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What drives plate tectonics?

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In the previous section we described how plate tectonics controls many things on the earth both today and in the past. Here we’ll describe the forces that drive the movements of plates, the patterns of how plates have moved and how computer modelling lets us understand the links between the deep mantle and the surface plates.

The earth is affected by gravitational forces from the sun and the moon. These move huge volumes of water every  day, causing the oceanic tides. They even cause small (but measurable) changes to the shape of the earth, the surface moving up and down by a few centimetres. As the earth and moon rotate quickly, these movements are quick and elastic, meaning the earth returns back to its original shape. It’s like a tall building swaying in the wind or during an earthquake. Elastic changes are where atoms move apart from each other, but the bonds within the material are not broken. Plate tectonics is like the entire building is moving, a permanent change caused by rocks slowly flowing or breaking.

Forces caused by plates

There are broadly two sets of forces that move the earth’s plates: those created by the plates themselves and those involving interactions with the mantle below.

Plate tectonics requires plates to be rigid and deform at the edges, they are strong enough that a force acting on one part of the plate pushes the entire plate. There are two sets of forces created at the edge of oceanic plates, one where they are created and another when they destroyed.

diagram of ridge push http://www.columbia.edu/~vjd1/driving_forces_basic.htm>

Ridge push is a force created at mid-ocean ridges where the flowing mantle (asthenosphere) rises up towards the surface. This occurs because the reduction in pressure allows melting and reduces the density of the material. As oceanic lithosphere (oceanic crust plus the stiff mantle material fixed to it) moves away from the ridge it cools and sinks as its density increases. This causes a slope and gravity acting on the higher ridge causes a horizontal force that pushes the entire plate horizontally. Some scientists calculate that a lot of the force pushing India into the Eurasian plate (creating the Himalayas) comes from the many ridges in the Indian ocean pushing on the plate.

Slab pull is a force associated with subducting oceanic lithosphere. Old cold oceanic lithosphere subducts because it’s denser than the surrounding mantle, therefore this negative buoyancy causes a force pulling on the edge of the plate. As it sinks it heats up, but also it is put under increasing pressure from the rock above it. This starts to drive metamorphic reactions that change minerals in the rock into different ones, more stable under the new conditions. Generally minerals with more compact, denser mineral lattices are stable and so the density of the rock is increased. The first transformation is called eclogitisation, but some oceanic plates reach the middle and lower mantle and so will undergo multiple transformations. 

We get a sense of the strength of this force by considering how these transformed subducted rocks – eclogites – reach the surface again. Eventually any subduction zone runs out of oceanic lithosphere, and the thin leading edge of the attached continent is pulled into the subduction zone where it is transformed at depth into eclogite. Contintental crust is much more buoyant and thicker than oceanic and resists subduction, meaning that subduction eventually stops. The deeper subducted oceanic lithosphere is pulling the other way and eventually it breaks in two. Once the force of the sinking oceanic lithosphere is removed, the buried edge of the continent, together with a stub of oceanic lithosphere, is quickly pulled back to the surface – bobbing back up like a balloon under-water.

The flowing mantle

Slab pull and ridge push are together one set of forces that act once plates are moving.  In addition forces will push onto plates from the convecting mantle below.

Convection is a physical property of bodies that can flow and are hotter below than above. Hotter material  is less dense than cool, so rises up and is replaced by cooler sinking material. You can see this sometimes in cooling soup, where patterns of flow affect the surface. 

Within the earth, sinking oceanic slabs will drag mantle material down with it and so become the downward flow part of convection. Similarly mid-ocean ridges are places where heat is released from the mantle and may correspond to an upward flow. However evidence from volcanic islands such as Hawaii suggests that mantle flow is more complicated than that. Most earth scientists believe in the existence of mantle plumes, long-lived flows of hotter mantle up towards the surface. The track of a mantle plume across the Pacific explains the pattern of the Hawaiian Islands. A mantle plume that caused volcanic activity in Greenland and the British Isles when the North Atlantic Ocean opened 60 million years ago is still active under Iceland, making that portion of the mid-Atlantic ridge above the surface.

These plumes appear to be unaffected by the passage of plates above them, and some scientists regard them as being fixed in location within the earth, being deep-seated structures. 

Seeing the effects of mantle convection on the surface movement of the plates is difficult. There are places on the earth, such as southern Africa which are much higher than we would expect. It seems that this is an area of upward mantle flow and this force is raising up the African continent, forming the high plateau that covers much of South Africa.

Supercontinents

Plate tectonic movements in the past show patterns where continents joined together into supercontinents, only to split apart again. The most famous supercontinent is called Pangea and it existed 270-200 million years ago. It contained all modern continents joined together. It’s breakup led to the creation of the continent shapes we are familiar with today. The Atlantic split apart the Americas from Europe and Africa. India, Antarctica and Australia were split apart by the creation of the Indian ocean. Also the Tethys ocean closed sending India colliding into Eurasia. 

Map of Pangea showing modern plate boundaries on it. Like https://www.worldatlas.com/articles/what-is-pangea.html. There are some wonderful examples on http://www.earthdynamics.org/earthhistory/Learn%20About%20Palaeogeography.html

Pangea formed late in the earth’s history. Before it the continents were separated from each other, but by different oceans that are now totally lost (the oceanic plates are now down in the mantle somewhere). We can trace the lines of the lost oceans by the traces of the collision when they formed or by patterns of fossils. Also slices of them called ophiolites may be found, lost within the centre of continents.

The line marking an ancient ocean can be found in countries around the North Atlantic. Called Iapetus, the join where it once was is often close to the modern Atlantic. By painstakingly  tracing traces of ancient oceans, combined with computer modelling, scientists have discovered other supercontinents older than Pangea. These are from times so far back that the shapes and names of continents are unfamiliar, but the processes are the same. Up to 13 supercontinents have been identified and named, going right back to earth’s earliest rocks. They appear to form and break-up at intervals of a few hundreds of millions of years. 

Whether or not the earth’s continents are all joined together or split into different parts affects many things. Continental shelves are great places for life, creating large areas of shallow water. If all continents are joined together, there is less continental shelf for creatures to live on. Cycles of supercontinent creation and break-up have been linked to changes in climate, patterns of the evolution of life, creation of continental crust, formation of ore deposits and many other things. 

Breaking up continents is not easy as continental lithosphere is stable and strong. The break-up of supercontinents seems to be linked to mantle plumes. A plume rising above a continent will heat it and push it up. Mantle flow away from the plume will start pulling the plate apart and the heat and volcanic activity make it easier to break. Some theories suggest that a supercontinent insulates the mantle below and eventually causes a hot plume to rise beneath it. This would explain why supercontinents form and are destroyed again and again in earth history.

Computer modelling goes “beyond plate tectonics”

The only real way to understand the complicated patterns of flow in the earth is by using computer modelling. Only computers than track the different types of force and the fact that this is all happening on a spherical earth. They can be used to try and bring together the theory and the real-life observations to reproduce patterns of plate tectonics over the history of the earth.

In building the models, scientists can use a lot of equations that describe the physics of how hot rocks flow – how they deform and how convection works. They also ensure that all the forces balance, that the earth remains the same size and that the surface plates are not spinning around the world. 

Into this theoretical model they add all the observations that led scientists to produce the theory of plate tectonics in the first place. Geological evidence of continental drift shows when continents were joined or moved apart. Magnetic stripes show how ocean basins opened, seismic tomography can see ancient subducted plates and help calculate where subduction zones were in the past. Techniques like palaeomagnetism show what latitudes rocks were at in the past.

Computer models now include all of this information and link it together in a consistent way. These models can describe both the movements of tectonic plates and patterns of ancient mantle plumes. Some say this is moving beyond plate tectonics and into a deeper understanding of how the entire earth works, not just the surface.

One example of the power of these models comes from studies of the distribution of diamonds at the surface of the earth. Diamonds form deep within the mantle, potentially over much of the earth, but they only come to the surface in particular places. Diamonds reach the surface within special types of volcanic eruptions called kimberlites. These are only found in very old parts of continents in Africa, North America, Australia and Asia. Deep under old continents, there is a thick and stable layer of cold and strong mantle attached to the crust. Kimberlites form when this old material is heated and molten rock rich in carbon dioxide is formed. This super-light material quickly shoots to the surface containing fragments of mantle rock within it, sometimes with diamonds. Computer modelling of past plate movements suggested that kimberlite eruptions occur when old continental lithosphere is heated by mantle plumes rising from below. Furthermore, mantle plumes tend to rise from the edges of mysterious structures at the core-mantle boundary called LLSVPs.

<<example of results of this modelling. first diagram in https://www.pnas.org/content/111/24/8735/tab-figures-data

This particular research is fairly recent and like most new studies is not accepted by all scientists. But it illustrates the power of these computer models. If it correctly explains where and when kimberlite eruptions occur, it could help mining companies find new kimberlites and so find new diamond mines.

These computer models are never complete. Scientists using computer modelling of complex systems like climate or the earth’s interior joke that all models are wrong, but the best ones are useful. They understand that new research will improve on existing models, but ones today can increase our understanding and suggest new areas of research.

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.

Plate tectonics

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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.


Earth’s layered structure

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Six thousand kilometres isn’t a long way. Along the surface of the earth it’s the distance from Beijing to the Middle East. That distance above your head is still well within earth’s orbit and many satellites are much further out. The Middle East and the earth’s orbit are places we understand reasonably well and that people have visited.

In 1863 Jules Verne, a French author wrote “Journey to the Centre of the Earth” a fictional story about a team of geologists and adventurers who climb down a hole in a volcano and reach a huge space within the earth full of prehistoric animals. Nothing he wrote was inconsistent with scientific knowledge at the time, because we knew virtually nothing about the deep earth. 

But now we know that 6000 km down below your feet is a place that humans can never reach, that’s very different from the surface. The deep interior of the earth is a place of unimaginably high pressures and temperatures, where solid rocks flow and from which come diamonds, huge volcanic eruptions that poison the earth and also a magnetic field that protects all life from harmful radiation.

Think of a plum. There’s a very thin skin at the surface, then the sweet juicy flesh making up the biggest part of the volume and a very different part in the middle. The earth’s internal structure is like a plum. The thin skin is the crust, the next layer down the mantle and the portion in the centre is the core.


Temperature and Pressure

The very centre of the earth is 6378km deep and is hotter than the surface of the Sun (estimated 6000°C). The earth is still hot after 4.5 billion years as it contains radioactive elements such as Uranium and Thorium that slowly produce heat as they decay into other elements. They’ve been doing this for billions of years and without this heating the earth would by now be much cooler. 

The temperature slowly increases down from the surface – this is known as the geothermal gradient. Miners have always known about this, rock temperatures in the world’s deepest mines (nearly 4km deep) reach 60°C and air conditioning is required to allow the miners to work safely. But this depth is a tiny way into the earth and temperatures keep going up right to the centre of the earth.

Pressure is caused by the weight of material pushing down on something. We live in air pressure every day (caused by the weight of air above us) and only notice it when it changes (like up a mountain or travelling by airplane). Air pressure is around 100,000 Pa (Pascal, the SI unit of Pressure). The most severe pressures humans have experienced are in the deepest parts of the ocean. To travel there we need specially made spheres of the strongest metals. This is needed to protect humans from the pressure of 10 kilometres of water (c. 110,000,000 Pa).  Within the deep earth the pressure is from the weight of thousands of kilometres of rock, which is 2-3 times denser than water. Pressures in earth’s core are estimated to be 330,000,000,000 Pa. Under these conditions, no materials behave like they do at the surface. Atoms are squashed together so tightly they form structures that are very different to those under lower pressures.

The Core

The very centre of the earth is known as it’s core. The core is made of Iron, mixed in with minor amounts of Nickel and other elements that mix easily with iron. We don’t know for sure as we have no samples (and never will, to get sample you’d have to destroy the earth first). It’s likely though it is similar to iron meteorites as these are from the cores of other broken up planets that formed in the same way as the earth. 

Seismology shows that the earth’s core is made of two parts, a liquid outer and a solid inner. Metals melt when they are hot, so the fact the outer is liquid is not a surprise. It flows and these movements cause the earth’s magnetic fields. Under intense pressure even materials that are very hot can form solids. High temperatures cause atoms to vibrate and move apart, but high pressures force the atoms back together where they form bonds and become a solid. In the inner core the effect of the incredible pressure forces the hot metal to become solid.

Patterns of seismic waves suggest there it’s not the same all the way through as they travel in different speeds along different directions. There’s no clear agreement though. Understanding the inner core is like trying to understand a room only by listening to sounds through a locked door: extremely difficult.

Rocks and minerals

The other layers of the earth, the mantle and crust, are made of rock and rock is made of minerals. Let’s talk about rocks and minerals before describing those layers in more detail.

The most common type of minerals on earth are called silicate minerals. These are formed from regular rows or grids of Silicon and Oxygen. A Silicon atom surrounded by four Oxygens is called a silica tetrahedron and it forms a strong stable pyramid structure. Quartz, which is common in the contintental crust and makes up sand, is made solely of silica tetrahedra. In other silicate minerals the silica tetrahedron are mixed with other metals, commonly Iron and Magnesium, Calcium and Aluminium. Also the tetrahedron can be joined together to form chains or sheets or many other structures. The variety of structures and number of other atomic elements that can be included together make an enormous range of minerals. Hundreds of minerals exist, with names from Amethyst to Zircon.

<diagram showing silicon tetrahedron https://en.wikipedia.org/wiki/Silicon%E2%80%93oxygen_tetrahedron>

All minerals are formed from regular structures which give them consistent properties. 

By contrast rocks are just mixtures of minerals bound together in all sorts of different ways. The mineral grains can be small or large, varied or all the same, randomly mixed or shaped into patterns. Most rocks are made of silicate minerals and only a few common minerals are not silicates. Important ones include calcite that forms limestones at the surface and diamond that is formed in the earth’s mantle.

The mantle

The mantle is the layer of the earth from the top of the crust (2285km depth) nearly to the surface (on average 10-30km depth). Chemically it is doesn’t vary very much, always being made of silicate minerals rich in Iron and Magnesium with only minor amounts of other elements. This type of composition is known as ultramafic.

Slices of mantle sometimes make it to the surface, where it’s usually made of a rock type called peridotite including minerals called olivine and pyroxene. It’s heavy and dark, usually looking unusual compared to the more familiar rocks of the crust.

Mantle rock deeper in the earth is chemically the same, but made of different minerals. Like between the inner and outer core, the reason for this is an increase in pressure.

As pressures increase deeper in the mantle different types of mineral stop being stable but instead are replaced by more exotic ones with a similar chemistry but different more compact structures and names like perovskite and bridgmanite. Under these higher pressures, silica tetrahedra are no longer stable, instead atoms are packed together more tightly in different ways. The change from one mineral type to another occurs over a small range of pressures and is called a ‘phase transition’.

Seismic data shows faint changes in rock properties at depths of 410 km and 660km depths. These ‘discontinuities’ occur not because the mantle composition changes, but because it’s where these phase transitions occur. The upper mantle is defined as that above 410km discontinuity which is where the pressure is reached at which olivine is transformed. The transition zone continues down to 660km where another complicated phase transition marks the top of the lower mantle.

Studying high-pressure minerals

We’re stuck at the surface, but we can study these exotic high-pressure minerals in two different ways, both involving diamonds.

Diamonds are a high-pressure form of pure Carbon that grow within the earth’s mantle and are brought to the surface in odd volcanic structures called kimberlite pipes. Some diamonds contain inclusions of these deep mantle minerals, which make them useless for jewelers but great for scientists. 

Scientists also reproduce conditions deep within the earth using diamonds in the laboratory. To create intense pressures they squash samples between two super-strong diamonds. Firing lasers through the diamonds increases the temperature of the sample also. These experiments allow us to reproduce the phase transitions and so link them to the discontinuities we see in the seismic data.

Mantle flow

One of the most remarkable things about rocks deep in the earth is that they flow, even while they remain solid. Only very small areas of the mantle are molten, but all of it flows. Hotter material at the base flows upwards, cool and sinks down. Remember that this happens very very slowly. Rocks move only a few centimetres a year so a journey from top of the mantle to the bottom takes 100s of millions of years. To understand how, we need to talk about minerals again.

Silicate minerals are made of regular rows and layers of atoms. Under pressure, at high temperatures, these rows or layers can move past each other, atom by atom slowly swapping around. This plastic deformation slowly changes the shape of the grains, turning spheres into pancake shapes, for example. Over millions of years, with small changes in countless numbers of mineral grains solid rocks can flow and move. Water ice is another type of mineral (just not one that forms rocks) and the flow of glaciers down hill is the same mechanism, only faster.

One consequence of the fact that rocks can flow is that the earth is not a perfect sphere. The rotation of the earth pushes material out around the equator and flattens the poles. This forms a shape called an oblate spheroid. The effect is small, but it means you could argue the world’s tallest mountain is Mt Chimborazo in Ecuador. It’s not the highest above sea-level (that’s Mt Everest) but since it is near the Equator the oblate spheroid shape means it is the furthest point from the centre of the earth.

Layers in the upper mantle

Towards the top of the upper mantle is the asthenosphere. This is a particularly soft layer that contains pockets of molten rock. Above this is the lithospheric mantle. This is material attached the base of crust and together they make up the lithosphere, rigid plates that drift slowly over the surface of the earth. The boundary between mantle and crust is known as the Mohorovičić discontinuity or “Moho”, named after the Croatian seismologist who first identified it over a hundred years ago. There are many places on earth where slices of deep rocks are found at the surface and you can stand on the Moho.

The Earth’s Crust

The top layer of the earth, the one we know best, is the crust. This comes in two types, oceanic and continental. Oceanic crust forms where plates move apart and mantle peridotite moves up towards the surface. This stays hot and as the pressure reduces it starts to melt. Melting peridotite produces magma with a different composition, more rich in Silica called mafic (meaning rich in Mg and Fe). This magma rises and cools forming the oceanic crust above. The cooling magma forms new crystals making a coarse rock called gabbro deep down and finer-grained dolerite and basalt above. Oceanic crust is only around 7-10 km thick. It’s a very thin skin indeed.

If basalt or gabbro melt, it in turn produces rocks even richer in silica called felsic (meaning rich in feldspar). Common felsic rocks are andesite and granite. Continental crust is on average of andesitic composition and much less dense and thicker than oceanic crust, on average 35 km thick. This is where all of the earth’s really old rocks are found.

The crust is where the most variety of rocks is seen as sedimentary rocks form only at the surface. Oxygen in the atmosphere and interactions with life have created many new minerals not found in the mantle. 

The Deep Earth may be somewhere that humans can never visit, but it’s different layers do interact together and sometimes affect what happens up here on the surface. We’ll discuss this more in the sections on plate tectonics.

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.

Earth as a planet

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Formation of the solar system

The earth, our home, is the third of 8 planets orbiting a star, our Sun. Together they make up the solar system. To understand how our planet formed, we need to know how solar systems are created and how they develop. 

Around 4.5 billion years ago there was a cloud of dust, ice and gas floating in space called a molecular cloud. A nearby old star exploded – a supernova explosion – and the shockwave disturbed the cloud making denser areas.  Slowly, gravity pulled material towards these dense parts, further increasing its mass and so pulling in yet more material, a process called gravitational collapse. The physics of conservation of angular momentum caused the material in the cloud it to spin faster and faster, to heat up and to flatten into a disc shape rotating about centre. 

Within the centre of the cloud hot hydrogen gas was squashed together until eventually the atomic nuclei started joining together and a process called nuclear fusion started. This produced heat and light and made a stable body hot bright body called a star. The Sun is the star at the centre of our solar system. Today astronomers can see stars being born in the same way within molecular clouds far away.

Formation of the Earth and other planets

The Sun is 99.86% of the mass of the solar system, but it’s worth talking about the remaining 0.14% as that’s what we are made of. As the Sun started to shine it was surrounded by a rotating disc, made up of dust, ice and gas called a solar nebula.

Within this disc a process called accretion started, where dust grains started clumping together in time forming larger bodies called planetisimals which are about 10km across. This is very like the way raindrops in clouds on earth, where tiny droplets of water join together until they are big enough to fall as rain. Scientists recently studied a planetisimal called 2014 MU19. It is very irregular and has a very low density: the dust and ice is only lightly joined together by gravity.

Planetisimals are rare today. Most collided together to form bodies around 100km in diameter called protoplanets. Some of these survive in the belt between Mars and Jupiter and are called asteroids. It’s thought that in total about a few hundred of these protoplanets were formed within the Solar System. Over time these protoplanets collided and when they did they tended to form larger and larger bodies. Eventually most material ended up in the limited number of planets were are familiar with today.

Not all planets are the same. The sun heated up the centre of the disc, forming a region called the Inner Solar System where it was too hot for water or methane in the solar nebula to form solid ice. Therefore planets that formed here (Mercury, Venus, Earth and Mars) were mostly formed from the dust and are now called the rocky planets. Further out where it is colder, the gas giants (Jupiter, Saturn, Neptune and Uranus) formed from cores of rock and ice together. Being large they also captured gas within the nebula  and are now much bigger, containing huge volumes of ice and gas. 

Planetary differentiation

As planetisimals and protoplanets collided and joined together, the force of impact made the material extremely hot. Also bigger protoplanets are able to stay hotter inside for longer. This heat meant that the dust of the nebula is compressed and melted to form dense rocky planets.

The original dust material was very rich in Iron, which together with smaller amounts of Nickel and Sulphur form dense metal alloys. Within hot and maybe molten planets, this denser material sank down into the centre of the planet formed a metal core, a process called planetary differentiation. The material left behind, surrounding the metal core was mostly made of Silicon, Oxygen, Iron, Magnesium, Calcium and Aluminium. These elements together form minerals based on regular lattices of Silicon and Oxygen. These silicate minerals form most of our planet – they are within most of the rocks we see. 

The exact mechanism by this the iron and rock separated out is not known. Perhaps following collisions the entire planet was molten and liquid iron separated out from liquid rock (magma). Even if the rock was solid, liquid metal may have slowly sunk between cracks and gaps to flow toward the centre of the planet. Whatever the details, we do know it happened, both from studies of our own earth and from meteorites.

Meteorites and Comets

Meteorites – small fragments from elsewhere in the solar system that fall to earth – are very varied. Some contain ancient dust grains from the original cloud. Others are pieces of small planets that were broken up by later impacts. Iron meteorites are from the cores and stony meteorites the remainder of these broken up planets. Pallasites, the most beautiful of meteorites, are a mixture of the two, with crystals of a silicate mineral called olivine set within metal. Without evidence from meteorites we would not be able to tell the story of how our planet was formed.

Not all things that hit the earth from outside are meteorites. Comets are bodies that come from the cold outer reaches of the solar system. They are mixtures of dust and ice and are common in the  Kuiper Belt and Oort cloud, areas of the solar system that sit beyond the orbits of the planets. Pluto, which until 2006 was classified as a planet, is now classified as a dwarf planet, along with other bodies that sit within the Kuiper Belt.

Formation of the Moon

Planets commonly have moons orbiting them, just like planets orbit the Sun. They may be meteorites captured by the planet’s gravity, or form from material orbiting the planet. But the earth’s moon is far too large to form that way. 

The most popular theory for how our moon formed is that is that there was an enormous collision between the earth and a protoplanet called Theia. Things move fast in space. We measure the speed of cars in units of kilometres per hour. The speed of objects in space is measured in kilometres per second. So when two planets collided, the impact was enormous. The planets were smashed into pieces and partly merged together. Huge volumes of the earth’s rocky mantle were pushed out into orbit around the planet. This then formed into our moon. The impact would have caused an enormous amount of heat to be released. Some studies suggest the entire earth could have become molten, with a global ocean of magma formed right up to the surface.

It’s very hard to know for certain to know if this theory is correct, as it happened so long ago. But it does explain the fact the Moon is very large and has a small core. Also we know from rock samples brought from the Moon that it’s geology and isotope chemistry is very similar to that of the Earth’s. Scientists are working hard to better understand how the Moon formed, by studying Moon rock chemistry with specialist equipment or by creating detailed computer models of the impact.

Late Heavy Bombardment

Over time the chaotic molecular cloud formed colliding planetisimals and then hot differentiated planets and then eventually the calm regular solar system we know today, with a small number of planets in stable orbits. This was a gradual process and collisions with large bodies were important for the earth’s early history even after the formation of the Moon.

The Late Heavy Bombardment is a period of time between 4.1 and 3.8 billion years ago when the rate of collisions was particularly high. The cause is not known, but one theory is that the large outer planets moved their orbits, disrupting the asteroid or Kuiper belts and so pushing many asteroids or comets out of stable orbits into paths that caused them to hit the earth, moon and other planets.

Impacts on Earth

Finding direct evidence of ancient impacts on earth is difficult as so few rocks from that time still exist. The earth is unusual in having plate tectonics and active erosion, processes that act to destroy old rocks at the surface. It’s only since the 1980s that earth scientists have worked out how to find traces of ancient impacts. 

When a large object hits the earth at huge speed the impact creates a huge hole in the ground, a crater. The material from the crater is thrown out over a large area forming layers of distinctive rocks. Some is molten rock that quick cools to form glass. Some, called tektites, were formed from relatively recent impacts (less than a million years ago) and can even be found at the surface today. 

Impacts create special minerals that cannot be formed by normal processes on earth and the meteorite may be rich in elements that are rare on the earth’s surface, such as Iridium. The Chicxulub impact, that hit when the dinosaurs became extinct, spread out Iridium to form a distinctive layer across the entire world. Within ancient sedimentary rocks we are finding more and more of these layers. 

When large meteorites or comets hit earth they make a big mess, but they don’t hit very often. Smaller pieces arrive every day. If you’ve ever seen a ‘shooting star’ you’ve seen a small fragment entering the atmosphere and heating up due to friction. Really tiny pieces are falling to earth all the time: one might be falling onto your roof right now! A recent study of material trapped in gutters on buildings found microscopic fragments that had fallen from space. 

Water and Life

We’ve talked about rocks and metal, but our earth is covered in water and teeming with life.

If the impact of Theia did create a global magma ocean, then all of the water on earth would have boiled away, leaving the planet as dry and lifeless as the moon. All life we know of is dependent on liquid water, but we don’t really know where earth’s water came from. It’s likely that much of it came from impacts of smaller meteoroids and especially comets, rich in water ice. Some of the water in your glass may have come originally from a frozen comet that hit the earth long ago.

As well as water, comets contain organic compounds. These weren’t formed by life, just that they are certain types of molecules that contain carbon. We don’t know where life formed or how, but we do know that it needs water and organic molecules, both of which were brought to earth by comets. 

When humans send probes to other planets, they try really hard to make them sterile to avoid contaminating other planets with earth organisms. That’s because travelling through space doesn’t necessarily kill such tiny and tough forms of life. Impacts can send rocks out into space and eventually onto other planets. We know of pieces of Mars that arrived here this way. In the early history of the solar system, tiny organisms may have travelled between planets hitching a lift on these fragments. Maybe life actually first started on Mars (when it was young and wet) and then came to earth via a meteorite. We’ve no direct evidence of this (yet) but it’s a great reminder that our planet is part of the solar system and still interacts with it in surprising and important ways.

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.