Plate tectonics

Posted on 30 October, 2020 by Metageologist

Deep in the earth, solid rocks can flow, but the surface layers are cold rigid plates that move across the surface. This means that continents are constantly drifting across the earth and oceanic crust is being created and destroyed.

Plate tectonics is one of the most successful scientific theories of the Twentieth Century. It explains the major structures of earth’s surface and interior, the distribution of earthquakes and volcanoes, location of coal and mineral deposits, even where we find different types of fossil.

With modern global positioning satellite technology, we can directly measure the movements of the plates. They move about the speed your fingernails grow, a few centimetres a year. This isn’t fast on human time-scales, but on geological time-scales it means things are always changing. A geographical map of the earth from 100 million years ago looks very different and from 500 million years it’s unrecognisable. This is still only about 11% of the earth’s history.

Wegener’s theory of continental drift explained geological evidence from continents very well, but by the end of the 1950s it still wasn’t fully accepted for two main reasons. Firstly scientists knew the deep earth was solid but didn’t yet realise that hot solid rock can flow. Secondly our understanding of the rocks under the deep oceans was very limited.

Discovery of sea-floor spreading

During the second world war, new technologies were developed to measure the earth’s ocean depths as a way of detecting enemy submarines. After the war the US Navy funded surveys of the ocean depths to continue this work. Marie Tharp, a scientist working in the USA was involved in mapping out data from these surveys. In 1952 her mapping she identified a huge ridge down the middle of the Atlantic Ocean with a narrow valley at the very top. She interpreted this as a place where the earth was moving apart and linked it with the then controversial theory of continental drift.

Many didn’t believe her, but soon huge quantities of data were collected confirming her idea. The surveys of the ocean also measured the earth’s magnetic field, as the Navy hoped it would help with detecting steel submarines. These data showed a clear pattern of stripes parallel to the mid-ocean ridges identified by Marie Tharp.

Rocks cooling on the sea-floor contain magnetic minerals that capture a record of the earth’s magnetic field. This affects modern measurements of magnetism made above the rocks. The stripes are explained because the earth’s magnetic field changes back and forth over time (the Poles switch round). As crust is gradually created at mid-ocean ridges and drifts apart it slowly records the changing magnetic field.

DIAGRAM SHOWING SEA-FLOOR SPREADING https://en.wikipedia.org/wiki/Vine%E2%80%93Matthews%E2%80%93Morley_hypothesis

The idea of ‘sea-floor spreading’ and that these mid-ocean ridges were creating new crust was developed in the early 1960s. For the Atlantic it was shown in 1965 that if you remove these stripes one by one and bring the two sides back together, the continents fit closely together. In plate tectonic theory these types of boundary where plates are moving apart are known as divergent. Mid-ocean ridges are not straight lines, but are offset by breaks in the oceanic plates called transform faults. The ridges are not just found within the Atlantic, but also within the Indian and parts of the Pacific oceans.

Discovery of subduction zones

In the 1960s governments invested in a world-wide network of seismometers as a way of tracking underground nuclear tests. The data captured transformed our understanding of the earth as it greatly increases our understanding of earthquakes happen.

Earthquakes are formed where rocks break and move along large surfaces called faults. Earthquakes in the Atlantic are focused on the mid-Ocean ridges, caused by the stretching and movement of rocks in the rift zone. But the places where earthquakes are most common and strongest are found not in the Atlantic but around most of the Pacific and mark not where crust is made but where it is destroyed.

The earth isn’t growing bigger, so if oceanic crust is being made in the Atlantic, it must be being destroyed elsewhere. The Pacific oceanic plate is surrounded by subduction zones where lithosphere (oceanic crust plus attached mantle that together forms the plate) sinks down into the earth’s mantle. These are a type of convergent plate boundaries. At the surface subduction zones can be recognised by deep trenches formed where the oceanic lithosphere bends down. The Marianas Trench is the deepest, but they exist all around SE Asia and also down most of the west side of the Americas.

As the oceanic lithosphere sinks into the earth there are sudden slips and jolts as it pushes its way down which cause large earthquakes that are very dangerous. Once scientists had enough data, they could see in the patterns of earthquakes the location of the plate as it sinks into the earth. The earthquakes were shallow near the trench and deeper further away forming a surface of earthquakes known as a Wadati-Benioff zone.

<<< Diagram of earthquakes coloured by depth. E.g. http://www.isc.ac.uk/ home page >>

This surface marks roughly the top of the oceanic plate and earthquakes form as it forces its way deep down into the earth. As it sinks it also heats up and water within the plate is forced out into the mantle above, which melts causing volcanoes at the surface.

diagram of subduction zone, e.g. https://en.wikipedia.org/wiki/Subduction

Volcanoes exist all around the Pacific, the so-called ‘ring of fire’, from New Zealand, through the Philippines, Japan, Alaska, Canada, USA and down through central and South America. Armed with a global set of data on earthquakes, scientists were able to trace subduction zones across the world and in turn show that the ring of fire volcanoes all sit above them.

In the early 1950s, Marie Tharp was not believed as continental drift was so controversial. But by 1967 the ‘plate tectonics revolution’ was complete. In that year, models showing the earth’s surface as 12 rigid plates moving across the surface were published. These explained all of the features and evidence we’ve mentioned so far in a consistent and powerful way. Plate tectonics theory now underlies all of modern Earth Science.

https://en.wikipedia.org/wiki/Plate_tectonics#/media/File:Plates_tect2_en.svg

Plate tectonics around the world

Looking at a map of plates and a topographical map of the world together is very interesting. Let’s go on a tour of the world and show how plate tectonics explains many things.

The south-western edge of Indonesia is a lovely example of a subduction zone. It has a deep trench, a clear Wadati-Benioff zone and a line of volcanoes that form the islands of Indonesia. This subduction zone caused an earthquake that in turn created a tsunami in December 2004 that killed 227 thousand people in 14 countries. The Australian plate is being subducted under a corner of the Eurasian plate but oceanic crust is also being created in a ridge down the middle of the Indian Ocean.

Patterns of plate movement are complicated. The earth is a sphere and plates are rotating on it, meaning that relative plate movements are different in different places and don’t necessarily make sense on a flat map. In the USA the Pacific NorthWest has subduction and volcanoes where the tiny Juan da Fuca plate is being subducted. But nearby in Southern California the plates are moving past each other (a transform plate boundary) so there are earthquakes but no volcanoes. The city of San Francisco was destroyed in 1906 by an earthquake on this plate boundary.

In some subduction zones the sediment sitting on top of the oceanic crust is scraped off and piles up above the subduction zone in a structure called an accretionary wedge. In the Caribbean the West Indies consists of two types of islands. First there is a curved line of volcanic islands stretching from Anguilla down to Grenada caused by subduction of the Atlantic under the Caribbean plate. The island of Barbados is linked to these islands culturally but sits further east. It’s is not volcanic but is a place where the accretionary wedge forms an island.

In some subduction zones oceanic lithosphere is sinking down below a different piece of oceanic lithosphere, rather than continental. Here the volcanoes form chains of islands and sometimes build up thick piles of crust called volcanic arcs. A lovely example of an arc of volcanoes is found in the Aleutian Islands in the North Pacific.

Eventually oceanic islands and arcs enter a subduction zone where they are far too thick to be subducted. They are scraped off and added to the other plate in a process called accretion. Japan and Alaska are both places where volcanic arcs have been added to continental crust multiple times in the past. This process is one way continental crust may be created.

Continental tectonics

Continental crust is very different from oceanic crust. All land on earth sits on continental crust, with the exception of volcanic islands like Iceland or Hawaii. It is different in composition, being much richer in Silica. It has a lighter density and is never subducted. It is not involved in sea-floor spreading or subduction, but it is affected by plate tectonics and not just because continents drift across the surface.

Continental crust is affected by all three types of plate boundary. The East Africa Rift is where a divergent plate boundary is being started. Two parts of Africa are being pulled apart, with the continental lithosphere being thinned and volcanic activity occurred. Within about 10 million years this will become a true plate boundary and oceanic crust will start forming in the wider rift as the fragments of continent completely break apart.

Transform plate boundaries are found in California, but also down the middle of the South island of New Zealand. Convergent plate boundaries involving continents are of two types. The western edge of South America is an example of oceanic crust converging with continental, where subduction causes volcanoes but also a large mountain range called the Andes.

Plate boundaries where two continents converge cause large mountain ranges. There used to be an oceanic plate called Tethys sitting between what is now the Eurasian plate (to the north) and the African and Indian plates. After this oceanic crust was fully destroyed, continents collided forming mountain belts. The Himalayan mountains were formed by India colliding into the Eurasian plate and the Alpine mountain chain in Europe, plus mountains in Turkey, Iraq and Iran from Africa hitting Eurasia. The Tethys oceanic crust was a complicated shape and some of it remains within the Mediterranean sea.

Plate boundaries within continents are not sharp or simple. The effects of the impact of the Indian and Eurasia plates extends all the way through China into Siberia. Plate tectonics describes rigid oceanic plates with sharp boundaries very well. Sometimes the term continental tectonics is used to describe the different ways in which continents behave.

At the same time as evidence to prove plate tectonics was building up, some scientists were thinking about how this theory could explain the earth’s history. They started to interpret old rocks in terms of plate tectonics. Big differences in fossils from locations now close together can indicate that an ancient ocean once existed between them. Slices of oceanic crust, known as ophiolites can be found within continents and also indicate where a now vanished ocean basin once was. Patterns of metamorphic and igneous rocks can also be used to trace ancient subduction zones (blueschist and eclogite rocks), volcanic arcs or contintental collision zones.

A geologist called Tuzo Wilson proposed the idea of a regular cycle, where oceans open and close again and again. Close around the edge of the North Atlantic, in both Europe and America there are the traces of an ancient continental collision zone called the Caledonides that marks where a now vanished ocean called Iapetus was destroyed. Some time in the future the Atlantic ocean will close and another collision zone be formed close to the old one.

Plate tectonics explains the modern earth very well and explains most of the earth’s history too. Modern research into plate tectonics looks to explain where it may or may not apply, for example on other rocky planets and the very early earth.

Mars and Venus are similar to earth in many ways, but neither have plate tectonics. The explanation may be simple: Mars be too small and cold for the rocks to mantle to flow properly. Venus may be too hot – it’s atmosphere is really effective at insulating the planet. Other explanations talk about the importance of water as a way of lubricating the subducting oceanic plates.

Another debate is around when plate tectonics started on earth. The early Earth had an internal temperature that was much hotter. For oceanic crust to subduct, it must be rigid enough to be pushed into the mantle and so if crust and mantle are hotter (like Venus now or the Earth in the distant past) then plate tectonics may not be possible. Instead blobs of crust sink down and hot plumes rising up are much more important. Rocks older than about 2.5 billion years old are different in many ways from those created now and maybe earth then was more like Venus now. Scientists are still debating these topics.

First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.

Beyond plate tectonics

Posted on 2 August, 2015 by Metageologist

Plate tectonics is the core unifying concept that has underpinned our understanding of the solid earth for over 50 years. To describe your research as moving “beyond plate tectonics” is quite a claim, but Trond Torsvik and the group he leads have some remarkable science to back it up. By tracking the movement of the earth’s plates over half a billion years they trace the effects of hot plumes of rock rising from the edges of structures sitting just above the earth’s core. Their research seeks to explain the origin of diamonds, immense volcanic eruptions linked to mass extinction events, the break-up of continents and how shifts in the earth’s axis caused glaciation in Greenland.

Dance of the plates

Trond Torsvik is a Norwegian scientist with a background in palaeomagnetism – studying fossils of the earth’s past magnetic field frozen in rocks – to trace the past locations of continents. Palaeomagnetism can tell you the latitude at which an ancient rock formed1. Torsvik worked with those in other disciplines – palaeontology and geology – to trace the slow joining and splitting of ancient continents.

This research (which involved many other scientists) has given us a pretty good view of how the earth’s plates moved around over the last 500 million years. But these movements are only the surface expression of the flow of the underlying rocks, the earth’s mantle. Now, as director of the Centre for Earth Evolution and Dynamics at the University of Oslo (CEED) Torsvik seeks to produce an integrated understanding of deep mantle flow – mantle dynamics – and how it drives plate tectonics and other surface processes.

Undoing subduction

The earth’s mantle convects. Although made of solid rock, over geological time-scales it flows like a liquid and we understand the physics of this process well enough to produce computer models of it. One important factor is subduction – as oceanic crust cools it sinks back into the mantle, changing the patterns of flow.

Based on our understanding of how the continents moved in the past, the CEED group (Bernhard Steinberger in particular) have calculated where ancient subduction zones were and therefore where the subducted plates ended up in the deep earth. These models of ancient mantle flow and subduction link our surface observations with deep-earth processes.

The diagrams below show how subduction zones have moved over time. The outline of the continents is fixed, representing a stable reference frame. The coloured lines show how subduction zones at the edges of plates have moved over time2.

The red lines correspond to modern subduction zones, but the colour coding shows how where they used to be in the past. Note how the western edge of the North America plate has moved east over time3. Also note how it shows the subduction zone that used to exist north of the Indian plate and which ceased around 60 million years ago as India and Asia collided (as the oceanic plate in between was completely subducted).

Steinberger, B., & Torsvik, T. (2012) figure 2b

Here we have the same picture, but starting from 140 million years ago and moving back to 300 million years ago, the beginning of the Permian. These are the subduction zones that surrounded the ancient continent of Pangea.

Steinberger, B., & Torsvik, T. (2012) figure 2a

The diagrams aren’t showing it directly, but they remind us that the oceanic crust that passed through these subduction zones is still down there in mantle; imagine the series of coloured lines as sheet descending down into the earth – that is a rough image of what is down there.

Deep structures affect the surface

Mantle plumes have long been suggested as the cause of chains of volcanic islands (like Hawaii). Many believe the concept has been overused and that some proposed plumes don’t exist – this is a controversial area. Torsvik and CEED have taken the debate forward by presenting a testable hypothesis – that big plumes form around the edge of structures at the base of the mantle and that this has been happening for hundreds of millions of years.

Seismic tomography shows mysterious lumps at the very base of the mantle. They are called Large Low Shear Velocity Provinces (LLSVPs) and one sits under Africa and another under the Pacific. They are probably patches of different composition, but no-one knows for sure.

The CEED group believe these LLSVPs haven’t moved for a long time, so they took their models of plate movements to show how surface features have moved over them over time. They also plotted the locations of unusual volcanic features called kimberlites and vast piles of lava called Large Igneous Provinces (LIPs). The diagram below shows an example from 160 million years ago – here they’ve plotted the ancient location of the continents, plus that of the LLSVPs (in red). Note that kimberlites are found where areas of craton (thick old continental plate shown as grey areas) are above the edges of an LLSVP. Kimberlites are the host rocks for diamonds, so this result is not of purely academic interest.

Torsvik, T., et. al (2010), figure 2

This pattern holds when the analysis is done for other periods in the past, also when looking at modern active hotspots. Put all the data together and the pattern is quite impressive. Note that kimberlites and hotspots are not shown in their current position4 but the continents are.

Torsvik, T., et. al (2010) figure 1

This is a startling result. The fit isn’t perfect (the white dots don’t fit the pattern) but nothing on this messy planet of ours ever is.

So why are LIPs and kimberlites associated with the edges of the LLSVPs? The linking factor is deep plumes, which interact with deep continental lithosphere to produce kimberlites (and bring diamonds to the surface). Big plumes cause LIPs and the one shown above around the location of modern-day St Petersburg is the Siberian Traps which caused the largest mass extinction ever know at the Permian-Triassic boundary.

Surface processes affect the deep earth

What links plumes and the edges of the LLSVPs? Think back to those diagrams of ancient subduction zones and those curtains of ancient oceanic crust sinking into the mantle. Modelling of mantle flow through time shows that the ancient subducted crust reaches the base of the mantle where it pushes up against the LLSVPs. The flow of heat from interior of the earth to the surface drives the hot material rising up through the mantle but the interaction between plate and LLSVPs provides plausible mechanisms to get plumes started – the sinking plate pushes on the edge of an LLSVP and creates domes that turn into plumes.

What I like about this work is that by presenting a clear mechanism and predictions of how the deep and surface earth work together it is eminently testable. If mantle plumes form at the edge of LLSVPs, how does this affect the chemistry of the molten rocks that reach the surface? Perhaps one side contains the LLSVP material and another not. Any new seismic tomography data can be compared with the computer models that underlie this research. Does this research give us a new way to find diamond deposits? Finding answers to any of these questions will either help confirm the hypothesis or take research in new and interesting directions.

Our wobbly world

So much science, so little time! But allow me to test your attention span a little more and talk about my favourite example of how research from CEED links the surface and the depths of this planet.

The presence or absence of ice on this planet is one of the longer-term climatic cycles observable in the fossil record. For all of the last half-billion years, glaciation has been restricted to the southern hemisphere – until the last few millions years. Climate is the major control over glaciation, but a paper this year points to three ways in which deep earth processes caused glaciation in Greenland to start.

Steinberger, B., et. al, figure 5

Firstly, Greenland is unusually high (and so cold) – this is because the deep plume now centred on Iceland thinned the Greenland lithosphere and, from five million years ago, fresh ‘plume pulses’ pushed it up. Secondly, standard plate-tectonics has caused it to drift north (blue points and lines in diagram) by 6 degrees. Thirdly and most mind-bogglingly, changes in the distribution of density of the earth’s interior have caused the earth’s pole of rotation to move closer to Greenland by 12 degrees (green points are observation, pink are theoretical calculations).

If you’ve ever pushed a barrel or ball part-full of water, you’ve some sense of what lies behind the third cause, known as “true-polar wander”. Classroom globes have have a solid rod down the earth’s axis, but the real earth does not – it rotates around an axis called the ‘maximum moment of inertia’ that is determined by the distribution of mass within the planet. If this distribution of mass changes over time, then the axis changes and the poles shift to compensate. Modelling suggests that the shift of the north pole towards Greenland was caused by increased subduction under East Asia and South America.

Plate tectonics explains subduction. But models that show subduction tweaking the earth’s axis to bring glaciers or tickling the deep earth to create mantle plumes that can kill off nearly all life, break up super-continents, and send diamonds tinkling up to the surface. That really is going beyond plate tectonics.

References & image credits

This post is necessarily a skim over large amounts of complicated research. If you don’t believe it’s true, at least read the papers yourself. All are available online.

Source of images are in the image text. All either from open-source papers or produced under fair-use.

This Nature paper links LLSVPs, diamonds, plumes and LIPs.
Torsvik, T., Burke, K., Steinberger, B., Webb, S., & Ashwal, L. (2010). Diamonds sampled by plumes from the core–mantle boundary Nature, 466 (7304), 352-355 DOI: 10.1038/nature09216

This details the mathematical models linking subduction, LLSVPs and the initiation of plumes.
Steinberger, B., & Torsvik, T. (2012). A geodynamic model of plumes from the margins of Large Low Shear Velocity Provinces Geochemistry, Geophysics, Geosystems, 13 (1) DOI: 10.1029/2011GC003808

This links deep-earth processes to the onset of glaciation in Greeland.
Steinberger, B., Spakman, W., Japsen, P., & Torsvik, T. (2015). The key role of global solid-Earth processes in preconditioning Greenland’s glaciation since the Pliocene Terra Nova, 27 (1), 1-8 DOI: 10.1111/ter.12133

This contains the detail about true polar wander.
Steinberger, B., & Torsvik, T. (2010). Toward an explanation for the present and past locations of the poles Geochemistry, Geophysics, Geosystems, 11 (6) DOI: 10.1029/2009GC002889

Hot spot volcanoes: no plumes required?

Posted on 1 February, 2015 by Metageologist

It’s a simple and well-known picture. Volcanoes form either at plate boundaries due to subduction or inside plates due to mantle plumes. Invoking plumes – columns of hot rock rising from deep in the mantle – is an awfully useful way of explaining oddly-placed volcanoes, both ancient and modern.

Too useful, many people think. The concept has been abused. See Erik Lundin’s excellent critique in “52 things you should know about Geology“: “A concept that is granted the freedom of perpetual ad hoc amendments has the ability to explain anything … But such a concept can neither be falsified not used predictively. In the long run it may be wiser to ask yourself ‘Is there an alternative explanation?’ rather than simply shrugging, ‘Plumes do that’.

Layers of volcanic debris and ash from the Newer Volcanics Province, Australia. Spot the bombs. With permission from Stephanie Sykora

How else to melt the mantle?

The best place to find alternative explanations is mantleplumes.org a site dedicated to “discussing the origin of “hotspot” volcanism”. The site lists many mechanisms, but I’m going to focus on just two: edge-driven convection and shear-driven upwelling.

It’s not that hard to melt the mantle. It’s everywhere fairly close to its melting point and it gets hotter the deeper you go. A key point to understand is that most of it only stays solid because of the intense pressure it is under. As mantle quickly rises up beneath mid-ocean ridges it melts because it stays hot as the pressure reduces. All the atoms that were squeezed tightly together in solid crystal lattices manage to break free into a liquid state, once the earth’s grip lessens a little.

Almost all matter behaves like this, but it doesn’t feel like common sense because we are most familiar with the freezing and melting of water, which is weird and works the opposite way round (which is why ice floats). I labour the point because both of today’s mechanisms are ways of creating upwellings1 – areas where hot mantle material rises up and so is prone to melting.

Edge-driven convection (EDC) is flow caused around the edges of continents. Continents have deep cold roots to them, like icebergs2. A convection cell is set up with a zone of upwelling about 600km from the craton edge. It wouldn’t be surprising if you find some volcanic rocks above here.

Diagram showing edge-driven convection. Reproduced with permission from mantleplumes.org

The above model assumes nothing is moving, but we know that there will often be flow of the mantle relative to the plates. If there is mantle flow across an edge (for example a craton edge) then material will flow up. This is one way of producing shear-driven upwellings (SDU)3

Diagram showing mechanisms of shear-driven upwelling. I discuss the left-hand example. Taken with permission from mantleplumes.org.

So far, so theoretical. Let’s go to Australia and look at some rocks.

Welling-up down-under

The Newer Volcanics Province is an active (but dormant) volcanic area in Victoria, Australia. To get a great overview of its many great volcanic features, check out this post (from which the photos here come). The lava is basaltic in composition – just what you’d expect from melting of mantle, but we are a long way from a plate boundary.

A recent paper in Geology studies what’s going on deep beneath the lava. Rhodri Davies and Nicholas Rawlinson of ANU, Canberra and Aberdeen universities start off with a spot of 3-D seismic tomography. Previous workers through they could dimly see a plume beneath, but armed with a new seismic data set from the (wonderfully-named) WOMBAT project they show there is no plume. Instead they show a shallow low-velocity anomaly underneath the NVP, consistent with region of hotter mantle, perhaps containing a small proportion of magma.

"Volcanic bomb with a olivine-rich xenoliths from the mantle at Mt. Noorat – Victoria, Australia" Courtesy of Stephanie Sykora

A piece of mantle that flowed upwards and melted: “Volcanic bomb with a olivine-rich xenoliths from the mantle at Mt. Noorat – Victoria, Australia” Courtesy of Stephanie Sykora

Having made the plume vanish, they turn to modelling of the mantle flow, based on their new improved knowledge of what is down there.

This area of Australia sits outside of the deep cratonic root. It’s like a thin ledge sticking out from the side of the iceberg. Therefore the edge of the deep root, that might cause EDC is to the north, allowing the upward return flow to sit directly beneath the NVP. Their models also include relative plate motion (how the plate is moving relative to the mantle beneath). This allows them to model the effects of SDU as well.

The modelling results produce a region of upwelling with velocities between 1 and 2 cm a year – fast moving for mantle – sitting directly underneath the NVP. This neatly explains the NVP, without any need to invoke plumes.

The mechanism is neat, but begs the question as to why there isn’t a line of volcanoes all around cratonic roots. Addressing this question, they point out the interaction of SDU and EDC. Under the NVP the two effects are complimentary – upwelling is increased where the mantle is flowing away from step. Also the edge here isn’t straight – 3D effects are important. Finally, mantle composition varies. So-called ‘fertile’ mantle may melt under conditions where mantle that’s already had melt extracted would not.

Are plumes dead?

There’s a compelling model here for explaining many volcanic hot-spots around the world with no need for plumes. Do we need plumes at all? Gillian Foulger, the force behind mantleplumes.org certainly doesn’t think so. Also Don Anderson of Caltech who recently had the posthumous last word at the AGU annual meeting last year.

Their views may prevail in time, but for the moment most of us still believe in plumes. Explaining how small-scale convection causes a minor volcanic field in one place doesn’t explain continental flood basalts like the Deccan or Siberian Traps. You know, the ones that cause mass extinctions and thickly cover vast areas.

But clearly plumes and not the only game in town. To progress, ideas involving plumes need to be anchored in our understanding of the deep earth, to be falsifiable and have predictive power. Recent research aims to do just that. Watch this space.

References

Davies D.R. (2014). On the origin of recent intraplate volcanism in Australia, Geology, 42 (12) 1031-1034. DOI: http://dx.doi.org/10.1130/g36093.1

Tasting the earth: mantle geochemistry

Posted on 11 January, 2015 by Metageologist

If seismologists listen to the earth then geochemists taste it.

Like experts blind-tasting a glass of wine and recognising where it came from, geochemists studying the deep earth aim to find out where a particular liquid came from. Their liquid – basaltic magma formed from melting of the mantle rocks – is now solid, so ‘tasting it’ involves dissolving it in Hydrofluoric acid or vapourising it in the bowels of a machine with an unlovely name.

A wine buff can sniff out where a wine came from because they’ve already sampled lots of known vintages. Geochemists have a much harder job – basalt samples don’t have labels. They are formed from melting of the rocks below, but was the material that melted from the deep earth or shallow? Is it from oceanic crust that’s been subducted and remelted or material that’s sat around since the earth was formed?

Mantle geochemists still have more questions than answers, but that’s because what they do is really hard. They are like first-time wine-tasters who’ve been given anonymous bottles and only a fuzzy satellite image of France to work with.

Image stolen from the Basalt winery. I’m sure they won’t mind.

What to sample?

Most basalt is produced at mid-ocean ridges, where oceanic plates move apart and the underlying shallow mantle rises up, decompresses and melts. Known as MORB1 this is plonk. Widely produced, homogenised and of little interest to the true connoisseur.

Basalt from oceanic islands (OIB) is mantle geochemists’ favourite tipple. Found only in select areas far from plate boundaries it has many flavours but can be distinguished from MORB by a trained nose. Thought to be formed by material rising up in hot plumes from the deep mantle it carries whiffs of what is lurking down there.

Of particular interest at the moment are dark intense picritic lavas. Formed under higher temperatures in smaller batches they tell us more about what happens when a mantle plume first nears the surface.

Tasting

The process of producing basalt from the mantle is complex, depending on the composition and mineralogy of the melting material plus the depth and pressure. Also a lot may happen to the magma before it cools as the surface as lava. Iceland has rhyolite lava flows – very different in composition to basalt, but ultimately formed from mantle melt2.

So to study the mantle that was melted tasting the basic chemistry of the lava is not enough as changes due to later processing can obscure the smell of the source material. More sensitive mechanical noses are required, that can sniff trace elements or isotopes that may be unchanged by later processing and hold the tang of the source mantle.

Terroir

Mantle composition, as inferred from basaltic melt, is very variable, leading to the identification of a ‘zoo’ of acronyms, from DMM and HIMU (sources for MORB) to EM and FOZO (for OIB).

A key concept is ‘enrichment’. Particular elements are ‘incompatible’ which means that if they are in a rock that melts, they are strongly partitioned into the melt. As the ‘enriched’ melt moves away you are left with a ‘depleted’ residue. Continental crust is extremely enriched, oceanic crust less so.

For this reason the churned up mantle contains portions which are depleted by having had oceanic crust melted from it (DMM) and other enriched portions which contain recycled oceanic crust (HIMU). Small amounts of continental crust may enter the mantle – perhaps the mantle frozen to the base of continents may fall off. Also continental material (sediment, stones frozen into icebergs, the Titanic) may end up on ocean floor destined to be subducted. EM and FOZO are sources that may have been enriched in this way.

Primitivo

Geochemists don’t just worry about the mantle, but the whole earth. Chondritic meteorites have long been thought to be a model of the bulk chemistry of the earth. Strip out iron and other elements into the core, account for the enriched crust and you can calculate the bulk composition of the mantle.3.

Compare known mantle compositions with the theoretical bulk composition and you get a gap, leading to the idea of a hidden reservoir of ‘primitive’ composition (e.g. closer to chondritic). Conceptually this is similar to the idea of ‘dark matter’ in physics – a thing invented to explain inconsistent pieces of evidence, but for which there is no direct evidence. Only time will tell if hidden reservoirs in the mantle will be found or go the way of the luminiferous aether.

Basalt in a vineyard

Paradoxes and problems

The idea of hidden reservoirs was extremely popular over 20 years ago, when it seemed that subducting plates stopped at 660km depth, where a ‘phase change’ in minerals alters the stiffness of the flowing mantle. This suggested that the lower mantle could be of very different composition. But modern seismic imaging suggests whole-mantle convection is possible, suggesting that over billions of years the mantle will have been thoroughly stirred – with the exception of a mysterious layer at the base of the mantle.

Mantle geochemists often talk of ‘paradoxes’ – patterns of ratios between elements and isotopes that aren’t consistent. There is a lead paradox, and an Argon one, plus a ‘heat-Helium imbalance’. Explaining these in terms of a primitive reservoir is one way, but others are possible. Let’s look at Helium.

Helium comes in two flavours. The first ³He is just two protons and a neutron and from the earth’s point of view it’s primoridal, it’s always been there and never changes. In contrast, when the great hulking nuclei of Thorium and Uranium fall apart they leave small fragments – making ⁴He in the alchemical process of radioactive decay.

The ratio of the two Helium isotopes is fairly consistent for MORB sources, but wildly variable for OIB. Material with a high ratio has been interpreted in terms of a primitive reservoir, rich in primoridal ³He. An alternative explanation is that the source is extremely low in ⁴He due to it being depleted in Uranium/Thorium. Or maybe the ³He bubbled up from the core.

Tasting the earth does not give you all the answers, but it is vital part of the picture. As I continue my tour of the deep earth, geochemistry will often have an important role to play. The difference between OIB and MORB is a powerful argument in the armoury of those who favour mantle plumes and as seismologists start to see odd things at the base of the mantle, getting a whiff of the chemistry here becomes very important.

Tasting ‘black cherries’, ‘tar’ or ‘cat-pee’ in wine is a clever trick. Tasting blobs of 4.5 billion year-old rock or recycled oceanic crust in basalt is even cleverer. Cheers!