The Earth Story

Apparent Polar Wander

This image shows one of the fundamental techniques geologists use to reconstruct the motion of plates on Earth. These plots are called “apparent polar wander” plots.

Certain rocks contain magnetic minerals, like the mineral magnetite. When these minerals either grow or move around freely (like in flowing water) they align themselves relative to the Earth’s magnetic field. A rock formed in the presence of a magnetic field will wind up with its own magnetic field that points towards the north/south pole at the time it forms.

If 2 continents are joined together, 2 rocks formed on those continents at the same time will point to the same North Pole. But the trick for geologists is…what happens if the continents move after the rocks form?

Once a rock is formed, the magnetic field is (mostly) locked in. If a scientist measures the magnetic field in rocks across time, that paleomagneticist (also sometimes nicknamed paleomagicians) will be able to see how the magnetic pole moves relative to the continent. Of course, the North Pole isn’t really what is moving, the continent itself is moving – hence the name “apparent polar wander”. The North Pole recorded by the rocks seems to change because the rocks themselves move relative to the North Pole.

Finally, go back to our case of 2 rocks forming on 2 continents that are together at one time and rift apart later. Once the continents separate, their rocks will record different magnetic poles as a result of the continental motion, but the tracks of the rocks can be treated like a jigsaw puzzle. In the 2nd image here the apparent polar wander paths from North America and Europe have been altered by assuming that the continents used to be joined together; if you make the assumption that the Atlantic Ocean once closed, suddenly the two curves plot on top of each other for the time when the continents were joined!

These kinds of paths are one of the key ways plate motions over time have been reconstructed.

-JBB

Image credit: Marshak Essentials of Geology textbook, licensed to me for teaching purposes

Flip Flops during the Jurassic

We are familiar with the Earth’s geomagnetic field and how it acts as a shield, protecting us from magnetic storms from the sun. Evidence of such storms can be witnessed during spectacular shows in the night sky in the northern and southern latitudes producing the Aurora borealis and the Aurora Australis, respectively (http://on.fb.me/1MfWBmb).

This geomagnetic field is the physical property of the Earth that enables magnetic compasses to point towards the north-pole. However, this has not been the case throughout our planet’s history. The polarity of the Earth’s geomagnetic field has undergone reversals – yes, our hiking compasses would point towards the south-pole during such periods. The geomagnetic field is a dynamic property that has the potential to change from milliseconds to millions of years. In 1906, geophysicists Bernard Brunhes and Motonori Matuyama identified the last time a geomagnetic reversal occurred – 780,000 years ago and named it the Brunhes-Matuyama reversal. Today, scientists are curious to understand reversals during the Jurassic period (145 – 200 million years ago), when a peculiar anomaly occurred. Scientists originally thought that during this period, no geomagnetic reversals happened. However, recent data from the volcanic sea floors of the Pacific Ocean reveals a different story.

Previously, measurements of the Earth’s magnetic field were carried out by towing magnetometers on a research vessel along the volcanic sea floors. This did not reveal sufficient information from deeper and harder to reach ocean areas. However, these days with a giant leap in underwater technology available for scientific study, scientists use autonomous underwater vehicles (AUV) to reach parts of the volcanic sea floors that have been out of reach previously.

The data using new technology reveals that geomagnetic reversals were ‘flipping and flopping’ so fast that it did not regain full strength as of today’s geomagnetic strength. This detail went unnoticed previously. The low strength of the geomagnetic field is distinctive to the Jurassic period and unique to the history of the Earth’s geomagnetic reversal. Understanding this period of the geomagnetic field’s behavior provides important data and clues for computer simulations (photo) to predict future reversals. This is a story how advances in technology aid in deeper understanding of our dynamic planet.

Image Credit: Gary A Glatzmeier http://bit.ly/1MMzkht

Source: http://1.usa.gov/1NTpdEv

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

Decoding Geomagnetic Reversals

The natural magnetic field of our planet is generated by complex motions of molten iron alloys - 2,900 kms deep in the outer core. The most fascinating feature of the Earth’s magnetic field is its ‘reversals’ that occur on irregular timescales ranging from days to millions of years. During a magnetic reversal – the north and south magnetic poles switch places. Reversals are rare events when we compare their duration (~1000 years) to the length of the polarity intervals (upto millions of years). Records of these reversals can be decoded from magnetic-grains in volcanic lavas and sediments as they lock-in the Earth’s magnetic field the time they were cooled on the surface of the Earth or were deposited at the bottom of the ocean. Paleomagnetists have spent the last 50 years going through these geological records and preparing a timescale of the magnetic reversals.

However, the paleomagnetic record lacks detail especially during the reversal event. These events are geologically ‘rapid’ that it lacks sufficient temporal resolution in the geological record – sediments. Furthermore, during such reversal events the Earth experiences a weak magnetic field intensity, and this affects the precision of the magnetic recording in the sedimentary records. On the other hand, volcanism is random in nature with lava flows unevenly distributed in time. Moreover dating lava flows comes with an error bar that exceeds the duration of the magnetic reversal and so cannot be accurately pinned down. Hence, the results from the geological record can be biased by artifacts and so such observations have generated controversy over the years.

Advances in computational mathematics and numerical simulations have enabled us to gain new insights into the mechanism of the geodynamo and its reversals. Numerical simulations have generated hundreds of reversals too and some of them are similar to the geological record. During such cases we can compare and analyze the data and simulations together.

Although magnetic reversals are poorly understood at the moment, most geophysicts would agree that the geomagnetic field is maintained by convection within the core which results in rapid motions of the iron-rich fluid in the outer core. We have also come to the agreement that no external forcing is required for a reversal and so a reversal is considered a fundamental property of the Earth’s dynamo.

Image: http://bit.ly/2cf2NAR

Source: http://bit.ly/2ckS2jK

Original Paper: http://bit.ly/2bJSbvp

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

EVIDENCE FOR A RAPID REVERSAL OF THE GEOMAGNETIC FIELD 41,000 YEARS AGO

Magnetic studies performed on sediment cores from the Black Sea by the GFZ German Research Centre for Geosciences show that Earth experienced a rapid (over 440 year period) and complete reversal of the geomagnetic field 41,000 years ago, during the last ice age. Geomagnetic reversals result in the interchanging of the positions of magnetic north and magnetic south. As this was a brief and unsustained pole reversal, it is known as a geomagnetic excursion and not a full reversal. The evidence for this brief pole reversal is further bolstered by data obtained from additional studies performed in the North Atlantic, the South Pacific and Hawaii; together this shows the polarity reversal was a global event. The sediment cores also provide further evidence for the last ice age and a supervolcano eruption.

Earth has had several pole reversals in the last 20 million years, with a complete pole reversal occurring about every 200,000 to 300,000 years. The last major pole reversal occurred approximately 780,000 years ago and is known as the Brunhes–Matuyama reversal. A reversal does not happen instantly, and typically takes between 1,000 and 10,000 years. The Brunhes–Matuyama reversal caused no drastic changes in plant or animal life as provided by evidence from the fossil record. By looking at oxygen isotope ratios in deep ocean sediment cores, no changes in glacial activity were found for this period either.

The field geometry of reversed polarity for the geomagnetic excursion of 41,000 years ago lasted about 440 years and the field strength was only about 25% of today’s field. The actual polarity lasted only 250 years, which is remarkably short. During this 250-year period, the magnetic field was only at 5% of today’s field strength. This significantly lowered Earth’s protection against hard cosmic rays, which in turn led to an increased exposure to radiation. Evidence for this is shown by peaks of radioactive beryllium (10Be) in ice cores recovered from the Greenland ice sheet.

10Be is formed in Earth’s atmosphere through the collision of cosmic rays with atoms. 10Be has a half-life of 1.36 million years before decaying to 10Boron. Periods of high solar activity decrease the flux of cosmic rays that hit Earth, so the production of 10Be is inversely proportional to solar activity and increased solar wind.

This geomagnetic excursion has been known about for 45 years, after the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central. The magnetisation of these lava flows differed significantly from the direction of the geomagnetic field today. This feature has since been known as the 'Laschamp event'. Before the latest work by the GFZ, the Laschamp event was shown only by point readings of the geomagnetic field during the last ice age. This new research gives a more complete view.

The sediment cores from the Black Sea not only show the geomagnetic excursion of 41,000 years ago, they also indicate several abrupt climate changes during the last ice age. These climate changes were already known from the Greenland ice cores, but now there is a high synchronisation between the data records from the Black Sea and Greenland. The sediment cores also document the largest volcanic eruption in the Northern hemisphere of the last 100,000 years: the eruption 39,400 years ago of a super volcano near Naples, Italy, known as the Campanian Ignimbrite super-eruption. About 350 cubic kilometres of rock and lava were expelled from the volcano and spread over the eastern Mediterranean and as far as central Russia.

Interestingly, one of the Neanderthal extinction hypotheses (Neanderthals became extinct around 30,000 years ago) involves climate change. During the last ice age, Europe changed into a semi-arid desert and it is speculated that Neanderthals did not adapt their hunting techniques to this new environment. Volcanic eruptions, such as the super eruption near Naples 39,400 years ago, also may have contributed to a reduction in food supply.

Excavations at Mezmaiskaya Cave in the Caucasus Mountains of southern Russia showed there was also a reduction in plant pollen at the time. Two distinct layers of volcanic ash were observed in the cave, which coincided with large-scale volcanic events that occurred around 40,000 years ago. The first volcanic event is the Campanian Ignimbrite super-eruption. The second volcanic layer coincides with the end of Neanderthal presence at Mezmaiskaya and coincides with a smaller eruption thought to have occurred around the same time in the Caucasus Mountains.

-TEL

More on pole reversals: http://www.nasa.gov/topics/earth/features/2012-poleReversal.html

Sources: http://www.alphagalileo.org/ViewItem.aspx?ItemId=124996&CultureCode=en http://www.sciencedaily.com/releases/2010/10/101006094057.htm

Nowaczyk, N. R.; Arz, H. W.; Frank, U.; Kind, J.; Plessen, B. (2012): “Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments” Earth and Planetary Science Letters, 351-352, 54-69. doi:10.1016/j.epsl.2012.06.050.

Liubov Vitaliena Golovanova, Vladimir Borisovich Doronichev, Naomi Elancia Cleghorn, Marianna Alekseevna Koulkova, Tatiana Valentinovna Sapelko, and M. Steven Shackley. Significance of Ecological Factors in the Middle to Upper Paleolithic Transition. Current Anthropology, 2010; 51 (5): 655 DOI: 10.1086/656185

Image: © Dr. habil. Norbert R. Nowaczyk / GFZ

Oriented Paleomag

When rocks form in the presence of Earth’s magnetic field, minerals within them (particularly magnetite) will store a record of the magnetic field they formed in. If geologists then measure the direction of the magnetic field preserved in the rocks, they can use the rock as an archive of how the rock moved through space. Paleomagnetic measurements therefore can be used as an archive of the motion of the continents around the surface of the planet. But how exactly is that done? We received this question to our blog at http://the-earth-story.com/

“Geologists use geomagnetism all the time to figure out where a rock's position was when it solidified. But how do they keep the sample pointing in exactly the same direction until they can get it back to their instruments to be analyzed? Or do they have to do it in situ in the field?”

Commonly, a single rock has been exposed to multiple magnetic fields: for example, one direction when it formed and a different direction later when it was altered. The best way to unravel this full history is through high precision paleomagnetic measurements in a laboratory setting. A small core of a sample is taken into the lab and under controlled conditions either heated or exposed to various magnetic fields to determine all of the components of the magnetic field, including the field direction when the rock formed.

To do that type of measurement, a rock has to be sampled in the field and then carried into the lab. Several companies produce drills that can extract cores from rocks, leaving holes like the ones seen in the first photo.

It’s literally impossible to keep a core of rock perfectly aligned on its trip from the field to the lab. Instead of trying to keep a rock core from shaking, the preferred technique is to measure the way the core was oriented out in the field.

In a 3-dimensional space, two directions/angles measured relative to a fixed coordinate system are enough to uniquely determine an orientation. Therefore, the best way to know the orientation of a rock core is to measure the inclination (angle with the horizontal) and declination (angle to true north).

The instrument pictured in these last two photos shared by the USGS is called an orienter. Basically, it can be placed in the hole after the core is drilled and allows both of those angles to be recorded after the drilling is complete. There are still complications, including damage or twisting of the core during drilling, which need to be accounted for, but this type of instrument is how that measurement is done.

Image credits: Richard Webb (CC licensed) http://www.geograph.org.uk/photo/1688057 USGS share: http://on.doi.gov/1QMtrS9

Read more: http://rogermarjoribanks.com/core-orientation-tools-best/

Origins of the World’s Largest Single Volcano

Hawaii’s Mauna Loa was regarded as the largest volcano in the Earth until 2013, when William Sager who studied the Tamu Massif for 20 years was able to prove that it was one single volcano. About 1600kms east of Japan in the placid waters of the North-Pacific Ocean is a bump in the ocean floor that is the size of the state of New Mexico/British Isles. This bump is the world’s largest single volcano, the second largest in the solar system (behind Olympus Mons on Mars), and is named Tamu Massif (http://on.fb.me/1R2dma4). This is another significant discovery from surveying our uncharted oceans. So what made this volcano so big? Recently, Sager and his team revisited the Tamu Massif to try and understand its origins, and came up with two hypothesis – the plume head hypothesis and the fertile mantle hypothesis. Firstly, the plume head hypothesis suggests that the Tamu Massif erupted because of a huge plume (giant blob of hot magma) slowly rose from the boundary between the Earth’s core and mantle, erupting at the surface. This theory states that the eruptions were short and fast with the magma spreading out in all directions. On the other hand, the fertile mantle hypothesis suggests that lava was oozing out of cracks and fissures resulting in gradual eruptions with lava that do not go far from the ridges.

To test out these theories, Sager and his team set out to map the Massif in more detail to figure out how it got so big. The Tamu Massif was formed 145 million years ago, and it was active for only a few million years. Tamu Massif was formed in the thin parts of the oceanic crust where three long mid-ocean ridges came together into a triple junction. Magnetic analysis of the Massif suggests that part of it formed through steady release of lava along the triple junction, while part of it (central peak) is harder to describe with the data available at the moment. However, a working theory suggests that a large plume of hot mantle rock may have contributed additional heat and material.

The recent magnetic field research shows part of the volcano with magnetic stripes with different magnetic properties. This suggests that lava flowed out evenly from the mid-ocean ridges over time and changed in polarity each time the Earth’s magnetic field reversed its direction. However, the central part of the peak is more complicated and its formation is not well understood yet.

Image Source: John Greene/Schmidt Ocean Institute, http://bit.ly/1I7AIZR

Source: http://bit.ly/1YqL0IM http://bit.ly/1QEY7Vi http://bit.ly/1Op070K

Videos: http://bit.ly/1YqE26s http://bit.ly/1S6CZVz

Rodinia Reconstructed

We received this interesting question submitted through our blog at http://the-earth-story.com/ (Seriously, visit & follow us, I just paid the fee to reregister the domain name).

“I have a question I was hoping you guys might be able to explain, please. I'm fascinated by plate tectonics, and reconstructing ancient continents, but how do they do it? I get the principle of 'rewinding' current continental movement to get to Pangaea, but how can they produce a map of, say, the Ordovician or the Precambrian, and say with any certainty that's how the continents were arranged so far back in time?”

To use that term, “rewinding” the Atlantic Ocean is pretty easy, you just have to move the continents back together like puzzle pieces. However, the Pacific and Indian Oceans are much tougher; you can’t easily tell exactly how places like Antarctica and Australia fit together, and parts of Asia were only assembled out of volcanic arcs over the last 250 million years.

To fit these plates back together, we use magnetic anomalies on the seafloor. Every few hundred thousand years on average, the Earth’s magnetic field flips, switching the North and South Poles. If we tow an instrument called a magnetometer behind a boat and measure areas where the magnetic field in the ocean is strong and weak, we get a measurement that tells us the orientation of mid-ocean ridges and how they were moving.

Igneous rocks contain minerals like magnetite that record the direction of the magnetic field when they formed. When a magnetic mineral cools through its Curie Temperature, it picks up whatever magnetic field is present and locks that magnetic field in. The rocks of the ocean floor therefore record the flips between north and south; if they are measured across the ocean floor they provide a record of the motion of any oceanic plate.

The oldest oceanic crust on Earth is Jurassic in age, about 200 million years old. By measuring oceanic magnetic anomalies we can therefore project the motion of the continents going as far back as 200 million years, about the time Pangaea began breaking apart.

This image, however, is a reconstruction of Rodinia, a proposed supercontinent 700 million years ago. How on Earth do we get information going back that far?

Part of the answer is continents. Igneous rocks that form on continents also record the ambient magnetic field and point to the North Pole as well. Sedimentary rocks on a continent also record the magnetic field since magnetic minerals in sediments adjust to the magnetic field while they flow in water. But, not only will those rocks tell you about magnetic flips, other properties of magnetism can be measured as well.

The magnetic field in a rock points towards the pole whenever it forms. If you measure the preserved magnetic field in rocks of different ages on a single continent, all of them point to the same North Pole. If the observed North Pole changes over time and a continent is actually solid, then any change in the North Pole direction is actually reflecting movement or rotation of the continent. Measuring the changes in North Pole direction within a continent produces a curve called an “apparent polar wander” curve – a measurement of how the continent has moved assuming the pole is actually fixed.

This gives us several new tools. For example, if 2 continents show a similar apparent polar wander curve over several hundred million years, you can project that they were likely joined together at that time (see more here: http://tmblr.co/Zyv2Js1LJUEUa).

Magnetics also gives another piece of information: inclination. We’re used to magnetic declination; that’s what a standard handheld compass measures, the angle to the North Pole. Although a normal compass can’t measure it, the Earth’s magnetic field also dips up and down. The dip of the magnetic field can tell the latitude of a continent. Steep magnetic inclinations means a rock formed at high latitudes near the pole, and shallow magnetic inclinations means a rock formed near the equator.

Therefore, using magnetic measurements on continents, we can tell where continents sat even without oceanic crust. However, none of these measurements are as good as oceanic north/south anomalies, so there are larger errors. Furthermore, sometimes you just don’t have the right rocks; if rocks are metamorphosed or eroded it will destroy previous magnetic information.

To build upon that measurement, you can use plate tectonic information. If you can match mountain ranges from one continent to another, you can come up with better estimates of which continents were in contact. If you find limestones, which commonly form in warm waters near the tropics, you can estimate that a continent must have been near the equator even if the rocks don’t have a strong magnetic signal. If you find glacial sediments, a continent must have been near the poles.

None of those are perfect. In fact, there is variation even in reconstructions of continental movement over the last 200 million years. Even when we have oceanic magnetic anomalies it isn’t always clear how continents moved; 2 different groups trying to reconstruct continental motion over the past 200 million years will often find important differences.

Even farther back in time the differences increase. There are some scientists who have argued that the supercontinent Rodinia didn’t even exist. However, enough scientists have argued for it that its name is well known and has its own Wikipedia entry.

That’s the way ancient continental movements have been reconstructed. Continent collisions leave a record in rocks. Continental motions leave a paleomagnetic record. However, there are gaps. There are proposals for multiple supercontinents before Rodinia, but those are even more poorly constrained and less accepted. One proposal even argues based on ancient paleomagnetic measurements that all the continental cores originally formed as one supercontinent and rifted apart, but ideas like that will remain controversial until we figure out ways to produce much better evidence than what we currently have.

-JBB

Image credit: https://en.wikipedia.org/wiki/Rodinia#/media/File:Rodinia_reconstruction.jpg

References: http://www.sciencedirect.com/science/article/pii/S0040195113000267 http://www.utm.utoronto.ca/~w3gibo/How%20to%20do%20field%20studies/properties_of_magnetic_field_at_.htm http://austhrutime.com/rodinia.htm http://www.earth.ox.ac.uk/~conallm/Rodinia.pdf

STONES FROM ANCIENT HANGI PITS MIGHT REVEAL EARTH'S PAST MAGNETIC FIELD Dr Gillian Turner from Victoria University, Wellington, New Zealand is studying the Earth's magnetic field using the stones that line Māori pit ovens, known as hāngi (sometimes called umu). Scientists have good palaeomagnetic data from around the world, recording field strength and direction, particularly from the Northern Hemisphere. There is a gap in detail however in the southwest Pacific; Dr Turner’s research hopes to fill that gap. Dr Turner’s project aims to retrieve information about changes in the Earth’s magnetic field over the past 10,000 years. Normally pottery would be used for data on the last few centuries. Pottery is used for such research as when the clay is fired in the kiln the minerals within the clay are heated above the Curie temperature and are demagnetised. As the clay objects cool down to become pottery, the minerals become magnetised again in the direction of the prevalent field; the strength of the magnetisation is directly related to the strength of that field. Māori did not use pottery, so an alternative source for data on the Earth’s magnetic field needed to be found. That alternative was the hāngi. Hāngi is the traditional Māori method of cooking food using heated rocks buried in a pit oven; Pacific Island nations use the same method. This method of cooking was and still is used in Chile, the Balkans, and certain parts of North Africa and the Middle East. To ‘put down a hāngi’ a pit is dug into the ground, stones are heated in the pit with a large fire and baskets of food are placed on top of the stones. Everything is then covered with Hessian bags, sheets, or flax mats and then covered with earth and leaves for several hours, before the hāngi is uncovered. For economical reasons, the traditional hāngi cooking principals are now mainly used for special occasions. Dr Turner, who has received funding from the Marsden Grant for this work, is undertaking an archaeological search in New Zealand to find ancient hāngi sites. The cooking stones used in these hāngi could give insight into Earth’s magnetic behaviour going back hundreds of years. Dating for these stones will be achieved through radiocarbon analysis of the charcoal left from the firewood used to light the pit oven. Dr Turner and her colleagues experimented with a modern-day hāngi to see if the stones at the base of the pit could reach over the Curie temperature and be re-magnetised, to prove hangi stones could be used as an alternative to pottery for their study. In Māori legend, the stones become ‘white hot’ with heat. Red hot heat is about 700ºC; Turner and her team put some thermocouples in the stones and were able to show they got as high as 1,100ºC. At such a high temperature, rock-forming minerals start to become plastic. Dr Turner’s team was able to show that a re-magnetisation had taken place by placing a compass on top of the cooled hāngi stones. Hāngi stones were carefully chosen; one of the most popular types used was an andesite boulder found in Central North Island. These volcanic boulders were preferred as they don’t crack or shatter in the fire. For the team of researchers, the volcanic boulders are the best because magnetically they behave better as they form with a high concentration of magnetite. Stones from hāngi sites will only provide data for the last 700-800 years. Dr Turner will also be studying volcanic rocks as well as lake and marine sediments in New Zealand. The outcome will be a detailed history of the southwest Pacific’s magnetic field over the last 10,000 years. For those interested: How to cook a hangi http://bit.ly/VM8qLH Guide on how to prepare a hangi http://bit.ly/ht49MP -TEL http://www.royalsociety.org.nz/2011/10/06/turner/ http://www.genuinemaoricuisine.com/Folders/Hangi.html http://www.bbc.co.uk/news/science-environment-20520454 http://www.livescience.com/25328-maori-stones-magnetic-field.html Image: http://www.hangiunderground.co.nz/gallery

Apparent Polar Wander This image shows one of the fundamental techniques geologists use to reconstruct the motion of plates on Earth. These plots are called “apparent polar wander” plots. Certain rocks contain magnetic minerals, like the mineral magnetite. When these minerals either grow or move around freely (like in flowing water) they align themselves relative to the Earth’s magnetic field. A rock formed in the presence of a magnetic field will wind up with its own magnetic field that points towards the north/south pole at the time it forms. If 2 continents are joined together, 2 rocks formed on those continents at the same time will point to the same North Pole. But the trick for geologists is…what happens if the continents move after the rocks form? Once a rock is formed, the magnetic field is (mostly) locked in. If a scientist measures the magnetic field in rocks across time, that paleomagneticist (also sometimes nicknamed paleomagicians) will be able to see how the magnetic pole moves relative to the continent. Of course, the North Pole isn’t really what is moving, the continent itself is moving – hence the name “apparent polar wander”. The North Pole recorded by the rocks seems to change because the rocks themselves move relative to the North Pole. Finally, go back to our case of 2 rocks forming on 2 continents that are together at one time and rift apart later. Once the continents separate, their rocks will record different magnetic poles as a result of the continental motion, but the tracks of the rocks can be treated like a jigsaw puzzle. In the 2nd image here the apparent polar wander paths from North America and Europe have been altered by assuming that the continents used to be joined together; if you make the assumption that the Atlantic Ocean once closed, suddenly the two curves plot on top of each other for the time when the continents were joined! These kinds of paths are one of the key ways plate motions over time have been reconstructed. -JBB Image credit: Marshak Essentials of Geology textbook, licensed to me for teaching purposes