GeoLog

Earth Magnetism and Rock Physics

Plate Tectonics and Ocean Drilling – Fifty Years On

Plate Tectonics and Ocean Drilling – Fifty Years On

What does it take to get a scientific theory accepted? Hard facts? A strong personality? Grit and determination? For many Earth Scientists today it can be hard to imagine the academic landscape before the advent of plate tectonics. But it was only fifty years ago that the theory really became cemented as scientific consensus. And the clinching evidence was found in the oceans.

Alfred Wegener had proposed the theory of continental drift back in 1912. The jigsaw-fit of the African and South American continents led him to suppose that they must once have been joined together. But in the middle of the century, the idea fell out of favour; some even referred to it as a “fairy-tale”.

It was not until the discovery of magnetic reversals on the seafloor in the early 1960s that the theory began to sound plausible again. If brand new ocean crust was being formed at the mid-ocean ridges, then the rocks either side of the ridge should show symmetrical patterns of magnetism. Fred Vine and Drummond Matthews, geologists at the University of Cambridge in the UK, were the first to publish on the idea of seafloor spreading in 1963.

But plate tectonics was still not the only theory on the market. The expanding Earth hypothesis held that the positions of the continents could be explained by an overall expansion in the volume of the Earth. Numerous twentieth-century physicists subscribed to such a view. Or, similarly, the shrinking Earth theory proposed that the whole planet had once been molten. Mountain ranges would then be formed as the Earth cooled and the crust crumpled.

Helmut Weissert, President of the EGU Stratigraphy, Sedimentology and Palaeontology Division, remembers the difficult exchanges that took place whilst he was a student at ETH Zürich in the late 1960s. “Earth-science-wise it was a hot time,” he recalls. “In Bern University they did not teach plate tectonics. We did not have a course on plate tectonics either. I probably first heard about plate tectonics in [my] second or third year.”

Weissert especially remembers Rudolf Trümpy, professor of Alpine geology at ETH at the time, saying that plate tectonics sounds interesting, but it does not work for the Alps. Meanwhile, younger voices at ETH, postdocs and lecturers, were becoming increasingly convinced by plate tectonic theory.

Weissert soon found himself in the midst of the controversy as his own research had a direct bearing on the debate. “I had an interesting diploma topic,” says Weissert. “I worked on continental margin successions and associated serpentinites.” Serpentinites are green-coloured rocks that are full of the water-rich mineral serpentine, and therefore must have formed on the ocean floor. The fact that Weissert was finding them in Davos, at the top of the Alps, was a good indication that modern-day Switzerland had once been part of the oceans. As Weissert succinctly puts it, “green rocks were ocean”.

The observed and calculated magnetic profile for the seafloor across the East Pacific Rise, showing symmetrical patterns of magnetism. (Image Credit: U.S. Geological Survey. Distributed via Wikimedia Commons)

By 1967, interest in the theory of plate tectonics had snowballed. When the Deep Sea Drilling Project (DSDP) was launched the following year, it had its sights firmly set on finding evidence that would definitively either confirm or reject the hypothesis of seafloor spreading.

The DSDP research vessel, the Glomar Challenger, set sail from Texas in March 1968. By its third leg it had drilled 17 holes at 10 sites along the mid-Atlantic ocean ridge and was already producing results that looked like they would confirm Wegener’s theory of continental drift. “After a few legs it was clear that the seafloor spreading hypothesis was tested and proven,” remembers Weissert.

There were only eight scientists on board, but two or three of them were working on the stratigraphy of the seafloor sediments. “The stratigraphy was superb,” explains Weissert. “You have the very young [sediments near the ridge] and then at the edges of the ocean the Jurassic sediments. If you have aging crust then you have aging sediment, so the hypothesis was very clear.” If the sediments got progressively older on moving away from the ridge, then so must the crust, a sure sign that new ocean floor was being created at the ridge.

Karen Heywood, EGU Division President in Ocean Sciences, remembers how her own fascination with the theory of plate tectonics ended up sparking her career in physical oceanography. Heywood began as a physics student at the University of Bristol in the 1980s. “They said we had to write an essay on the historical development of an idea in physics,” she recalls. “I did the development of the theory of plate tectonics and seafloor spreading. I wrote this essay all about Alfred Wegener.”

“This essay inspired me to think about earth sciences,” she says. “The idea that you could apply physics to the real world was amazing. It got me into oceanography.”

Heywood went on to establish her career at the University of East Anglia (UEA), where she became the first female professor of Physical Oceanography in the UK. “I went to the UEA and Fred Vine was there. It brought me back full circle. I could not believe that this was Fred Vine, who had discovered the magnetic stripes. This was the real person and that was amazing… it was the same person that I had read about and written about in my essay as an undergraduate in the 80s.”

There were clearly strong personalities on both sides of the debate about plate tectonics, but Weissert is pragmatic about the progress of science. “You have to accept that you are part of a scientific development. Everybody makes hypotheses… We all make mistakes. We all learn. We all improve.”

Indeed, many years later, in 2001, Trümpy wrote what Weissert calls “a beautiful small article” entitled Why plate tectonics was not invented in the Alps. Trümpy magnanimously writes, “Shamefacedly, I must admit that I was not among the first Alpine geologists to grasp the promise of the new tectonics.”  And yet, he continues, “to the Alps, plate tectonics brought a better understanding”. The humans and the science move on together.

By Tim Middleton, EGU 2018 General Assembly Press Assistant

References

DSDP Phase: Glomar Challenger, International Ocean Discovery Program

Trümpy, R., Why plate tectonics was not invented in the Alps, International Journal of Earth Sciences, Volume 90, Issue 3, pp 477–483, 2001.

Imaggeo on Mondays: River in a charoite schist

Imaggeo on Mondays: River in a charoite schist

Polarized light photomicrograph of a thin section of a charoite-bearing schist.

Charoite is a rare silicate found only at one location in Yakutia, Russia. For its beautiful and uncommon purple color it is used as a semi-precious stone in jewelry.

Under the microscope charoite-bearing rocks give an overall feeling of movement, with charoite forming fibrous mats that swirl and fold as a result of deformation during metamorphism.

Due to the variable orientation with respect to the polarized light the charoite may exhibit different interference colors. It may be difficult to conceive, but these microstructures tell us that solid rocks can flow! Width of view: 5,4 mm.

Description by Bernardo Cesare, as it first appeared on imaggeo.egu.eu

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/.

 

April GeoRoundUp: the best of the Earth sciences from the 2018 General Assembly

April GeoRoundUp: the best of the Earth sciences from the 2018 General Assembly

The 2018 General Assembly took place in Vienna last month, drawing more than 15,000 participants from 106 countries. This month’s GeoRoundUp will focus on some of the unique and interesting stories that came out of research presented at the Assembly.

Mystery solved

The World War II battleship Tirpitz was the largest vessel in the German navy, stationed primarily off the Norwegian coastline as a foreboding threat to Allied armies. The ship was 250 metres in length and capable of carrying around 2,500 crewmates.

Despite its massive size, the vessel’s presence often went unnoticed as it moved between fjords, masked by a chemical fog of chlorosulphuric acid released by the Nazi army.

Ultimately the ship sank and the war ended, but evidence of the toxic smog still lingers today, in the tree rings of Norway’s nearby forests.

Claudia Hartl, a dendrochronologist from the Johannes Gutenberg University in Mainz, Germany, made this discovery unexpectedly while sampling pines and birches near the Norwegian village Kåfjord. She and her research team presented their findings at the General Assembly in Vienna last month.

The German battleship Tirpitz partly covered by a smokescreen at Kaafjord. (Image Credit: Imperial War Museums )

Hartl had been examining wood cores to draw a more complete picture of past climate in the region when she noticed that some trees completely lacked rings dating to 1945,” reported Julissa Treviño in Smithsonian Magazine.

The discovery was odd since it is rare for trees to have completely absent rings in their trunks. Tree ring growth can be stunted by extreme cold or insect infestation, but neither case is severe enough to explain the missing tree rings from that time period.

“A colleague suggested it could have something to do with the Tirpitz, which was anchored the previous year at Kåfjord where it was attacked by Allied bombers,” explains Jonathan Amos from BBC News.

The researchers indeed found physical and chemical evidence of the smokescreen damage on the trees, demonstrating the long-lasting impact warfare can impart onto the environment.

 

What you might have missed

Seismicity of city life

Researchers use seismometers to record Earth’s quakes and tremors, but some seismologists have employed these instruments for a different purpose, to show how humans make cities shake. “This new field of urban seismology aims to detect the vibrations caused by road traffic, subway trains, and even cultural activities,” reports EGU General Assembly Press Assistant Tim Middleton on GeoLog.

With seismometers, Jordi Díaz and colleagues at the Institute of Earth Sciences Jaume Almera in Barcelona, Spain have been able to pick up the seismic signals of major football games and rock concerts, like footballer Lionel Messi’s winning goal against Paris Saint-Germain and Bruce Springsteen’s Barcelona show.

Seismic record captured by the seismometer during the Bruce Springsteen concert. The upper panel shows the seismogram, while the lower panel shows the spectrogram where it is possible to see the distribution of the energy between the different frequencies. (Image Credit: Jordi Díaz)

Díaz’s project first began as an outreach campaign, to teach the general public about seismometers, but now he and his colleagues are exploring other applications. For example, the data could help civil engineers with tracking traffic and monitoring how buildings withstand human-induced tremors.

Antarctica seeing more snow

Meanwhile in Antarctica, snowfall has increased by 10 percent in the last 200 years, according to new research presented at the meeting. After analysing 79 ice cores, a research team led by Liz Thomas from the British Antarctic Survey discovered that Antarctica’s increased snowfall since 1800 was equivalent to 544 trillion pounds of water, about twice the volume of the Dead Sea.

It has been predicted that snowfall increase would be a consequence of global warming, since a warmer atmosphere can hold more moisture, thus resulting in more precipitation. However, these ice core observations reveal this effect has already been happening. The new finding implies that Earth’s sea level has risen slightly less than it would have otherwise, but only by about a fifth of a milimetre. Though overall, this snowfall increase is not nearly enough to offset Earth’s increased ice loss.

Ocean’s tides create a magnetic field

Also at the Assembly, scientists presented new data collected from a team of ESA satellites known as Swarm, In particular, the satellite observations recently mapped magnetic signals induced by Earth’s ocean tides. As the planet’s tides ebb and flow, drawn by the Moon’s gravitational pull, the salty water generates electric currents. And these currents create a tiny magnetic field, around 20,000 times weaker than the global magnetic field.

Scientists involved with the Swarm project say the magnetic view provides new insight into Earth’s ocean flow and magnetic field, can improve our understanding of climate change, and help researchers build better Earth system models.

When salty ocean water flows through Earth’s magnetic field, an electric current is generated, and this in turn induces a magnetic signal. (Credit: ESA/Planetary Visions)

 

Other noteworthy stories:

 

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: Chilean relics of Earth’s past

Imaggeo on Mondays: Chilean relics of Earth’s past

As Earth’s environment changes, it leaves behind clues used by scientists to paint portraits of the past: scorched timber, water-weathered shores, hardened lava flows. Chile’s Conguillío National Park is teeming with these kind of geologic artifacts; some are only a few years old while others have existed for more than 30 million years. The photographer Anita Di Chiara, a researcher at Lancaster University in the UK, describes how she analyses ancient magnetic field records to learn about Earth’s changing crust.

Llaima Volcano, within the Conguillío National Park in Chile, is in the background of this image with its typical double-hump shape. The lake is called Lago Verde and the trunks sticking out are likely remnants from one of the many seasonal fires that have left their mark on this area (the last one was in 2015).

The lake sits on pyroclastic deposits that erupted from the Llaima Volcano. On these deposits, on the side of the lake, you can even track the geologic record of seasonal lake level changes, as the layers shown here mark the old (higher) level of the lake during heavy winter rains.

The lake also overlaps the Liquiñe-Ofqui Fault, which runs about 1000 kilometers along the North Patagonian Andes. The fault has been responsible for both volcanic and seismic activity in the region since the Oligocene (around 30 million years ago).

I was there as field assistant for Catalina Hernandez Moreno, a geoscientist at Italy’s National Institute of Geophysics and Volcanology, studying ancient magnetic field records imprinted on rocks. We examined the rocks’ magnetised minerals (aligned like a compass needle to the north pole) as a way to measure how fragmented blocks of the Earth’s crust have rotated over time along the fault.

From this fieldwork we were able to examine palaeomagnetic rotation patterns from 98 Oligocene-Pleistocene volcanic sites. Even more, we concluded that the lava flows from the Llaima Volcano’s 1958 eruption would be a suitable site for studying the evolution of the South Atlantic Anomaly, an area within the South Atlantic Ocean where the Earth’s magnetic field is mysteriously weaker than expected.

By Anita Di Chiara, a research technician at the Lancaster Environment Centre in the UK 

References

Hernandez-Moreno, C., Speranza, F., & Di Chiara, A.: Understanding kinematics of intra-arc transcurrent deformation: Paleomagnetic evidence from the Liquiñe-Ofqui fault zone (Chile, 38-41°S), Tectonics, https://doi.org/10.1002/2014TC003622, 2014.

Hernandez-Moreno, C., Speranza, F., & Di Chiara, A.: Paleomagnetic rotation pattern of the southern Chile fore-arc sliver (38°S-42°S): A new tool to evaluate plate locking along subduction zones. Journal of Geophysical Research: Solid Earth, 121(2), https://doi.org/10.1002/2015JB012382, 2016.

Di Chiara, A., Moncinhatto, T., Hernandez Moreno, C., Pavón-Carrasco, F. J., & Trindade, R. I. F.: Paleomagnetic study of an historical lava flow from the Llaima volcano, Chile. Journal of South American Earth Sciences, 77, https://doi.org/10.1016/j.jsames.2017.04.014, 2017.

 

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submittheir 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/.