Marion Ferrat

What’s Geology got to do with it? 3 – Christmas! Part 2

Marion Ferrat December 24, 2013

Dear Readers,

Welcome to the last Four Degrees post of 2013! I’m back home with family and here the Christmas festivities happen today, on Christmas eve. So before I focus my attention on wrapping my last present and stuffing the goose for our family meal, here is the second instalment of our Christmas special of ‘What’s Geology got to do with it’! What has geology got to do with the Three Wise Men, popular Christmas presents or Rudolph the reindeer? Find out here… and a very merry Christmas!

Source: Kris de Curtis, Wikimedia Commons.

The Gifts of the Three Wise Men
Christmas Wishlist – Tablets!
Reindeer

1. The Gifts of the Three Wise Men

The Visit of the Three Wise Men, ca. 1900 – Source: Wikimedia Commons.

In Christian tradition, the Three Wise Men, known as Melchior, Caspar and Balthazar, visited Jesus after his birth. Each brought a gift from his land: gold (from Persia), frankincense (from India) and myrrh (from Arabia). These gifts were common offerings and presented to Jesus. Gold was seen as valuable, myrrh was used as an anointing oil, and frankincense as a perfume.

Gold –

Gold is a precious metal and was already used for coinage, jewelry and art before written historical records began. Some of the oldest gold artefacts were found in graves in Bulgaria, dating back to the 5^th millennia BC.

Gold is found as ores in rocks, typically as a mixture of gold and silver, It generally occurs as tiny particles embedded in the rock, accompanied by other minerals such as quartz and pyrite (‘Fool’s gold’), or as grains, flakes or nuggets that have eroded from the rock and can be found in rivers or loose soil deposits.

Gold nugget – Source: USGS, Wikimedia Commons.

It is estimated that a total of 165,000 tonnes of gold has been mined in the world, roughly half of that from South Africa. In 2007, China became the world’s largest gold producer.

The gold brought to Jesus by Melchior could have originated from Persia, modern-day Iran. Iran contains several gold-rich regions, and total gold reserves are estimated to be 320 tonnes. Until 2012, the city of Takab contained Iran’s largest gold mine, with over 4 tonnes of gold reserves. Recently, three new gold mines have been discovered in the city of Saqqez in the West of the country.

Frankincense –

Frankincense from Somalia – Source: Snotch, Wikimedia Commons.

Frankincense is an aromatic resin that comes from four main species of Boswellia trees. It is used in perfumes and aromatherapy and is considered valuable for its healing abilities. Frankincense is also used in religious rituals in many Christian churches. It is thought that the biblical frankincense brought by Caspar was extracted from the tree Boswellia sacra.

Frankincense can be found in different grades, depending on the time of harvesting of the resin. It is extracted by scratching the bark of the tree so that the resin bleeds out and hardens, forming frankincense ‘tears’. There are different sorts of resin, depending on the tree species producing it as well as on the geology of the soil upon which the tree grows and the climate under which it develops.

Flowers from a Boswellia sacra tree – Source: Scott Zona, Wikimedia Commons.

The Boswellia sacra tree is native from the Arabian peninsula and northeastern Africa. It is abundant in Somalia and the arid woodlands of the slopes of the Dhofar mountains of Oman and Yemen. The tree is famous for its ability to grow in very unforgiving environments. It typically develops on calcareous soils (limy or chalky soils mostly composed of calcium carbonate) and is found on rocky slopes and ravines. The trunk is made of disk-shaped bulbs, which ensures that the tree remains firmly anchored in the rock, even in violent storms.

Myrrh –

Man collecting myrrh in Somalia – Source: Somalia Ministry of Information and National Guidance, Wikimedia Commons.

Myrrh is another type of aromatic resin. It was very popular in the ancient world and was used as medicine in ancient China and Egypt, as well as for Egyptian religious rituals and mummification. It was also used in cosmetics, and Greek soldiers used it on the battlefield as an oil to stop wounds from bleeding.

Myrrh comes from the thorny tree Commiphora myrrha. The tree is native from the Arabian peninsula (mainly Yemen), Somalia, Eritrea and Ethiopia. Like frankincense, myrrh is also extracted by cutting through the bark and sapwood to bleed the tree. Commiphora myrrha grows best in thin soils containing limestone, at altitudes of 250 m to 1,300 m, with a mean annual rainfall of 23 to 30 cm.

2. Christmas Wishlist – Tablets!

Be it standard or mini, Microsoft, Nexus or Apple, the tablet remains a very popular Christmas present. So as we are handling our shiny new toys, let’s ask ourselves: what does geology have to do with it?

A typical tablet is made of aluminium, copper, silicon, gold, nickel, glass, steel and plastic. The raw metals used must be mined before making their way to the factory and I will use aluminium and copper as examples.

Bauxite – Source: Wikimedia Commons.

Aluminium is a silvery-white metal. It is tough, conducts electricity and is resistant to corrosion. Most metallic aluminium is produced from an ore called bauxite. Bauxite is formed in tropical climates when rocks containing little iron and silica are weathered by the elements. This ore is primarily mined from Australia, China, Guinea, Brazil, India or Russia. Other possible sources of aluminium include rocks such as shales or clays.

The production of aluminium takes place in three stages. First, the ore is mined, then it is refined to recover alumina (aluminium oxide) from bauxite, and finally it is smelted to produce aluminium from alumina. The ore is mined by what is called open cut mining, i.e. surface methods of mining where the soil is removed by bulldozers and scrapers. The bauxite lying below is then removed before being loaded into trucks or conveyor belts to be transported to refineries.

Weipa bauxite mine in Australia – Source: Wikimedia Commons.

Copper is a soft and malleable red-orange metal. It conducts heat and electricity and is used in many metal alloys. It has been used as a material for thousands of years but nearly all of the copper ever extracted has been mined since 1900.

Copper nugget – Source: Jurii, Wikimedia Commons.

Most copper is extracted from large open pit mines in geological deposits containing up to 1% copper. The largest producers of copper are Chile, the United States, Indonesia and Peru. Although the Earth contains large amounts of copper, only a small fraction of this is economically viable and extractable.

The ores containing copper are knows as porphyry copper deposits. Porphyry is a type of igneous rock – a rock that is formed through the cooling and solidifying of magma – that forms from a column of hot magma rising from inside the Earth to the surface. It is characterised by the fact that it contains larger crystals in a much finer surrounding.

Porphyry, with large crystals visible in a finer surrounding – Source: Piotr Sosnowski, Wikimedia Commons.

The size of crystals in a rock is determined by how fast the rock cools down and solidifies. If the rock cools slowly, the crystals have time to grow from the liquid magma, forming large crystals that can be seen with the naked eye. If the rock cools quickly, the crystals do not have time to grow and solidify as tiny grains that are only visible under a microscope. Porphyry deposits must therefore have cooled in two stages, first very slowly deep in the Earth’s crust, and then very rapidly as the magma reaches shallow depths and rises to the surface very rapidly, for instance in a volcano. As the magma cools, fluids are driven off and carry with them dissolved metals such as gold, lead, tin, zinc, and of course copper.

The Chino open-pit copper mine in New Mexico – Source: Eric Guinther, Wikimedia Commons.

3. Reindeer

When they are not driving Santa’s sleigh on Christmas night, reindeers live in different regions of northern Eurasia, including Scandinavia.

Over the past few decades, reindeer activities and reindeer farming in specific areas have had important ecological impacts. Reindeer graze and trample the vegetation covering the ground and the release of faeces and urine provides specific nutrients to the soil. This has damaged the lichen and moss-rich vegetation originally present, slowly replacing it with ‘lawns’ of nutrient-rich and digestible forage. The new vegetation type leads to what is called a positive feedback, where reindeer grazing leads to more digestible foliage, which enhances reindeer grazing, and so on. The loss of lichens also enhances the growth of coniferous trees.

Reindeer herding in Sweden – Source: Mats Andersson, Wikimedia Commons.

These on-going disturbances have ultimately created a new stable ecological state in regions of reindeer herding. But they also have consequences for local climate, through changes in the surface properties of the land.

When radiation from the Sun reaches the surface of the Earth, it can be both absorbed by the land or reflected back to the atmosphere – generally a mixture of the two. The proportion of energy absorbed versus reflected depends on the properties of the ground, including its colour. This is what is called albedo. Ice, for example, reflects a majority of solar energy and has a high albedo. Darker surfaces such as oceans absorb more energy and have a low albedo. The more energy is absorbed, the warmer that particular region (although water and land will warm up differently).

High-resolution satellite image of the border zone shared by Norway (northern half) and Finland (southern half), June 2001. A reindeer fence mirrors the border between the two countries. The difference in whiteness is due to more lichen coverage in Norway, with reindeer herding in Finland causing loss of lichen cover – Source: Grid Arendal Maps and Graphics Library.

The loss of lichen cover in reindeer herding areas has slowly reduced the whiteness and therefore the albedo of the land surface, changing the balance of solar energy reflected and absorbed in these regions. Reindeer can therefore have both ecological and climatic consequences, and studies have only recently started to investigate the potential positive or negative impacts of these changes in northern Scandinavia.

On these geological notes, a very merry Christmas to all!

Marion

Snacking on climate

Marion Ferrat December 10, 2013

ClimateSnack is a new initiative for early-career climate scientists around the world to improve their writing and communication skills. Snackers get to write tasty climate blogs and discuss them in a friendly and interactive environment. Marion talked to three members of the Imperial College London group for the latest issue of GeoQ!

Good written and oral communication skills are quickly becoming a pre-requisite for early career scientists. Writing, presenting, interacting and collaborating are important for making contacts, developing research proposals, applying for fellowships and communicating one’s work. This is particularly true in a very publicised field such as climate change research, where inter-disciplinarity reigns, and the ability to convey ideas to wide ranging audiences is crucial.

But gaining these skills is not always straightforward. Writing and publishing online can be daunting, so can interacting with researchers outside of one’s field.

Born in January 2013 at the University of Bergen, ClimateSnack brings together postdoctoral and PhD scientists across climate change disciplines, and helps them improve the way they communicate their work in a friendly, interactive environment. In July, Imperial College London became the second institution to join what has now become a global network of hungry climate snackers.

Panorama of Bergen – Source: Sindre, Wikimedia Commons.

I joined ClimateSnack back in August and have really enjoyed chatting about climate change research with so many PhD and postdoctoral students across the college departments and climate disciplines. When thinking of what to write for the Young Scientists section of the GeoQ issue on climate change, I decided that it would be great to discuss this initiative that has taught me very much about communicating climate change research. So I interviewed three core members of the London snacking team and asked them to tell me more about what ClimateSnack is all about. Here is what came out of our interview!

“ClimateSnack is essentially designed to help early-career researchers develop their writing skills and their communication skills in general”, says Dr Will Ball, a postdoctoral researcher in the department of Physics and the founder of the ClimateSnack group at Imperial College London.

“At each institute that we have set up a ClimateSnack group, we physically bring together people in different areas of climate research. They will write thousand-word blogs about their work, keeping it very simple. In fact you want to keep it at the level that any other climate scientist in a different area of climate research would be able to understand. So as a solar physicist, I should be able to communicate my work to somebody working on, say, atmospheric dust”.

These blog pieces are the climate “snacks” that eventually get published online:

“Then we have a centralised hub that all the institutes publish through, which is the website”, Will continues. “Through that, people will be able to interact, get to know each other and give feedback on the actual writing. So they get better at writing, and also learn about the science that’s going on around them. That’s the concept”.

Sian Williams, a PhD student in atmospheric physics looking at dust plumes and land-atmosphere interactions, runs the day-to-day climate snacking affairs in London:

“We have a meeting once a month where people from different departments across Imperial College come together”.

London snackers Rachel White, Will Ball and Sian Williams.

“Every time, we have a few snacks. I try to encourage people to write them and then send them out to anyone who is coming to the meeting in advance, so that people get a chance to read what has been written and give feedback”.

Writing a snack can be a daunting but rewarding experience. Each author reads out his or her piece and the floor is then open to discussion. I remember that reading my own piece out loud was really quite scary! But it helped very much with improving the post, because one instantly picks up on sentences or expressions that don’t quite fit or contain too much jargon.

“People who have come together from different institutions say what they like about the articles, how they think they can be improved. Normally when you write something, be it for a journal or a website, you never really get that direct feedback, so I think it’s a really great opportunity”, Sian continues.

Dr Rachel White, a postdoctoral researcher in regional climate modelling, has recently published her very first snack, writing about the difficulties of simulating global rainfall patterns: “I actually found that it was easier to write than I thought it would be”.

Detail of the portrait of a young woman with writing pen and wax tablets, Museo Archeologico Nazionale di Napoli – Source: Wikimedia Commons.

But putting pen to paper is just the first step: “Trying to check that you have really written what you wanted to write, and that people are going to understand what you meant, is the really interesting process”, Rachel adds. “That’s where the ClimateSnack meetings come in. Different people will have got different things from your article. You have to be quite careful so that everybody understands what you meant. That is a really interesting concept to learn and try and get you head around”.

Will is now an experienced snacker: “Publishing online was nerve-racking, but I developed a better sense of confidence in what I’m doing and in my writing”.

These meetings are not just useful for improving one’s writing, but also for placing early-career researchers in a safe, productive environment where they can hone their discussion and personal engagement skills.

“It’s not just writing. At these meetings you have to communicate, debate, argue, discuss, and you get better at that. And it’s in a safe environment. That’s where you build the confidence and then start moving out”, Will explains.

“Important, imaginative work comes out of collaborating with people who aren’t in your field”, Rachel adds. “Being able to discuss your research and describe it clearly to someone who is in a different field is incredibly important, at conferences, over the internet, everywhere.”

For Will, these communication skills are valuable even within one’s own field: “How many abstracts, how many summary papers have you read that are difficult to understand, even in your own field? [ClimateSnack] makes you more aware of the phrases and the words you use. I’ve noticed that in the way I write. I’m just a little bit more aware of what might confuse somebody.”

Source: Daniel Schwen, Wikimedia Commons.

ClimateSnack has grown at an incredible pace since January. “We are setting up at many other institutes in the UK, and have interest from several others in Europe and in the United States”, Will tells me. “So it’s going to expand very quickly in the next coming months”.

The success and uniqueness of ClimateSnack lies, I think, in its open and constructive environment, and in the opportunities it creates for early-career researchers to forge international collaborations with other climate scientists.

Concluding our interview, Sian adds: “There are opportunities for climate snackers to go on residential courses across Europe, which is really exciting because it’s not only building skills but again building collaborations with different people. And I think the main exciting thing is more people from different universities getting involved”.

Source: ISS Expedition 34 crew, Wikimedia Commons.

I have certainly loved being part of this exciting group and have learned so much about other branches of climate research. It has been fantastic to meet so many climate scientists from different departments and universities and I look forward to hearing about upcoming snacks at the next meeting!

Marion

Radioactive waters

Marion Ferrat November 25, 2013

As the decommissioning of the damaged Fukushima nuclear power station begins, Marion Ferrat takes a look at how radioactive elements make their way to the world’s oceans – and how scientists can use them to study important processes that go on in our waters.

Fukushima Dai-ichi nuclear station – Source: Asacyan, Wikimedia Commons.

Early last week, work began to remove spent fuel rods at the disused Fukushima Dai-ichi power station, more than two years after the plant suffered a triple meltdown caused by the powerful earthquake and tsunami of March 2011. Over 1,500 fuel assemblies will be removed over the course of one year, but decommissioning of the entire site will take decades.

Like Chernobyl before it, the Fukushima disaster released large amounts of radioactive elements to the environment, many of which made their way into natural waters.

But such accidents are not the only sources of anthropogenic radionuclides – the radioactive products of nuclear reactions – to the environment. Radioactive elements have also been released from nuclear reprocessing plants, as well as from the hundreds of nuclear tests that took place between the 1940s and 1980s.

In this post, I want to explore what happens to these radioactive elements when they enter the aqueous environment, and how oceanographers use them as unique tracers to study ocean circulation patterns.

Where does the radioactivity come from?

The main sources of anthropogenic radionuclides to natural waters are:

Nuclear weapons tests in the 1940s-1980s:
This is what is called global nuclear fallout. Radioactive elements released during the hundreds of cold war weapons tests fell back into the oceans after the detonation of the nuclear devices. As most of the tests were carried out in the Pacific, this ocean became the largest repository of radioactive fallout. Only the longer-lived isotopes released from these tests remain in the environment today.

Radionuclides released during the Chernobyl reactor accident (April 1986):
The accident released 400 times more radioactive material than the Hiroshima bomb and is considered the worst nuclear accident in history. Adjacent regions in Belarus, Ukraine and Russia were the worst hit but a radioactive cloud travelling westward also contaminated many regions of Europe, the North Sea and the North Atlantic Ocean.

The Japan earthquake of 11 March 2011 – Source: Maximilian Dörrbecker, Wikimedia Commons.

Radionuclides released from the Fukushima Dai-ichi power plant (March 2011):
The tsunami generated by the Tōhoku earthquake of 11 March 2011 damaged the Fukushima nuclear facility, leading to equipment failure and a subsequent nuclear meltdown. This accident released between 10-30% of the radiation caused by Chernobyl. Fukushima and Chernobyl are both classified as level 7 (the maximum level) in the International Nuclear Event Scale.

Discharges from nuclear fuel reprocessing facilities:
Nuclear reprocessing plants involve the extraction of plutonium and uranium from used nuclear fuels. The chemical processes involved generate large quantities of waste, which makes its way into the environment. Authorised releases from facilities such as Sellafield in Cumbria and Cap de la Hague in France have contributed to the contamination of the Atlantic Ocean and Irish Sea. Though discharges have been heavily reduced since their peak in the 1970s and 80s, these can still be observed in the environment. The Centre for Environment, Fisheries and Aquaculture science (CEFAS) releases annual or multi-annual environmental monitoring reports assessing the impact of this contamination in Britain.

Aerial view the Sellafield nuclear reprocessing plant in Cumbria – Source: Simon Ledingham, Wikimedia Commons.

What are the main radionuclides in waters today?

A whole range of radioactive elements are released from these different sources. While some radionuclides produced are very short-lived (they decay with half-lives of hours to days), others can remain in the environment for tens to tens of thousands of years.

The main radioactive isotopes from these sources found in the oceans today include tritium (3-H), radiocarbon (14-C) and isotopes of strontium (90-Sr), technetium (99-Tc), caesium (134-Cs and 137-Cs) and plutonium (239-Pu and 240-Pu). They have half-lives ranging from 2 to 200,000 years.

These radionuclides are the most commonly studied both for monitoring (because of their radiological impacts) and research purposes: as it turns out, radioactive waters have a lot to say about the ways our oceans work.

Anthropogenic radionuclides: fingerprinting the oceans

Anthropogenic radionuclides are very strong tracers of oceanic, biogeochemical and sedimentary processes.

Once they make their way to the waters, they don’t just sit there; they swim, they sink, and they behave in a variety of different ways. Because some of these radioactive isotopes were absent from the natural environment until the mid 20^th century, they only have a relatively small number of possible sources and can act like “fingerprints”, pinpointing the movement of particular water currents. So scientists can sample ocean water on large research ships, look at radionuclide concentrations in the water today, and see how far they have moved around in the oceans.

There are two types of interesting radionuclides: those that dissolve in the water, such as tritium or caesium, and those that react with particles, like plutonium. These two types of behaviour allow oceanographers to study different oceanic processes.

The Pacific Ocean – Source: NASA, Wikimedia Commons.

Soluble radionuclides are useful because they get transported with water masses: where the water goes, the radioactive element follows. Because scientists know with relatively high confidence where the elements entered the water, they can look at how ocean circulation has re-distributed them worldwide. For example, high caesium concentrations were recently found in the Tasman Sea. These must have come from elsewhere as very few nuclear tests were carried out in the Southern Hemisphere. Scientists were able to trace this caesium back to its source in the North Pacific Ocean, allowing them to study in detail the way that Pacific waters get transported across the equator over decades.

Particle-reactive elements like plutonium are equally useful. They will tend to attach themselves to larger particles (what is called being scavenged) rather than dissolve in the water, so they will slowly sink to the bottom of the ocean along their transport route. A few years ago, scientists from the Marine Environmental Laboratories of the International Atomic Energy Agency in Monaco carried out a global study of plutonium in the oceans of the southern hemisphere. They found that plutonium from the North Pacific region had made its way to the South Pacific, through to the Indian Ocean, and onto the South Atlantic. They also noticed lower and lower concentrations in the surface waters along the way, so they were able to study both the exact route of water transport, and the rate at which particles in the ocean scavenged this plutonium.

Gallons and gallons of water

Though these radioactive elements are clearly present in the waters, their concentrations once they have travelled the world can be extremely low. This is a challenge for oceanographers wanting to measure them, because instruments cannot detect radioactivity below a certain level and measurements can be hampered by other sources of radiation. So scientists need to sample enough water to be able to confidently determine the radioactivity of their target radionuclides.

A rosette system for sampling seawater from research vessels – Source: NOAA Photo Library, Wikimedia Commons.

For tritium, this is relatively straightforward as it is present in high enough concentrations (but with little radiological impact). Approximately 1L of water is sufficient to determine its activity. But other radionuclides, such as technetium or caesium, have typically required up to hundreds of litres of water for one sample. This, of course, is problematic when sampling on a research ship: space is quickly taken up by huge containers of sampled water, and all of this water gets very heavy! Until recently, sampling ships have had to restrict themselves to certain elements or to a small sampling area in order to cope with the large volumes of water required.

Recent developments have allowed scientists to get around some of these problems. Radionuclides can be pre-concentrated on board the ship with clever feats of chemistry: the sampled water is passed through pumping systems containing certain reagents that ‘fix’ the radionuclide of interest (for example caesium), removing it from the bulk of the water. Once this is done, the remaining water can be emptied back into the ocean.

Developments in detector efficiencies and new ‘radiation-proof’ underground laboratories have also substantially increased the sensitivity of instruments and allowed the determination of important radionuclides in samples of tens rather than hundreds of litres.

Most of these radionuclides entered the oceans nearly half a century ago (this interesting video gives an idea of the scale of cold war nuclear testing over the Pacific Ocean). They will continue to decay with time and some will eventually disappear in a few decades. The longer-lived isotopes, however, will continue to swim the oceans for thousands of years to come. In any case, scientists will be able to use these nuclear products to study the world’s oceans.

A momentous discovery deep below: Earth’s inner core

Marion Ferrat September 30, 2013

For the Accretionary Wedge blog festival with the theme of ‘Momentous Discoveries in Geology’, Marion Ferrat discusses how a pioneering lady discovered what lies deepest inside our planet.

We know a lot about our planet today: its position in the solar system, its age, its composition and its internal workings and structure. Many laborious experiments, observations and hypotheses have helped scientists piece together its mysteries bit by bit.

Earth within the inner solar system - Source: NASA, Wikimedia Commons.

Mercury, Venus, Earth and Mars – Source: NASA, Wikimedia Commons.

One branch of Earth Science in particular has revolutionised geoscientists’ understanding of the interior of the Earth: that branch is seismology.

Seismology is the study of seismic waves. In other words, the study of the energy released by earthquakes. Once released, this energy travels in all directions, moving from the ‘source’ point (this can be a natural earthquake or a man-made detonation), through the interior of the Earth, and back up to the surface again.

Seismology is useful because the seismic waves travel differently, and at different speeds, depending on the material they travel through. When a wave reaches a boundary between two different materials or layers within the Earth, it will be deflected: it can either be transmitted to the layer below (but in a slightly different direction), it can travel along the boundary itself, or it can be reflected back to the surface. When a wave passes through the boundary and into the next layer, the amount and direction of the deflection will depend on whether the material below is more or less dense than that above.

Seismic waves travelling through a layer of the Earth – Source: Julia Schäfer, Wikimedia Commons.

These multitude of possible pathways mean that, by looking at how and where on Earth a seismic wave arrives back at the surface, scientists can take a good guess as to what it has travelled through. By building up this information for more and more waves, they can start to paint a good picture of what is going on beneath our feet. Studying seismic waves for geoscientists is a little bit like carrying out a CAT scan for doctors: it allows them to scan the interior of something they cannot see from the outside.

Seeing to the centre of the planet

For my ‘Momentous Discovery in Geology’, I chose to look at a huge moment in the history of seismology: the discovery of the Earth’s inner core. And along with a momentous discovery, comes a momentous discoverer: Danish seismologist Inge Lehmann.

Internal structure of the Earth – Source: Kelvinsong, Wikimedia Commons.

The Earth is a little bit like an onion, in that it has layers. The outermost layer, which we live on, is called the crust. It can be as thin as a 10 kilometres under the oceans and as thick as 70 kilometres under large mountains such as the Himalayas.

Below the crust is the mantle, which makes up over 80% of our planet’s volume. The mantle is mainly solid but can behave in a viscous way when deformed very slowly, over geological timescales.

At the centre of the Earth lies the dense, metallic core. It is predominantly made of iron and nickel. The outer part of the core is liquid and plays an important role in influencing the Earth’s magnetic field.

The core lives nearly 3000 km beneath the surface and has a temperature of nearly 6000°C. It is too deep, too hot and too far to explore with any kind of instrument. This is where seismology steps in.

A liquid ball of molten metal?

Towards the beginning of the 20^th century, seismologists realised that the core must be liquid, thanks to the precious seismic waves they were observing.

When an earthquake occurs, energy is released in the form of two distinct types of seismic waves. Surface waves travel, as their name suggests, along the surface of the planet. These are the waves that cause the damage to human life and infrastructure. Body waves, on the contrary, travel inside the Earth and get deflected by the different layers they travel through, depending on whether each layer is more or less dense than its predecessor.

P- and S-waves travelling through a medium – Source: Actualist, Wikimedia Commons.

Body waves can be further split into two types, distinguishable by the way in which they displace the medium they travel through: Primary waves, or P-waves, and Secondary waves, or S-waves.

These two wave types travel differently through the Earth. One of the important characteristics of S-waves is that they cannot travel through liquid. P-waves can do but slow down considerably when not travelling through solid material.

These properties are what alerted scientists that there was something molten down in the centre of the Earth:

When seismic waves are released from an earthquake, they travel in all directions and should therefore be able to reach back to the surface all around the planet. However, seismologists noticed that seismic waves generated by an earthquake somewhere on the surface of the planet were not being observed at every seismometer on the surface. This no-wave zone is what is called the P- or S-wave shadow zone, where no arrivals can be recorded for a given earthquake.

Paths of P- and S-waves through the Earth's core: the liquid outer core cases a shadow zone - Source: USGS, WIkimedia Commons.

Paths of P-waves through the Earth’s core: the liquid outer core causes a shadow zone – Source: USGS, Wikimedia Commons.

The presence of this shadow zone meant that our P- and S-waves must be affected by something liquid, deep inside the Earth. And so arose the hypothesis of a liquid core.

Something more to the story

In 1929, a large earthquake occurred near New Zealand. Seismologists were quick to study the seismic wave arrivals at seismic stations around the world but Inge Lehmann studied them a little more closely than her peers.

She was puzzled by what she saw: seismometers located within the P-wave shadow zone of the earthquake, where no arrivals should be recorded, were showing signs of the earthquake’s waves. If the core was one large ball of liquid material, this should not be possible.

Lehmann suggested that these waves had travelled some distance inside the liquid core before bouncing off some other, previously unknown, boundary. This bouncing deflected the waves in another direction and meant that they found themselves arriving within the shadow zone.

This hypothesis was the basis of careful studying by Inge Lehmann of more seismic arrivals around the world and she eventually published her results in her revolutionary 1936 paper P’ (or P-prime). Today, the boundary between the outer and inner core is commonly known as the ‘Lehmann discontinuity’.

Inge Lehmann’s theory was later confirmed with the development of more sensitive instruments.

Lehmann was a pioneer in the world of seismology and among women scientists, establishing a new theory about the Earth in a very much male-dominated world.

In 1971, the American Geophysical Union awarded her the William Bowie medal, its highest honour. Inge Lehmann went on to live to the age of 105 and published her last paper in 1987, at the age of 99.

A momentous discoverer and scientist indeed.