Earth’s lost history of planet-altering eruptions revealed

Mar 17, 2017

By Alexandra Witze

Enormous volcanoes vomited lava over the ancient Earth much more often than geologists had suspected. Eruptions as big as the biggest previously known ones happened at least 10 times in the past 3 billion years, an analysis of the geological record shows.

Such eruptions are linked with some of the most profound changes in Earth’s history. These include the biggest mass extinction, which happened 252 million years ago when volcanoes blanketed Siberia with molten rock and poisonous gases.

“As we go back in time, we’re discovering events that are every bit as big,” says Richard Ernst, a geologist at Carleton University in Ottawa, Canada, and Tomsk State University in Russia, who led the work. “These are magnificent huge things.”

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3 comments on “Earth’s lost history of planet-altering eruptions revealed

  • @OP link –

    Surprisingly, the ancient eruptions lurk almost in plain sight.
    The lava they spewed has long since eroded away, but the underlying plumbing that funnelled molten rock from deep in the Earth up through the volcanoes is still there.

    Ernst and his colleagues scoured the globe for traces of this plumbing.
    It usually appears as radial spokes of ancient squirts of lava, fanned out around the throat of a long-gone volcano.
    The geologists mapped these features, known as dyke swarms, and used uranium–lead dating to pinpoint the age of the rock in each dyke.
    By matching the ages of the dykes, the researchers could connect those that came from a single huge eruption.
    During their survey, they found evidence of many of these major volcanic events.

    Each of those newly identified eruptions goes into Ernst’s database.
    “We’ve got about 10 or 15 so far that are probably comparable to the Siberian event,” Ernst says, “that we either didn’t know about or had a little taste, but no idea of their true extent.”

    They include a 1.32-billion-year-old eruption in Australia that connects to one in northern China.
    By linking dyke swarms across continents, scientists can better understand how Earth’s crust has shuffled around over time, says Nasrrddine Youbi, a geologist at Cadi Ayyad University in Marrakesh.

    This sounds like a great geological tool for understanding continental drift and splitting crustal plates over millions or billions of years!

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  • As a means of tracking mid ocean lava flows, the symmetrical stripes showing magnetic polarity in the solid rock of ocean floors, show reversals in the Earth’s magnetic field and displacement of pieces of crust.

    This has traditionally been mapped by ships pulling magnetometers along, but there is now a new space based method using satellites.

    Space view of Earth’s magnetic rocks

    It is the best depiction yet of the magnetism retained in Earth’s rocks, as viewed from space.

    The map was constructed using data from Europe’s current Swarm mission, combined with legacy information from a forerunner satellite called Champ.

    Variations as small as 250km across are detectable.

    Clearly seen are the “stripes” of magnetism moving away from mid-ocean ridges – the places on the planet where new crust is constantly produced.

    This pattern – the consequence of periodic changes in Earth’s polarity being locked into the minerals of cooling volcanic rock – was one of the key pieces of evidence for the theory of plate tectonics.

    On land, the signal tends to reflect the composition and thickness of the different rock layers that make up the continents.

    Generally speaking, younger crust will be thinner and have a low content in magnetic minerals. Whereas, the old cratons, those stable interior sections of continents, will tend to be thicker and have a higher magnetic mineral content.

    Detecting any of this from orbit is a challenge because the signals are dwarfed by that part of the global magnetic field coming from the dynamo – the movement of liquid iron in the Earth’s outer core.

    Stand on the surface of the planet and the intensity of the global field may be between 25,000 and 65,000 nanoTeslas (a fridge magnet is a thousand times stronger).

    “Again it depends where you are but the lithospheric signal is well below 100nT, even 50nT, on average,” explained Nils Olsen from the Technical University of Denmark and one of the scientists who created the new space map.

    “But we have certain regions where it can reach up to 2,000nT, and one of these regions is the Bangui anomaly in western Africa,” he told BBC News.

    This sharp signal in the Central African Republic is the possible impact site of a large iron asteroid more than 500 million years ago.

    Another high intensity region in the crust is the famous Kursk anomaly in central Russia where substantial reserves of iron ore have been mined.

    The German Champ spacecraft measured Earth’s magnetic field from orbit between 2000 and 2010.

    It was succeeded in 2013 by the Swarm trio of satellites operated by the European Space Agency (Esa).

    Champ was lower in the sky than Swarm is currently, and so found it easier to detect the lithospheric magnetism. However, the sophistication of the new mission and the subtle differences the newer satellites can sense in side-by-side observations mean further detail still can be extracted from the data.

    Champ alone was getting a resolution of 300-330km, so the combined model is a big step forward.

    Our very best global view of crustal magnetism is the World Digital Magnetic Anomaly Map (WDMAM), which was put together by scientists over many years, and includes much high-resolution aero- and ship-borne measurements.

    But the WDMAM would be very patchy if it did not also include space data, said Mike Purucker from the US space agency.

    “The World Digital Magnetic Anomaly Map is a 5km grid at 5km altitude around the world. The longwave component is provided by satellite observations and the shorter wavelengths by aero and marine surveys.

    “[The satellite data] defines better the magnetic field under the auroral ovals where it has been very difficult to separate internal from external fields,” he told the BBC.

    Future versions of the WDMAM will now make use of the updated Champ/Swarm view.

    Having maps of crustal magnetism is important for investigating the geological history of Earth and for understanding the distribution of commercially important mineral resources, says Kathy Whaler from Edinburgh University, UK.

    “[The new Swarm/Champ] model should mean estimates to the depth at which magnetisation is lost are better.

    “Our assumption is this is where the temperature reaches the Curie point. There has previously been a suggestion that this is deeper in some subduction regions, as well as possibly over old, thick, stable cratons, for example.

    “We should be able to estimate these depths as a function of position on the surface more accurately, and thereby understand some of the large-scale tectonics better.”

    Swarm itself is trying to provide greater insights on all of the different contributors to the global magnetic field.

    As well as the rocks and that dominant signal coming from the swirling convection of molten iron in the core, there are other inputs pulling on the needle of every compass.

    These include the magnetism generated by electric fields high above the Earth, and even a very subtle effect derived from the movement of salt water ocean currents.

    Long-term observations will tease apart the size of each contribution and how it varies through time.

    “The Bravo satellite is still at slightly above 500km and the Alpha and Charlie satellites are at 443km, roughly. So we are still at a good altitude,” said Esa’s Swarm mission manager, Rune Floberghagen.

    “Considering the fuel situation, we are set for a very, very long mission – far beyond the upcoming solar minimum and following solar maximum, and perhaps even up to the solar minimum after that.”

    This would mean at least one Swarm satellite still working in 2031.

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  • It seems that Earth is not the only planet where the geology is revealing information about major past events!

    Scientists have located an impact crater linked to powerful tsunamis that swept across part of ancient Mars.

    The team believe an asteroid triggered 150m-high waves when it plunged into an ocean thought to have existed on northern Mars three billion years ago.

    Lomonosov crater in the planet’s northern plains fits the bill as the source of tsunami deposits identified on the surface.

    Details were outlined at the 48th Lunar and Planetary Science Conference.

    Although the idea has lost some of its currency in recent years, some scientists think an ocean might once have filled the vast lowland region that occupies the Red Planet’s northerly latitudes.

    Growing evidence that tsunami waves washed over the boundary between the southern highlands and northern lowlands help strengthen the hypothesis.

    François Costard, Steve Clifford and colleagues identified and mapped the distribution of sediment that apparently originated in the northern plains and flowed onto a possible ancient shoreline to the south.

    “We found typical tsunami deposits along the dichotomy between the northern hemisphere and southern hemisphere of Mars,” Dr Costard, from Université Paris-Sud and CNRS, told BBC News.

    “It supports that there was, at that time, a northern ocean.”
    Climbing high

    One type of feature seen on the dichotomy boundary is a lobate flow deposit. Dr Clifford, from the Lunar and Planetary Institute in Houston, explained the evidence.

    “These lobate deposits propagate uphill from the northern plains and do so in close association with a potential palaeo-shoreline. The predictions of the numerical modelling that François and his colleagues have done provide a very persuasive case for an ocean at this time,” he told BBC News.

    “There’s also a second set of landforms that we see along the coastline called thumbprint terrain…. the reflection of the tsunami waves from the coast and their interaction with a second set of tsunami waves, predicted by the numerical modelling, would have resulted in sediment deposition that’s very similar to what we actually observe on Mars.”

    This terrain has previously been interpreted as having been caused by mud flows, mud volcanoes, or glaciers.

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