The Earth Story

Tectonic Timeline Out of the solid, rocky worlds in our solar system, only 1 has a crust broken into pieces that change position relative to each other: Earth. We call that motion plate tectonics, and over time it has redistributed and reshaped the continents and oceans again and again. But, the Earth formed extremely hot, as a giant, molten ball of rock out in space, and so there must have been a time when plate tectonics didn’t exist and a point where it started. Since no humans were around 4 billion years ago and none of the other worlds in the solar system show anything like plate tectonics on Earth, how and when tectonics began on Earth remains a geological mystery.

Tangled magnetic currents The VIIRS instrument on the NOAA-operated Suomi-NPP satellite is designed to look down at Earth during the night, taking photos in visible and infrared light. It turns out that while doing so, it is actually able to take images of the tangled lines of light seen during the aurora borealis, captured here over alaska (map lines added in later) -JBB Image credit: http://bit.ly/1vxSQzh Read more: http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00221.1

Magnetite and magnet hourglass

This beautiful image shows the northern lights (aurora borealis) as well as a shooting star (meteor) over Hecla/Grindstone Provincial Park in Manitoba, Canada. Auroras occur when solar activity in the form of highly charged electrons are blown towards earth in what is called “solar wind”. The electrons interact with elements in the earth's atmosphere. Solar winds stream away from the sun at speeds of about 1 million miles per hour. When they reach the earth, some 40 hours after leaving the sun, they follow the lines of magnetic force generated by the earth's core and flow through the magnetosphere. As the electrons enter the earth's upper atmosphere, they encounter atoms of oxygen and nitrogen at different altitudes, ranging from 20 to 200 miles above the earth's surface. The colour of the aurora depends on which atom is first struck and at what altitude. For example, the green colour in this image is the result of electrons interacting with oxygen molecules at an altitude up to 241Km (150 miles). The meteor on the other hand is visible as a result of what is called ram pressure; the pressure exerted on a body which is moving through a fluid medium, like air. As the meteorite is soaring through the atmosphere a shock wave is formed as a result of the compression of air. This in turn heats the air and subsequently heats the meteor as it flows around it. The intense heat vaporizes most meteors, creating what we call shooting stars, as in this image. -Jean Picture courtesy of Federico Buchbinder

The Lamont-Doherty Earth Observatory tells the story of the collection of one set of data on a ship in 1960 and how it suddenly provided strong evidence for the theory of plate tectonics - the idea that Earth’s surface is fractured and moves around as rigid bodies, reshaping the arrangement of continents and islands over time.

Candy wrapper core

That's the title of a paper published in Scientific Reports this last week. The lead author, Maurizo Mattesini, a geophysicist from Madrid, proposes a new model for the make-up of Earth's inner core. The paper title is captivating, but its explanation for the core's structure is complicated.

The inner core is known to be formed of crystalline iron, but the exact atomic arrangement of the iron is uncertain, and remains an enigmatic puzzle in the most inaccessible part of our planet.

Data from seismic waves, passing through Earth's inner core, have suggested differences between the eastern and western hemispheres of the inner core, which sits within the molten liquid metal outer core. The inner core first began to crystallise from within the outer core more than a billion years ago, as Earth cooled. Since that time it has grown to its present size, around 1,220 km radius, and it continues to solidify and grow at a rate of about 0.5 mm per year.

While seismic waves tell us about the inner cores density and elasticity, they do not distinguish the precise arrangement of atoms. The conditions of extreme pressure and temperature of the inner core - more than 6000 K and well over three million atmospheres - make it very difficult to replicate in the laboratory, and quantum mechanical calculations of the physical properties of iron under those conditions remain the best approach. But various calculations differ.

Mattesini and colleagues suggest that the differences seen in more than 1000 seismic events with waves passing through different parts of the core are due to differences in the atomic structure of iron in the various parts. It builds a picture of a heterogeneous inner core, with variations in structure and mixing of material on one side, more than the other.

The reasons for the differences in the east and west sides of the core are themselves uncertain, but a prevailing explanation is that following the last major asteroid impact event the inner core suffered an impulse as the Earth rattled, so that still the inner core has an eastward drift with the front side continually melting as the trailing side gradually crystallises, so that dynamically the sphere remains central.

The core certainly appears intriguing, with potential stories yet to tell. The approach adopted in this week's report is promising, because it links mineral physics directly to seismology, and provides a way of reconciling disparate results and building a consistent picture of this, the deepest depths of the Earth.

http://www.bbc.co.uk/news/science-environment-23180271 http://www.nature.com/srep/2013/130628/srep02096/full/srep02096.html

Dysprosium, A Rare Earth Metal

Discovered in 1886, dysprosium is primarily used in control rods in nuclear reactors. The metal is a very good neutron absorber, hence its presence in the cement of nuclear reactor control rods.

Of all the elements on the periodic table, dysprosium is also the most magnetic element. Furthermore, it is even able to stay magnetized under extremely low temperatures.

This lanthanide is very rare. In fact, its name comes from Greek origins that translates to "hard to obtain". It took 80 years after its discovery to create Dysprosium in its pure, isolated form. By weight, dysprosium exists in a quantity of 6ppm in the earth's crust.

The alloyed form of this rare earth metal is used in sonar systems and sensors, primarily existent in ships. In a magnetic field, this alloy can expand or contract, depending on the strength of the field. Dysprosium is also on of the elements used to construct CDs.

--Sam J.

Image Credit:

References: Images of Elements

Time Magazine, "Rare Earth Metals"

http://www.chemicool.com/elements/

The world's oldest chunk of oceanic crust, hiding in plain sight in the Mediterranean?

The Middle Sea is the last remnant of a long lived ocean called Tethys that formed an embayment in the supercontinent Pangea before widening as volcanic rifts began to tear it apart and create the present day oceans surrounding the modern dispersed continents. Sea crust is very different to land crust, much thinner and denser and it forms at spreading centre where rifting is creating space for molten basalt to rise into the gaps and create new crust. At the other end of the cycle the crust moves away, cools and sinks (creating the deep basins) and eventually subducts somewhere back into the mantle. As a result most oceanic crust is young, with the current oldest dating from some 200 million years ago.

As it erupted and froze under the water, crystals of magnetic minerals such as the iron oxide magnetite aligned themselves with the prevailing magnetic field, which has a clear direction. During the history of our planet the direction of the magnetic poles has reversed many times (for largely unknown reasons, but thought to be due to the sloshing of liquid iron in the outer core), recording these reversals as stripes of opposite alignment in the oceanic crust, which essentially acts as a slowly unrolling and vanishing magnetic tape. This tape was the key that brought about the plate tectonic revolution of the 1960's. Lavas on land do this as well, so the record of reversals has been elucidated further into the past than any extant oceanic tapes, with occasional gaps in the record.

The boundaries between continents and ocean are messy, no resemblance to the neat lines in geology textbook diagrams, and the Med is no exception, recording the slow grind westwards of the Anatolian Plate and northwards of the African, with the Alps as living testament to the immense forces involved. A team working with magnetic data and the 'underground strata maps' discernible in seismic reflections to reveal a patch of crust that they think is 340 million years old (the next youngest off Japan weighs in at 190 million, though older chunks have been pushed onto the continents as ophiolites during mountain building events), back in the middle Carboniferous as Pangaea was just finishing its assembly.

Sited between Turkey and Egypt, the stripes showed a previously unknown spreading centre, running from north to south, though it was once part of southern Tethys, where the African plate edge sits under the sea in a complex region known as the Herodotus basin. The feature is buried under 10km of recent sediments, shed off the growing mountains in the lands around the sea as the continents collide. The dating was done by comparing the magnetic signals with the known movements of the plates during the past 400 million years and the record in land lavas, with the closest match being the Carboniferous.

Exploring this piece of crust should reveal some interesting information about the early days of Pangaea, and some even speculate that the chunk of crust could even be a bit older than the supercontinent, giving us a new glimpse into a a part of the geological record in a very different world to today's. It suggests that the supercontinent may have started breaking up earlier than thought, and that this piece of crust may be a record of it, or that it might even predate the final assembly of the supercontinent.

Loz

Image credit: graphic: Roi Granot, Photo superwallpapers http://bit.ly/2bhDrq6 http://bit.ly/2bhCu14 http://bit.ly/2bhDZfP Original paper, paywall access: http://go.nature.com/2bQ6Raq

There’s a hole in the bottom of the sea

Geologists are able to reconstruct the motion of plates in the ocean using what we call “magnetic anomalies”. Oceanic crust forms as molten rock that cools and solidifies when it touches ocean water. As it cools, it forms some minerals that respond to Earth’s magnetic field as natural magnets. These minerals record the magnetic direction at the time they formed – they become small magnets pointing to the current North and South poles.

Every few hundred thousand years, the planet’s North and South Poles switch spots – what we call a magnetic reversal. When one of these happens, rocks that were once pointing towards the North and South poles are suddenly pointing the wrong way, creating an “anomaly”. In the 1950s and 1960s, geoscientists measuring magnetic fields in the ocean discovered that around mid-ocean ridges, there were parallel stripes on each side where the magnetic field alternates – some lines point towards the modern north pole, others point the opposite way. By measuring these lines, geologists can backtrack how ocean crust was generated at a mid-ocean ridge.

A mid-ocean ridge creates plates on two sides. As the plates separate, magma wells up in-between and sticks to both sides, creating the parallel pattern on both. If mid-ocean ridges always worked this way, then every part of every plate should have one of these sets of lines on it, even if the plate on the other side has been completely subducted. This works everywhere on Earth except one spot – the Western Pacific.

In the Western Pacific Ocean, some of the oldest ocean crust on Earth doesn’t have the normal pattern of anomalies. They are a bit chaotic because of volcanoes that erupted later, but there’s a general pattern like the one outlined in the first image – it’s triangular. Just to the east of the Mariana trench, the oldest crust on the Pacific Plate has anomalies that have the wrong shape. No modern-day mid-ocean ridge setup will produce anomalies like the one seen there, so a new paper proposes a different setup actually led to the formation of the Pacific Plate.

Geometrically, it is possible for 3 plates to come together at a single spot, forming a linkage we call a triple-junction. Only certain types of triple junctions are stable – 3 mid-ocean ridges can come together at one spot and sit there constantly, but other types can’t exist permanently. The second image illustrates how one of these unstable assemblages could create the Pacific Plate.

If the setup in the Pacific Ocean started with two transform faults and a subduction zone, the setup is geometrically stable. The subduction zone has the “teeth” symbols on it; the transform faults have arrows showing how they move. In the setup shown in that diagram, the subduction zone is actually shrinking – the part of the plate sinking downwards is getting cut off as the transform faults “zip up”.

At some point, the subduction zone could completely vanish. At that time, 3 transform faults will run into each other. This setup is an unstable triple junction – 3 transform faults cannot hit each other and stay in the same place, something has to get out of the way. If the transform faults are oriented correctly, one of the possible outcomes is that a gap will open in the middle.

A gap opening between 3 transform faults will allow mantle rocks to rise up. Those rocks will start melting and generate magma that fills in the gap. This set of eruptions will create a newly formed plate in the hole and that plate will have a triangular shape.

These images come from a new paper by Utrecht University scientist Lydian Boschman and show their proposed setup for how the Pacific plate was generated. At the same time as Pangaea was breaking up on the opposite side of the world, three transform faults ran into each other on the floor of the Panthallasa Ocean. That temporary setup led to the opening of a gap that filled in with magma, creating the triangular shaped magnetic anomaly found today in the Western Pacific. As time went on, the plates involved in this intersection – the Farallon Plate, the Izanagi Plate, and the Phoenix plate, were mostly subducted away as the Pacific Plate in the middle grew. The Pacific formed classic spreading centers on each side, creating parallel magnetic anomalies that are our only evidence of these earlier plates.

Today, this unusual plate tectonic interaction is preserved in the Western Pacific Ocean. At one point, 190 million years ago, some of the oldest ocean crust on Earth was generated when there was, for a time, a hole in the bottom of the sea. And I think I have hands-down the winning title for any article on this paper.

-JBB

Image credits and original paper: http://advances.sciencemag.org/content/2/7/e1600022.full

Reference with magnetic measurements because I wanted to see them myself: http://bit.ly/2av49cz

Press release version: http://go.nature.com/2axELnC

So much beauty in dirt

This hand is holding a magnet that was dipped into the sand at Piha black sand beach, North Island, New Zealand. Although there are exposed dark, igneous rocks at that beach, much of the sediment at that site apparently comes from farther inland in the Taupo Volcanic Zone. The rocks erupted by the Taupo Caldera contain the mineral titanomagnetite, which is a dark, dense crystal. As the volcanic rocks are eroding, the denser minerals are progressively separating out as the water flows downstream in the Waikato River. By the time it gets to this beach, the sediment is dark and enriched enough in the magnetic grains that a magnet stuck into the beach will come out covered in those grains.

The sand deposits concentrated near these beaches have been developed as ironsand, an iron resource used as feedstock for steel making. The same process that concentrated the iron here also created New Zealand’s richest iron ore resources.

-JBB

Image credit: https://flic.kr/p/8LozJf

References: https://www.volcanodiscovery.com/taupo.html http://www.piha.co.nz/volcanic-past-of-piha/ http://www.piha.co.nz/piha-beach/ http://www.tandfonline.com/doi/pdf/10.1080/00288306.1985.10421203 http://www.nzsteel.co.nz/new-zealand-steel/the-story-of-steel/the-history-of-ironsand/

An ancient subduction zone is buried beneath this spot.

This spot is called the Demon City. It’s a set of currently eroding sedimentary layers on the west side of the Junggar Basin, Xinjiang Province, China. The Junggar is a sedimentary basin that at its lowest sits below sea level, and it is almost completely surrounded by major mountains including the Tian Shan range. The mountains produce a rain shadow desert, leaving this area currently dry and barren. Much of this area, including the Tian Shan itself, are the remnants of collisions between island arcs and small chunks of continents during the assembly of modern-day Asia. A new paper proposes that beneath these sedimentary layers they have discovered a remnant of one of those collisions; an ancient subduction zone, buried beneath the dirt.

The continental chunks that were put together to assemble China assembled hundreds of millions of years ago, so today very little of it is exposed at the surface. This part of the basin does contain a few scattered, highly altered ophiolites, which are assemblages of rocks that could have once been part of the ocean crust, but they’re broken up by modern day faults and hard to interpret. To get around this difficulty, the scientists applied geophysical techniques.

When geologists can’t see the rocks they want to investigate, they often turn to geophysical tools. If a scientist can measure a physical property of the ground, they may be able to interpret boundaries between rocks many kilometers, or even thousands of kilometers below the surface.

For this study, scientists from the China University of Geosciences (Wuhan) applied magnetotellurics. When the sun sends charged particles at the Earth in the solar wind, or lightning strikes the surface, it is a moving electrical current. The material that makes up the ground actually can respond to these currents – imagine a salty, brine fluid in the ground and you can imagine the kind of conductor that will rapidly respond to a bolt of lightning. Rocks also respond to these currents depending on their conductivity, and different rock types have different conductivities. The scientists measured electrical resistance and the local magnetic field across this region and found that there were several, deeply-buried structures beneath these layers.

One spot showed a very sharp offset in electrical resistivity deep beneath a modern day fault zone; they interpreted that spot as the remnant of the ancient subduction zone itself. Moving away from the subduction zone, they found rocks with unusually low resistivity, which is common in areas like volcanic arcs as water coming off of a subducting plate will reduce the resitivity of the rocks it is added to. They could even find that these rocks had been preferentially oriented by flowing upwards, as they would if they were moving up towards the surface above a subduction zone.

A recent study suggested that much of the uplift in these mountains was happening on ancient, preserved features buried in the mantle. This study therefore uses geophysical techniques to characterize one of those features, found at the boundary between the Tian Shan mountains to the west and the Junggar Basin to the east. This discovery illustrates the kind of complexity that exists in the upper mantle and lower crust thoughout Asia as a result of that continent’s assembly during the Phanerozoic.

-JBB

Image credit: https://flic.kr/p/8ErbfN

References/original paper: http://onlinelibrary.wiley.com/doi/10.1002/2015JB012394/full http://bit.ly/28Q8g2F

Babingtonite

While I normally share colourful and transparent minerals, I am personally fond of opaques, or those like this one that are so dark green that they appear black. For those who like colour and gemminess I selected a cluster of dark crystals from China (measuring: 71 mm x 55 mm) sitting on a bed of pale green Prehnite (see http://bit.ly/27ROIRj). Make no mistake about it though it's the dark stumpy prisms that make this specimen rare and of interest to any serious collector, as they are quite rare, especially in large attractive specimens.

It often forms in a process called late stage hydrothermal alteration, in which hot element charged waters cycle through volcanic or metamorphic rocks in convection systems powered by the residual heat of a cooling and just past geological event. Most commonly it is found in basalts in association with zeolite minerals, a group that forms in these events at the edge of low intensity metamorphism. The mineral is weakly magnetic due to a high iron content.

The mineral was first described in 1824 and named after an Irish mineralogist and founder member of the London Geological Society. It is also the official mineral of Massachusetts.

China has produced the best specimens, though the 65 million year old flood basalts of the Deccan Plateau have also produced their share of beauties. Even a few dark crystals of Babbingtonite increases a specimen's value considerably.

Loz

Image credit: Carles Millan

http://www.mindat.org/min-478.html http://www.minerals.net/mineral/babingtonite.aspx http://www.galleries.com/Babingtonite http://bit.ly/1TVmZ87

The search for the unicorn’s other horn ….

In the dim and distant past, the days when I was a young scientist, I had a theoretical physicist friend who was trying to predict the behaviour of magnetic monopoles. He likened his project to the quest for the unicorn’s second horn. No one expected to see it any time soon.

With the passage of time it seems that the quest has moved from theoreticians to experimentalists. In 2013, a group of scientists reported the first results of their search for magnetic monopoles, which they suggest, should be concentrated in rocks derived from Earth’s deep mantle. Needless to say (sorry, my cynical side is coming out here), they have not yet found one. But they still think they might.

So, why the interest in magnetic monopoles? Well, we know that electric dipoles (a positive and negative charge separated from each other) exist – materials rich in these dipoles are used in your camera flash, in passive infrared burglar detectors, and even in the memory card of your Sony PlayStation. Electric monopoles also exist – as isolated positive or negative electric charge (giving rise to static electricity, for example). But their magnetic equivalents seem different. A magnetic north pole seems always to be accompanied by a magnetic south pole, to form a magnetic dipole (like a bar magnet or a compass needle). Can norths or souths exist in isolation, in the same way that positive or negative electric charges do? Well, experience says no.

In a paper entitled “Search for Magnetic Monopoles in Polar Volcanic Rocks” a team assembled from Sweden, Switzerland, Iceland, Denmark, USA and the UK explain that magnetic monopoles formed in the very early Universe, and predicted by grand-unification theories, may persist by attaching themselves to magnetic nuclei. It has been suggested that they would be present in cosmic rays, so material subjected to cosmic ray bombardment – Moon rocks, rocks from Earth’s crust, and meteorite samples, have been the focus of earlier (fruitless) searches. Bendt and co-workers point out that (heavy) monopoles in Earth’s interior would accumulate towards the core, but would end up trapped at the core-mantle boundary where Earth’s geodynamo forces them to the magnetic axis in the polar regions. Mantle convection then brings them back toward the surface, eventually (over a time scale of around half a billion years) appearing in igneous rocks generated from mantle hot spots.

This thesis has been the reasoning behind the group’s quest for magnetic monopoles in igneous rocks from high latitudes. They have, for example, analysed rocks from Antarctic and Arctic flood basalts and intrusions, such as the Skaergaard intrusion shown here. More than 20 kg of rock have yet to yield any sign of a monopole. While this negative result may seem unremarkable to many, it does set limits on the likely mass of the elusive magnetic monopole.

~SATR

Image: Sødalen scientific camp, near the Sgaergaard intrusion, Eastern Greenland (credit: “submanant”, Flickr).

Links: http://arxiv.org/abs/1301.6530

http://physics.aps.org/articles/v6/34

http://www.spacedaily.com/reports/Searching_for_magnetic_monopoles_in_polar_rocks_999.html