculation of palaeolatitutdes [the past latitude of a place some time in the past]. Accurate interpretation of the direction of the Earth’s magnetic field recorded by rocks is essential to reconstructing the positions of continents throughout time.

This was my first taste of palaeomagnetism and opened the doors to the world of research.

So, then you moved onto a MSci where one of your study areas is Tristan da Cunha, a volcanic island in the South Atlantic. The location of the island means that you’ve dedicated some time to studying the South Atlantic Anomaly (SAA). So, what is it and why is it important?

The SAA is a present day feature of the magnetic field and has existed for the past 400 years, at least, based on observations. It is a region in the South Atlantic Ocean where the magnetic field is weaker than it is expected to be at that latitude.

The Earth’s magnetic field protects the planet and satellites orbiting around Earth from charged particles floating around in space, like the ones that cause aurorae. The field in the SAA is so weak that space agencies have to put special measures in place when their spacecraft orbit over the region to account for the increased exposure to radiation. The Hubble telescope, for example, doesn’t take any measurements when it passes through the SAA and the International Space Station has extra shielding added to protect the equipment and astronauts.

If you picture the Earth’s magnetic field:  it radiates from the poles towards the Earth’s equator, like butterfly wings extending out of the planet. In that model, which is what palaeomagnetic theory is based on, it is totally unexpected to have a large area of weakness.

Earth’s magnetic field connects the North Pole (orange lines) with the South Pole (blue lines) in this NASA-created image, a still capture from a 4-minute excerpt of “Dynamic Earth: Exploring Earth’s Climate Engine,” a fulldome, high-resolution movie. Credit: NASA Goddard Space Flight Center

We also know that the Earth’s magnetic field reverses (flips its polarity), on average, every 450,000 years. However, it has been almost twice as long since we have had a flip, which means we are ‘overdue’ a reversal. People like to look for signs that the field will reverse soon; could it be that the SAA is a feature of an impending (in geological time!) reversal? So, it becomes important to understand the SAA in that respect too.

So, how do you approach this problem? If the SAA is something you can’t see, simply measure, how do you go about studying it?

Palaeomagnetists can look to the rock record to understand the history of the Earth magnetic field.

Volcanic rocks best capture Earth’s magnetic field because they contain high percentages of iron bearing minerals, which align themselves with the Earth’s magnetic field as the lavas cool down after being erupted. They provide a record of the direction and the strength of the magnetic field at the time they were erupted.

In particular, I’ve been studying lavas from Tristan da Cunha (a hotspot island) in the Atlantic Ocean similar in latitude to South Africa and Brazil. There are about 300 people living on the island, which is still volcanically active. The last eruption on the island was in 1961. In 2004 there was a sub-marine eruption 24 km offshore.

Jürgen Matzka (GFZ Potsdam) collected hundreds and hundreds of rock cores from Tristan da Cunha on sampling campaigns back in 2004 and 2006.

We recently established the age of the lavas we sampled as having erupted some 46 to 90 thousand years ago. Now that we know the rock ages, we can look at the Earth’s magnetic field during this time window.

Why is this time window important?

These lavas erupted are within the region of the present day SAA, so we can look to see whether any similar anomalies to the Earth’s magnetic field existed in this time window.

So, what did you do next?

Initial analyses of these rocks focused on the direction of the magnetic field recorded by the rocks. The directional data can be used to trace back past locations of the Earth’s magnetic poles.

Then, during my master’s research dissertation I had the opportunity to experiment on the rocks from Tristan da Cunha with the focus on palaeointensity [the ancient intensity of the Earth’s magnetic field recorded by the rocks]. We found that they have the same weak signature we observe today in the SAA but in this really old time window.

The rocks from Tristan da Cunha, 46 to 90 thousand years ago, recorded a weaker magnetic field strength compared to the strength of the magnetic field of the time recorded by other rocks around the world.

Some of the lavas sampled on Tristan da Cunha. Credit: Jürgen Matzka

What does this discovery tell us about the SAA?

I mentioned at the start of the interview that, as far as we thought, the anomaly didn’t extend back more than 400 years ago – it’s supposed to be a recent feature of the field. Our findings suggest that the anomaly is a persistent feature of the magnetic field. Which is important, because researchers who simulate how the Earth’s magnetic field behaved in the past don’t see the SAA in simulations of the older magnetic field.

It may be that the simulations are poorly constrained. There are far fewer studies (and samples) of the Earth’s magnetic directions and strengths from the Southern Hemisphere. This inevitably leads to a sampling bias, meaning that the computer models don’t have enough data to ‘see’ the feature in the past.

However, we are pretty certain that the SAA isn’t as young as the simulations indicate. You can also extract information about the ancient magnetic field from archaeological samples. As clay pots are fired they too have the ability to record the strength and direction of the magnetic field at the time. Data recorded in archaeological samples from southern Africa, dating back to 1250 to 1600 AD also suggest the SAA existed at the time.

Does the fact that the SAA is older than was thought mean it can’t used be to indicate a reversal?

It could still be related to a future reversal – our findings certainly don’t rule that out.

However, they may be more likely to shed some light on how reversals occur, rather than when they will occur.

It’s been suggested that the weak magnetic anomaly may be a result of the Earth’s composition and structure at the boundary between the Earth’s core and the mantle (approximately 3000 km deep, sandwiched between the core and the Earth’s outermost layer known as the crust). Below southern Africa there is something called a large low shear velocity province (LLSVP), which causes the magnetic flux to effectively ‘flow backwards’.

These reversed flux patches are the likely cause of the weak magnetic field strength observed at the surface, and could well indicate an initiating reversal. However, the strength of the Earth’s magnetic field on average at present is stronger than what we’ve seen in the past prior to field reversals.

The important thing is the lack of data in the southern hemisphere. Sampling bias is pervasive throughout science, and it’s been seen here to limit our understanding of past field behaviour. We need more data from around the world to be able to understand past field behaviour and to constrain models as well as possible.

Sampling bias is pervasive throughout science, and it’s been seen here to limit our understanding of past field behaviour. This image highlights the problem (black dots = a sampling location). Modified from an image in the supporting materials of Shah, J., et al. 2016. Credit: Jay Shah.

You are coming towards the end of your PhD – what’s next?

So I moved far away from Tristan da Cunha for my PhD and have been looking at the magnetism recorded by meteorites originating from the early Solar System. I’d certainly like to pursue further research opportunities working with skills I’ve gained during my PhD. I want to continue working in the magical world of magnetism, that’s for sure! But who knows?

Something you said at the start of the interview struck me and is a light-hearted way to round-off our chat. You said that palaeomagnetism are often referred to as ‘paleaomagicians’ by others in the Earth sciences, why is that so?

Over the history of the geosciences, palaeomagntists have contributed to shedding light on big discoveries using data that not very many people work with. It’s not a big field within the geosciences, so it’s shrouded in a bit of mystery. Plus, it’s a bit of a departure from traditional geology, as it draws so heavily from physics. And finally, it’s not as well established as some of the other subdisciplines within geology and geophysics, it’s a pretty young science.  At least, that’s why I think so, anyway!

Interview by Laura Roberts Artal, EGU Communications Officer

References and further reading

Shah, J., Koppers, A.A., Leitner, M., Leonhardt, R., Muxworthy, A.R., Heunemann, C., Bachtadse, V., Ashley, J.A. and Matzka, J.: Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South AtlanticEarth and Planetary Science Letters441, pp.113-124, doi: 10.1016/j.epsl.2016.02.039, 2016.

Shah, J., Koppers, A.A., Leitner, M., Leonhardt, R., Muxworthy, A.R., Heunemann, C., Bachtadse, V., Ashley, J.A. and Matzka, J.: Paleomagnetic evidence for the persistence or recurrence of the South Atlantic geomagnetic Anomaly. Geophysical Research Abstracts, Vol. 19, EGU2017-7555-3, 2017, EGU General Assembly 2017.

May GeoRoundUp: the best of the Earth sciences from around the web

May GeoRoundUp: the best of the Earth sciences from around the web

Drawing inspiration from popular stories on our social media channels, as well as  unique and quirky research news, this monthly column aims to bring you the best of the Earth and planetary sciences from around the web.

Major Story

In the last couple of weeks of May, the news world was abuzz with the possibility of Donald Trump withdrawing from the Paris Agreement. Though the announcement actually came on June 1st, we’ve chosen to feature it in this round-up as it’s so timely and has dominated headlines throughout May and June.

In withdrawing from the agreement, the United States becomes only one of three countries in rejecting the accord, as this map shows. The implications of the U.S joining Syria and Nicaragua (though, to be clear, their reasons for not signing are hugely different to those which have motivated the U.S withdrawal) in dismissing the landmark agreement have been widely covered in the media.

President Trump’s announcement has drawn widespread condemnation across the financial, political and environmental sectors. Elon Musk, Tesla and SpaceX CEO, was one of many in the business sector to express their criticism of the President’s decision. In response to the announcement, Musk tweeted he was standing down from his duties as adviser to a number of White House councils. While in early May, thirty business CEOs  wrote an open letter published in the Wall Street Journal to express their “strong support for the U.S. remaining in the Paris Climate Agreement.”

In a defiant move, U.S. States (including California, New York and Vermont), cities and business plan to come together to continue to work towards meeting the targets and plans set out by the Paris Agreement. The group, coordinated by former New York City mayor Mark Bloomberg, aims to negotiate with the United Nations to have its contributions accepted to the Agreement alongside those of signatory nations.

“We’re going to do everything America would have done if it had stayed committed,” Bloomberg, said in an interview.

Scientist and learned societies have also been vocal in expressing their criticism of the White House decision. Both Nature and Science collected reactions from researchers around the globe. The EGU, as well as the American Geophysical Union, and many in the broader research community oppose the U.S. President’s decision.

“The EGU is committed to supporting the integrity of its scientific community and the science that it undertakes,” said the EGU’s President, Jonathan Bamber.

For an in-depth round-up of the global reaction take a look at this resource.

What you might have missed

This month’s links you might have missed take us on a journey through the Earth. Let’s start deep in the planet’s interior.

The core generates the Earth’s magnetic field. Periodically, the magnetic field reverses, but what caused it to do so? Well, there are several, competing, ideas which might explain why. Recently, one of them gained a bit more traction. By studying the seismic signals from powerful earthquakes, researchers at the University of Oxford found that regions on top of the Earth’s core sometimes behave like a giant lava lamp. It turns out that blobs of rock periodically rise and fall deep inside our planet. This could affect the magnetic field and cause it to flip.

Meanwhile, at the planet’s surface, the Earth’s outer solid layer (the crust) and upper layer of the molten mantle,  are broken up into a jigsaw of moving plates which pull apart and collide, generating earthquakes, driving volcanic eruptions and raising mountains. But the jury is still out as to when and how plate tectonics started. The Earth is so efficient at recycling and generating new crustal material, through plate tectonics, that only a limited record of very old rocks remains making it very hard to decipher the mystery. A recently published article explores what we know and what yet remains to be discovered when it comes to plate tectonics.

Tectonic plate boundaries. By Jose F. Vigil. USGS [Public domain], distributed by Wikimedia Commons.

Oil, gas, water, metal ores: these are the resources that spring to mind when thinking of commodities which fuel our daily lives. However, there are many others we use regularly, far more often than we realise or care to admit, but which we take for granted. Sand is one of them. In the industrial world it is know as ‘aggregate’ and it is the second most exploited natural resource after water. It is running out. A 2014 United Nations Environment Programme report highlighted that the “mining of sand and gravel greatly exceeds natural renewal rates”.

Links we liked

  • Earth Art takes a whole new meaning when viewed from space. This collection of photographs of natural parks as seen from above is pretty special.
  • This round-up is usually reserved for non-EGU related news stories, but given these interviews with female geoscientists featured in our second most popular tweet of the month, it is definitely worth a share: Conversations on being a women in geoscience – perspectives on what being a female in the Earth sciences.
  • We’ve shared these previously, but they are so great, we thought we’d highlight them again! Jill Pelto, a scientist studying the Antarctic Ice Sheet and an artist, uses data in her watercolous to communicate information about extreme environmental issues to a broad audience.

The EGU story

Temperatures in the Arctic are increasing twice as fast as in the rest of the globe, while the Antarctic is warming at a much slower rate. A new study published in Earth System Dynamics, an EGU open access journal, shows that land height could be a “game changer” when it comes to explaining why temperatures are rising at such different rates in the two regions. Read the full press release for all the details, or check out the brief explainer video, which you can also watch on our YouTube channel.

 

And don’t forget! To stay abreast of all the EGU’s events and activities, from highlighting papers published in our open access journals to providing news relating to EGU’s scientific divisions and meetings, including the General Assembly, subscribe to receive our monthly newsletter.

Imaggeo on Mondays: A lava layer cake

Imaggeo on Mondays: A lava layer cake

Brekkuselslækur, a small river, carves its way across Iceland’s ancient volcanic landscape. At Hengifoss, Iceland’s third-highest waterfall, it tumbles fiercely down thick, dark layers of lavas erupted from volcanoes some 18 to 2.58 million years ago, during a period of geological time known as the Tertiary.

Eruptions are rarely continuous; during hiatuses in the extrusion of lavas, ash is able to settle atop the smoldering layers. If the pauses are long enough and the conditions just right (a warm and humid climate is needed) the ashes, through the addition of clay and iron minerals, slowly turn to soil . When new lavas are layered over the top of the ash-rich soils, a chemical reaction takes place between the iron trapped in the soil and the oxygen transported by the lavas, to form iron oxide. In essence, the soils rust and turn a distinctive red colour.

As the process is repeated time and time again, layers of alternating black lavas and red soils are built up to form a giant ‘mille feuilles’ cake.

In the summer months, tourists flock to this popular site. An unspoilt view of the 188m high torrent means an early morning hike to beat the crowds. For a bird’s eye view of Hengifoss, the adventurous can even scarmble to the cliff tops too.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: In the belly of the beast

In the belly of the beast . Credit: Alexandra Kushnir (distributed via imaggeo.egu.eu)

Conducting research inside a volcanic crater is a pretty amazing scientific opportunity, but calling that crater home for a week might just be a volcanologist’s dream come true, as Alexandra postdoctoral researcher at the Institut de Physique du Globe de Strasbourg, describes in this week’s Imaggeo on Mondays.

This picture was taken from inside the crater of Mount St Helens, a stratovolcano in Washington State (USA). This particular volcano was made famous by its devastating explosive eruption in 1980, which was triggered by a landslide that removed most of the volcano’s northern flank.

Between 2004 and 2008 Mount St Helens experienced another type of eruption – this time effusive (where lava flowed out of the volcano without any accompanying explosions). Effusive eruptions produce lava flows that can be runny (low-viscosity) like the flows at Kilauea (Hawaii) or much thicker (high viscosity) like at Mount St Helens. Typically, high viscosity lavas can’t travel very far, so they begin to clump up in and around the volcano’s crater forming dome-like structures.  Sometimes, however, the erupting lava can be so rigid that it juts out of the volcano as a column of rock, known as a spine.

The 2004 to 2008 eruption at Mount St Helens saw the extrusion of a series of seven of these spines. At the peak of the eruption, up to 11 meters of rock were extruded per day. As these columns were pushed up and out of the volcanic conduit – the vertical pipe up which magma moves from depth to the surface – they began to roll over, evoking images of whales surfacing for air.

‘Whaleback’ spines are striking examples of exhumed fault surfaces – as these cylinders of rock are pushed out of the volcano their sides grind against the inside of the volcanic conduit in much the same way two sides of a fault zone move and grind past each other. These ground surfaces can provide scientists with a wealth of information about how lava is extruded during eruption. However, spines are generally unstable and tend to collapse after eruption making it difficult to characterize their outer surfaces in detail and, most importantly, safely.

Luckily, Mount St Helens provided an opportunity for a group of researchers to go into a volcanic crater and characterise these fault surfaces. While not all of the spines survived, portions of at least three spines were left intact and could be safely accessed for detailed structural analysis. These spines were encased in fault gouge – an unconsolidated layer of rock that forms when two sides of a fault zone move against one another – that was imprinted with striations running parallel to the direction of extrusion, known as slickensides. These features can give researchers information about how strain is accommodated in the volcanic conduit. The geologist in the photo (Betsy Friedlander, MSc) is measuring the dimensions and orientations of slickensides on the outer carapace of one of the spines; the southern portion of the crater wall can be seen in the background.

Volcanic craters are inherently changeable places and conducting a multi-day field campaign inside one requires a significant amount of planning and the implementation of rigorous safety protocols. But above all else, this type of research campaign requires an acquiescent mountain.

Because a large part of Mount St Helens had been excavated during the 1980 eruption, finding a safe field base inside the crater was possible. Since the 2004-2008 deposits were relatively unstable, the science team set up camp on the more stable 1980-1986 dome away from areas susceptible to rock falls and made the daily trek up the eastern lobe of the Crater Glacier to the 2004-2008 deposits.

Besides being convenient, this route also provides a spectacular tableau of the volcano’s inner structure with its oxidized reds and sulfurous yellows. The punctual peal of rock fall is a reminder of the inherent instability of a volcanic edifice, and the peculiar mix of cold glacier, razor sharp volcanic rock, and hot magmatic steam is otherworldly. That is, until an errant bee shows up to check out your dinner.

By Alexandra Kushnir, postdoctoral researcher at the Institut de Physique du Globe de Strasbourg, France.

This photo was taken in 2010 while A. Kushnir was a Masters student at the University of British Columbia and acting as a field assistant on the Mount St Helens project.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.