Tag Archives: earth

Earth’s layered structure

Posted on by

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

Posted on by

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.