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Throughout Earth’s long geologic history, the magnetic pole has not remained stable. For reasons that are still little understood, the Earth’s magnetic field can suddenly – and without warning – weaken, start to shift around, and even completely reverse direction.

Credit: Shutterstock

Records indicate that over the last 160 million years, the magnetic pole has reversed its polarity at least several hundred times. Called a “geomagnetic field reversal,” this has resulted in the poles swapping positions, with magnetic north becoming magnetic south, and vice versa. The magnetic pole has also undergone what are called “excursions.” During an excursion event, the earth’s magnetic field weakens and begins to drift but does not reverse itself. The field re-strengthens and the poles finally return to their initial position.

UC Santa Cruz geology professor Robert Coe will be presenting his paper, “What We Know and Don’t Know about Reversals” during the upcoming American Geophysical Union (AGU) meeting in Washington, D.C. this December.

Coe is an emeritus professor of geophysics, and has had a long and distinguished career. He has received numerous awards and recognition for his many research accomplishments. He received his Ph.D. at the University of California, Berkeley, and did postgraduate work in Australia before returning to the States where he joined the UC Santa Cruz faculty in 1968. He has made significant contributions in a number of areas, including volcanology, geochemistry, and tectonics. In the 1970s, he developed a method of more accurately measuring the intensity of the magnetic field in rocks – a method that bears his name. Perhaps his most significant contribution, however, has been in paleomagnetism, where he has been a pioneer in the study of magnetic field reversals.

“The Earth’s magnetic field is restless,” said Coe in a recent interview.

Evidence for this restlessness first came to light in the early twentieth century when geologists recognized that certain rocks exhibited magnetism that was different in orientation from the Earth’s then current magnetic field. While given little consideration at the time, geologists eventually acknowledged the significance of this observation and became interested in investigating the phenomenon. A number of influential papers published in the 1960s – including a number of papers authored by Coe – shed considerable light on the process by identifying polarity transitions in both lava flows and sediments.

The process by which rocks get magnetized occurs when they are formed, Coe explained. Scientists know much more about how volcanic rocks become magnetized than they do about sedimentary rocks. As igneous rocks cool, for example, they become magnetized in the direction of the field prevailing at the moment. This process may take a few days or a few years and provides a “snapshot” of the Earth’s magnetic field, he added. Consequently, by studying many different rocks formed during different geologic periods, researchers can create a record of the Earth’s history of magnetic wanderings.

One of the best records of the earth’s magnetic reversals comes from Steens Mountain in southeastern Oregon. Here, a series of overlapping Miocene-age basaltic lava flows record a complex history of several thousand years of Earth’s geomagnetic history. Significantly, the Steens Mountain record bears evidence of a complete magnetic reversal that occurred at an extraordinarily rapid pace (between 3 and 8 degrees per day) some15.5 million years ago.

Unfortunately, magnetic reversals can be far more complex than even the best, most detailed paleomagnetic record on hand. Volcanic records are limited by the “discontinuous and episodic nature of volcanic eruptions,” Coe said. To better understand Earth’s geomagnetic history, Coe emphasized, we need to have excellent records not only from volcanic contexts but also from sedimentary data.

Highly detailed deep-sea cores recently obtained during drilling operations in the North Atlantic might provide the lynchpin. These were obtained from a vertical section of the ocean floor and provide a continuous sequence of magnetized strata.

“The North Atlantic records give one hope,” Coe said.

Today, much has been learned about reversals in the Earth’s magnetic field.

It is now known, for example, that magnetic reversals happen much more frequently than previously surmised, and that they can often occur at incredibly rapid clips. It is also known that the last complete reversal, which occurred 770,000 years ago, occurred over a span of less than 100 years.

Much more, however, is still to be learned.

Most importantly, geologists continue to debate the cause of these reversals.

“A magnetic reversal certainly has a large random or chaotic aspect,” Coe said. “And it’s active over many time scales.”

Although a number of theories have been posited, the prevailing explanation suggests that reversals are ultimately tied to the Earth’s convective motion.

“The Earth’s magnetic field comes from a dynamo-like action arising from the motion of the metallic fluids in the outer core,” he said.

Scientists have also debated whether a reversal can cause major hazards, especially to technology. Some have argued that a reversal would cause the failure of the worldwide electronic and communication systems. The question, however, is controversial and remains unanswered.

Author: Tom Garlinghouse | Source: University of California - Santa Cruz [December 11, 2018]

Scientists long believed that the lower mantle was composed of Bridgmanite (Mg,Fe)SiO₃ and magnesiowüstite (Mg,Fe)O, in which Fe2+ dwells. This view changed when experiments showed that Fe²⁺ simply can't exist at the pressure and temperature of the lower mantle. What is present is Fe³⁺. The two phases (Mg,Fe)SiO₃ and (Mg,Fe)O both shed Fe²⁺ and, in turn, MgSiO₃ and MgO remain. However, what mineral hosts Fe³⁺ had remained a secret.

Maohokite [Credit: CHEN Ming]

Now scientists have a possible answer: Maohokite, a newly discovered high-pressure mineral. It may be what composes the Earth's lower mantle along with Bridgmanite MgSiO₃ and magnesiowüstite MgO. The study reporting this new mineral was published in Meteoritics & Planetary Science.

Maohokite was discovered by CHEN Ming's team from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and SHU Jinfu from the Center for High Pressure Science and Technology Advanced Research. The mineral was named after Hokwang Mao, in honor of his great contribution to high-pressure research.

The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association under the designator IMA 2017-047.

Natural minerals can be divided into two types: low-pressure minerals and high-pressure minerals, depending on their formation pressures. The pressure and temperature required for the formation of high-pressure minerals can only be provided by the environment of the mantle or the hypervelocity collision between celestial bodies.

Maohokite is the second case. It was found in shock-metamorphosed rocks from the Xiuyan impact crater in China.

This high-pressure mineral was formed from the decomposition of ferromagnesian carbonate via a self-oxidation-reduction reaction at a temperature >900 °C and impact pressure >25 GPa (a pressure range found at depths more than 670km below Earth's surface). In this reaction,  Fe²⁺ oxidizes into Fe³⁺ and then later combines with  Mg²⁺ to form maohokite, thus making it a possible important constituent of the lower mantle.

Maohokite, with a composition of MgFe₂O₄, has an orthorhombic CaFe₂O₄-type structure. The existing mineralogical model of the Earth's mantle shows that the ferromagnesian lower mantle is mainly composed of Bridgmanite (Mg,Fe)SiO₃ and magnesiowüstite (Mg,Fe)O. Therefore, the fact that Maohokite contains Mg and Fe, two major components of the lower mantle, only makes the case stronger that Maokohite is a key mineral in the lower mantle.

Source: Chinese Academy of Sciences [December 05, 2018]

Volcanoes are not fed by molten magma formed in large chambers finds a new study, overturning classic ideas about volcanic eruptions.

Volcano Fuego in Antigua, Guatemala [Credit: fboudrias/Shutterstock]

Instead, the study suggests that volcanoes are fed by so-called 'mush reservoirs' - areas of mostly solid crystals with magma in the small spaces between the crystals.

Our understanding of volcanic processes, including those leading to the largest eruptions, has been based on magma being stored in liquid-filled 'magma' chambers - large, underground caves full of liquid magma. However, these have never been observed.

The new study, by researchers at Imperial College London and the University of Bristol and published in Nature, suggests the fundamental assumption of a magma chamber needs a re-think.

Lead author Professor Matthew Jackson, from the Department of Earth Sciences and Engineering at Imperial, said: "We now need to look again at how and why eruptions occur from mush reservoirs. We can apply our findings to understanding volcanic eruptions with implications for public safety and also to understand the formation of metal ore deposits associated with volcanic systems."

In order to erupt, volcanoes need a source of magma - melted, liquid rock - containing relatively few solid crystals. Traditionally, this magma was thought to be formed and stored in a large underground cave, called a magma chamber.

The traditional picture of a magma chamber needs a re-think [Credit: Artur Balytskyi/Shutterstock]

Recent studies of magma chemistry have challenged this view, leading to the suggestion of the mush reservoir model, where smaller pools of magma sit in the small gaps between solid crystals. However, the mush reservoir model could not explain how magmas containing relatively few crystals arise and are delivered to volcanoes in order for them to erupt at the surface.

Now, with sophisticated modelling of mush reservoirs, the research team has come up with a solution. Within the mush reservoir scenario, the magma is less dense than the crystals, causing it to rise up through the spaces between them.

As it rises, the magma reacts with the crystals, melting them and leading to local areas containing magma with relatively few crystals. It is these short-lived areas of increased magma that can lead to eruptions.

Co-author Professor Stephen Sparks, from the University of Bristol's School of Earth Sciences, said: "A major mystery about volcanoes is that they were thought to be underlain by large chambers of molten rock. Such magma chambers, however, were very difficult to find.

"The new idea developed by geologists at Imperial and Bristol is that molten rock forms within largely crystalline hot rocks, spending most of its time in little pores within the rock rather than in large magma chambers. However, the rock melt is slowly squeezed out to form pools of melt, which can then erupt or form ephemeral magma chambers."

As well as the initiation of eruptions, the new mush reservoir model can help explain other phenomena in volcanic systems, such as how the magma chemical composition evolves and how much older crystals can be erupted within younger magmas.

Author: Hayley Dunning | Source: Imperial College London [December 04, 2018]

Researchers from Wits and colleagues from the University of Antananarivo in Madagascar are on a "detective mission" to describe, classify and trace the history of a meteorite that landed in and around the small town of Benenitra in southwestern Madagascar shortly before the lunar eclipse on 27 July 2018.

A fragment of the meteorite showing the black fusion crust and thumbprint-like depressions (called regmaglypts) formed by melting during its entry into the atmosphere. The small bumps on the surface are grains of nickel-iron alloy [Credit: Wits University]

News of the event in this remote area was brought to the attention of a Wits Geosciences graduate, Tim Marais, who was travelling in the area a few days after the meteorite fall. He collected some preliminary eyewitness accounts that reported a bright meteor fireball, a loud explosion and a rain of rock fragments that fell in and around Benenitra that, fortuitously, appear to have missed all people and buildings, and he was able to acquire several small fragments that residents had managed to locate.

He delivered these to Professors Roger Gibson and Lewis Ashwal in the School of Geosciences at Wits and asked them to verify their extra-terrestrial origin. The signs of a dark fusion crust and small spheres in the rock matrix that were visible on broken surfaces appeared promising and the School's Senior Technician, Caiphas Majola, was immediately commissioned to prepare a thin section of one of the fragments for microscopic analysis.

Tracing the history

Assessment of the thin section confirmed that it was, indeed, a meteorite and, more specifically, a relatively common type called a chondrite, referring to the small spherical chondrules that it contains. This established that the meteorite dates from the formation of our Solar System about 4.56 billion years ago.

At the same time, a news report appeared in the local Triatra Gazette newspaper on 4 August regarding the eyewitness reports and showing a large specimen with a similar black fusion crust. To corroborate the event scientifically, the team approached Dr Andry Ramanantsoa of the Laboratory of Seismology and Infrasound at the Institute and Observatory of Geophysics at the University of Antananarivo to investigate if there was any evidence that a significant explosion occurred in the atmosphere above Madagascar sometime in the evening of 27 July.

Ramanantsoa was able to confirm, using infrasound data from the international Comprehensive Nuclear Test Ban Treaty Infrasound Station IS33 outside Antananarivo, that there had, indeed, been an "upper atmosphere energy release event" at 5.16 p.m. GMT (7.16 p.m. local time). Furthermore, he was able to identify that it occurred in a direction south-southwest of Antananarivo - the exact bearing of Benenitra.

This is a close-up of the meteorite fragment showing the fusion crust [Credit: Wits University]

The next step was to see if the blast wave from the atmospheric detonation was sufficiently large to have caused a ground vibration that could be detected by geophysical seismometers. For this the team turned to Dr Andriamiranto (Ranto) Raveloson, a Postdoctoral Fellow and Technical Manager of the Africa Array Seismic Network that is co-ordinated from Wits. He was able to confirm a very faint seismic tremor at 5.17 p.m. GMT on the same night.

The final confirmation that the fragment was related to a fall on 27 July was obtained from Dr Matthias Laubenstein from the Laboratori Nazionali del Gran Sasso at the Istituto Nazionale di Fisica Nucleare in Italy, who measured the meteorite for rare cosmogenic nuclides that are created when an asteroid in Space is bombarded by high-energy cosmic rays. His measurements showed high levels of cosmogenic nuclides, consistent with the meteorite having entered Earth's protective atmosphere only within the past few months.

Based on these findings, the team has submitted a request to the international Meteoritical Society to officially name and register Africa's newest meteorite - Benenitra - on its database.

Classifying Africa's newest meteorite

Ashwal and Gibson have refined the meteorite classification as an L6 chondrite. The name refers to the fact that it contains a low amount of iron (the "L"), and that it contains recognizable chondrules. Chondrules are the original building blocks of rocky bodies - such as asteroids and the Inner Planets - in Space. As these bodies grew larger, heat built up inside them - partly through gravitational collapse and partly because of radioactive elements - which caused the metals and chondrules to recrystallise and maybe even melt.

Where temperatures were sufficiently high to melt the body, the denser metals were then able to settle towards the core of the body, with the less dense silicate melts rising towards the surface. Eventually the body would have cooled down sufficiently to solidify. The Benenitra meteorite appears to have got quite hot (the number "6" refers to the high amount of recrystallisation that occurred within the chondrules owing to this heating process) but only melted partially, allowing some of the chondrules to survive.

Poor Man's Space Probe

The meteorite also has a thin shock-melt vein that is most likely related to a collision with another asteroid that shattered the original body and sent the fragment spinning off on its eventual collision course with Earth.

"Meteorites are commonly called The Poor Man's Space Probe, because they deliver rocks from Outer Space to our door for free, where we can study the birth and history of other parts of our Solar System" says Gibson. The Benenitra meteorite fragment is being subjected to a range of other tests to establish properties such as its density and magnetism, and the team plans to write the results up soon in a scientific paper.

"The Benenitra meteorite is a new Space rock, but it is also a witnessed fall, which makes it part of special group of meteorites. It is part of our collective heritage as a species, planet and Solar System. It fell in a remote area that will henceforth be recognised internationally for the event on the evening of 27 July. Our project provides an opportunity to further strengthen scientific collaboration in the SADC region. Ultimately, one of our goals is to inform the people of Benenitra about the significance of what they witnessed and thus build greater awareness of science," says Gibson.

Source: University of the Witwatersrand [November 27, 2018]