Who Was Democritus?

As the philosopher Nietzsche famously said “He who would learn to fly one day must first learn to stand and walk and run and climb and dance; one cannot fly into flying.” This is certainly true when it comes to humanity’s understanding of the universe, something which has evolved over many thousands of years and been the subject of ongoing discovery.

And along the way, many names stand out as examples of people who achieved breakthroughs and helped lay the foundations of our modern understanding. One such person is Democritus, an ancient Greek philosopher who is viewed by many as being the “father of modern science”. This is due to his theory of universe that is made up of tiny “atoms”, which bears a striking resemblance to modern atomic theory.

Though he is typically viewed as one of Greece’s many pre-Socratic natural philosopher, many historians have argued that he is more rightly classified as a scientist, at least when compared to his contemporaries. There has also been significant controversy – particularly in Germany during the 19th century – over whether or not Democritus deserves credit for atomic theory.

This argument is based on the relationship Democritus had with contemporary philosopher Leucippus, who is renowned for sharing his theory about atoms with him. However, their theories came down to a different basis, a distinction that allows Democritus to be given credit for a theory that would go on to become a staple of the modern scientific tradition.

Hendrik ter Brugghen - Heraclitus, 1628. Credit: rijksmuseum.nl
Democritus, by Hendrik ter Brugghen – Heraclitus, 1628. Credit: rijksmuseum.nl

Birth and Early Life:

The precise date and location of Democritus birth is the subject the debate. While most sources claim he was born in Abdera, located in the northern Greek province of Thrace, around 460 BCE. However, other sources claim he was born in Miletus, a coastal city of ancient Anatolia and modern-day Turkey, and that he was born in 490 BCE.

It has been said that Democritus’ father was from a noble family and so wealthy that he received the Persian king Xerxes on the latter’s march through Abdera during the Second Persian War (480–479 BC). It is further argued that as a reward for his service, the Persian monarch gave his father and other Abderites gifts, and left several Magi among them. Democritus was apparently instructed by these Magi in astronomy and theology.

After his father had died, Democritus used his inheritance to finance a series of travels to distant countries. Desiring to feed his thirst for knowledge, Democritus traveled extensively across the known world, traveling to Asia, Egypt and (according to some sources) venturing as far as India and Ethiopia. His writings include descriptions of the the cities of Babylon and Meroe (in modern-day Sudan).

Upon returning to his native land, he occupied himself with the study of natural philosophy. He also traveled throughout Greece to acquire a better knowledge of its cultures and learned from many of Greece’s famous philosophers. His wealth allowed him to purchase their writings, and he wrote of them in his own works. In time, he would become one of the most famous of the pre-Socratic philosophers.

The ruins of the ancient Greeof Abdera, with the west gate shown. Credit:
The ruins of the ancient Greek city of Abdera, with the west gate shown. Credit: Wikipedia Commons/Marysas

Leucippus of Miletus had the greatest influence on him, becoming his mentor and sharing his theory of atomism with him. Democritus is also said to have known Anaxagoras, Hippocrates and even Socrates himself (though this remains unproven). During his time in Egypt, he learned from Egyptian mathematicians, and is said to have become acquainted with the Chaldean magi in Assyria.

In the tradition of the atomists, Democritus was a thoroughgoing materialists who viewed the world in terms of natural laws and causes. This differentiated him from other Greek philosophers like Plato and Aristotle, for whom philosophy was more teleological in nature – i.e. more concerned with the purpose of events rather than the causes, as well things like essence, the soul, and final causes.

According to the many descriptions and anecdotes about Democritus, he was known for his modesty, simplicity, and commitment to his studies. One story claims he blinded himself on purpose in order to be less distracted by worldly affairs (which is believed to be apocryphal). He was also known for his sense of humor and is commonly referred to as the “Laughing Philosopher” – for his capacity to laugh at human folly. To his fellow citizens, he was also known as “The Mocker”.

Scientific Contributions:

Democritus is renowned for being a pioneer of mathematics and geometry. He was among the first Greek philosophers to observe that a cone or pyramid has one-third the volume of a cylinder or prism with the same base and height. While none of his works on the subject survived the Middle Ages, his mathematical proofs are derived from other works with contain extensive citations to titles like On Numbers, On Geometrics, On Tangencies, On Mapping, and On Irrationals.

Right circular and oblique circular cones. Credit: Dominique Toussaint
Right circular and oblique circular cones. Credit: Dominique Toussaint

Democritus is also known for having spent much of his life experimenting with and examining plants and minerals. Similar to his work in mathematics and geometry, citations from existing works are used to infer the existence of works on the subject. These include On the Nature of Man, the two-volume collection On Flesh, On Mind, On the Senses, On Flavors, On Colors, Causes concerned with Seeds and Plants and Fruits, and to the three-volume collection Causes concerned with Animals.

From his examination of nature, Democritus developed what could be considered some of the first anthropological theories. According to him, human beings lived short lives in archaic times, forced to forage like animals until fear of wild animals then drove them into communities. He theorized that such humans had no language, and only developed it through the need to articulate thoughts and ideas.

Through a process of trial and error, human beings developed not only verbal language, but also symbols with which to communicate (i.e. written language), clothing, fire, the domestication of animals, and agriculture. Each step in this process led to more discoveries, more complex behaviors, and the many things that came to characterize civilized society.

In terms of astronomy and cosmology, Democritus was a proponent of the spherical Earth hypothesis. He believed that in the original chaos from which the universe sprang, the universe was composed of nothing but tiny atoms that came together to form larger units (a theory which bears a striking resemblance to The Big Bang Theory and Nebular Theory). He also believed in the existence of many worlds, which were either in state of growth or decay.

In a similar vein, Democritus advanced a theory of void which challenged the paradoxes raised by his fellow Greek philosophers, Parmenides and Zeno – the founders of metaphysical logic. According to these men, movement cannot exist because such a thing requires there to be a void – which is nothing, and therefore cannot exist. And a void cannot be termed as such if it is in fact a definable, existing thing.

To this, Democritus and other atomists argued that since movement is an observable phenomena, there must be a void. This idea previewed Newton’s theory of absolute space, in which space exists independently of any observer or anything external to it. Einstein’s theory of relativity also provided a resolution to the paradoxes raised by Parmenides and Zeno, where he asserted that space itself is relative and cannot be separated from time.

Democritus’ thoughts on the nature of truth also previewed the development of the modern scientific method. According to Democritus, truth is difficult, because it can only be perceived through senses-impressions which are subjective. Because of this, Aristotle claimed in his Metaphysics that Democritus was of the opinion that “either there is no truth or to us at least it is not evident.”

However, as Diogenes Laertius quoted in his 3rd century CE tract, Lives and Opinions of Eminent Philosophers: “By convention hot, by convention cold, but in reality atoms and void, and also in reality we know nothing, since the truth is at bottom.”

Diogenes Laërtius: Lives and Opinions of Eminent Philosophers. A biography of the Greek philosophers. Title page from year 1594. Credit: Public Domain
Diogenes Laertius, Lives and Opinions of Eminent Philosophers, makes mention of Democritus and his theories. Credit: Public Domain

Ultimately, Democritus’ opinion on truth came down to a distinction between two kinds of knowledge – “legitimate” (or “genuine”) and bastard (or “secret”). The latter is concerned with perception through the senses, which is subjective by nature. This is due to the fact that our sense-perception are influence by the shape and nature of atoms as they flow out from the object in question and make an impression on our senses.

“Legitimate” knowledge, by contrast, is achieved through the intellect, where sense-data is elaborated through reasoning. In this way, one can get from “bastard” impressions to the point where things like connections, patterns and causality can be determined. This is consistent with the inductive reasoning method later elaborated by Renee Descartes, and is a prime example of why Democritus is considered to be an early scientific thinker.

Atomic Theory:

However, Democritus greatest contribution to modern science was arguably the atomic theory he elucidated. According to Democritus’ atomic theory, the universe and all matter obey the following principles:

  • Everything is composed of “atoms”, which are physically, but not geometrically, indivisible
  • Between atoms, there lies empty space
  • Atoms are indestructible
  • Atoms have always been, and always will be, in motion
  • There are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.

He was not alone in proposing atomic theory, as both his mentor Leucippus and Epicurus are believed to have proposed the earliest views on the shapes and connectivity of atoms. Like Democritus, they believed that the solidity of a material corresponded to the shape of the atoms involved – i.e. iron atoms are hard, water atoms are smooth and slippery, fire atoms are light and sharp, and air atoms are light and whirling.

Democritus' model of an atom was one of an intert solid that ineracted mechanically with other atoms. Credit: .science.edu.sg
Democritus’ model of an atom was one of an inert solid that interacted mechanically with other atoms. Credit: .science.edu.sg

However, Democritus is credited with illustrating and popularizing the concept, and for his descriptions of atoms which survived classical antiquity to influence later philosophers. Using analogies from our sense experiences, Democritus gave a picture or an image of an atom that distinguished them from each other by their shape, size, and the arrangement of their parts.

In essence, this model was one of an inert solid that excluded other bodies from its volume, and which interacted with other atoms mechanically. As such, his model included physical links (i.e. hooks and eyes, balls and sockets) that explained how connections occurred between them. While this bears little resemblance to modern atomic theory (where atoms are not inert and interact electromagnetically), it is more closely aligned with that of modern science than any other theory of antiquity.

While there is no clear explanation as to how scholars of classical antiquity came to theorize the existence of atoms, the concept proved to be influential, being picked up by Roman philosopher Lucretius in the 1st century CE and again during the Scientific Revolution. In addition to being indispensable to modern molecular and atomic theory, it also provided an explanation as to why the concept of a void was necessary in nature.

If all matter was composed of tiny, indivisible atoms, then there must also be a great deal of open space between them. This reasoning has also gone on to inform out notions of cosmology and astronomy, where Einstein’s theory of special relativity was able to do away with the concept of a “luminiferous aether” in explaining the behavior of light.

Early atomic theory stated that different materials had differently shaped atoms. Credit: github.com
Early atomic theory stated that different materials had differently shaped atoms. Credit: github.com

Diogenes Laertius summarized Democritus atomic theory as follows in Lives and Opinions of Eminent Philosophers:

“That atoms and the vacuum were the beginning of the universe; and that everything else existed only in opinion. That the worlds were infinite, created, and perishable. But that nothing was created out of nothing, and that nothing was destroyed so as to become nothing. That the atoms were infinite both in magnitude and number, and were borne about through the universe in endless revolutions. And that thus they produced all the combinations that exist; fire, water, air, and earth; for that all these things are only combinations of certain atoms; which combinations are incapable of being affected by external circumstances, and are unchangeable by reason of their solidity.”

Death and Legacy:

Democritus died at the age of ninety, which would place his death at around 370 BCE; though some writers disagree, with some claiming he lived to 104 or even 109. According to Marcus Aurelius’ book Meditations, Democritus was eaten by lice or vermin, although in the same passage he writes that “other lice killed Socrates”, implying that this was meant metaphorically. Since Socrates died at the hands of the Athenian government who condemned him, it is possible that Aurelius attributed Democritus death to human folly or politics.

While Democritus was highly esteemed amongst his contemporaries, there were also those who resented him. This included Plato who, according to some accounts, disliked him so much that he wished that all his books would be burned. However, Plato’s pupil Aristotle was familiar with the works of Democritus and mentioned him in both Metaphysics and Physics, where he described him as a “physicist” who did not concern himself with the ideals of form or essence.

Democritus meditating on the seat of the soul by Léon-Alexandre Delhomme (1868). Credit: Pubic Domain
Democritus meditating on the seat of the soul, by Léon-Alexandre Delhomme (1868). Credit: Pubic Domain

Ultimately, Democritus is credited as being one of the founders of the modern science because his methods and theories closely resemble those of modern astronomers and physicists. And while his version of the atomic model differs greatly from our modern conceptions, his work was of undoubted value, and was a step in an ongoing process that included such scientists as John Dalton, Neils Bohr and even Albert Einstein.

As always, science is an process of continuing discovery, where new breakthroughs are built upon the foundations of the old and every generations attempts to see a little farther by standing on the shoulders of those who came before.

We have many interesting articles about atomic theory here at Universe Today. Here’s one about John Dalton’s atomic model, Neils Bohr’s atomic model, the “Plum Pudding” atomic model.

For more information, check out The History of the Atom – Democritus.

Astronomy Cast has a wonderful episode on the subject, titled Episode 392: The Standard Model – Intro

What are the Earth’s Layers?

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

There is more to the Earth than what we can see on the surface. In fact, if you were able to hold the Earth in your hand and slice it in half, you’d see that it has multiple layers. But of course, the interior of our world continues to hold some mysteries for us. Even as we intrepidly explore other worlds and deploy satellites into orbit, the inner recesses of our planet remains off limit from us.

However, advances in seismology have allowed us to learn a great deal about the Earth and the many layers that make it up. Each layer has its own properties, composition, and characteristics that affects many of the key processes of our planet. They are, in order from the exterior to the interior – the crust, the mantle, the outer core, and the inner core. Let’s take a look at them and see what they have going on.

Modern Theory:

Like all terrestrial planets, the Earth’s interior is differentiated. This means that its internal structure consists of layers, arranged like the skin of an onion. Peel back one, and you find another, distinguished from the last by its chemical and geological properties, as well as vast differences in temperature and pressure.

Our modern, scientific understanding of the Earth’s interior structure is based on inferences made with the help of seismic monitoring. In essence, this involves measuring sound waves generated by earthquakes, and examining how passing through the different layers of the Earth causes them to slow down. The changes in seismic velocity cause refraction which is calculated (in accordance with Snell’s Law) to determine differences in density.

Model of a flat Earth
Model of a flat Earth, with the continents modeled in a disk-shape and Antarctica as an ice wall. Credit: Wikipedia Commons

These are used, along with measurements of the gravitational and magnetic fields of the Earth and experiments with crystalline solids that simulate pressures and temperatures in the Earth’s deep interior, to determine what Earth’s layers looks like. In addition, it is understood that the differences in temperature and pressure are due to leftover heat from the planet’s initial formation, the decay of radioactive elements, and the freezing of the inner core due to intense pressure.

History of Study:

Since ancient times, human beings have sought to understand the formation and composition of the Earth. The earliest known cases were unscientific in nature – taking the form of creation myths or religious fables involving the gods. However, between classical antiquity and the medieval period, several theories emerged about the origin of the Earth and its proper makeup.

Most of the ancient theories about Earth tended towards the “Flat-Earth” view of our planet’s physical form. This was the view in Mesopotamian culture, where the world was portrayed as a flat disk afloat in an ocean. To the Mayans, the world was flat, and at it corners, four jaguars (known as bacabs) held up the sky. The ancient Persians speculated that the Earth was a seven-layered ziggurat (or cosmic mountain), while the Chinese viewed it as a four-side cube.

By the 6th century BCE, Greek philosophers began to speculate that the Earth was in fact round, and by the 3rd century BCE, the idea of a spherical Earth began to become articulated as a scientific matter. During the same period, the development of a geological view of the Earth also began to emerge, with philosophers understanding that it consisted of minerals, metals, and that it was subject to a very slow process of change.

Edmond Halley's model of a Hallow Earth, one that was made up of concentric spheres.
Illustration of Edmond Halley’s model of a Hallow Earth, one that was made up of concentric spheres. Credit: Wikipedia Commons/Rick Manning

However, it was not until the 16th and 17th centuries that a scientific understanding of planet Earth and its structure truly began to advance. In 1692, Edmond Halley (discoverer of Halley’s Comet) proposed what is now known as the “Hollow-Earth” theory. In a paper submitted to Philosophical Transactions of Royal Society of London, he put forth the idea of Earth consisting of a hollow shell about 800 km thick (~500 miles).

Between this and an inner sphere, he reasoned there was an air gap of the same distance. To avoid collision, he claimed that the inner sphere was held in place by the force of gravity. The model included two inner concentric shells around an innermost core, corresponding to the diameters of the planets Mercury, Venus, and Mars respectively.

Halley’s construct was a method of accounting for the values of the relative density of Earth and the Moon that had been given by Sir Isaac Newton, in his Philosophiæ Naturalis Principia Mathematica (1687) – which were later shown to be inaccurate. However, his work was instrumental to the development of geography and theories about the interior of the Earth during the 17th and 18th centuries.

Another important factor was the debate during the 17th and 18th centuries about the authenticity of the Bible and the Deluge myth. This propelled scientists and theologians to debate the true age of the Earth, and compelled the search for evidence that the Great Flood had in fact happened. Combined with fossil evidence, which was found within the layers of the Earth, a systematic basis for identifying and dating the Earth’s strata began to emerge.

Credit: minerals.usgs.gov
The growing importance of mining in the 17th and 18th centuries, particularly for precious metals, led to further developments in geology and Earth sciences. Credit: minerals.usgs.gov

The development of modern mining techniques and growing attention to the importance of minerals and their natural distribution also helped to spur the development of modern geology. In 1774, German geologist Abraham Gottlob Werner published Von den äusserlichen Kennzeichen der Fossilien (On the External Characters of Minerals) which presented a detailed system for identifying specific minerals based on external characteristics.

In 1741, the National Museum of Natural History in France created the first teaching position designated specifically for geology. This was an important step in further promoting knowledge of geology as a science and in recognizing the value of widely disseminating such knowledge. And by 1751, with the publication of the Encyclopédie by Denis Diderot, the term “geology” became an accepted term.

By the 1770s, chemistry was starting to play a pivotal role in the theoretical foundation of geology, and theories began to emerge about how the Earth’s layers were formed. One popular idea had it that liquid inundation, like the Biblical Deluge, was responsible for creating all the geological strata. Those who accepted this theory became known popularly as the Diluvianists or Neptunists.

Another thesis slowly gained currency from the 1780s forward, which stated that instead of water, strata had been formed through heat (or fire). Those who followed this theory during the early 19th century referred to this view as Plutonism, which held that the Earth formed gradually through the solidification of molten masses at a slow rate. These theories together led to the conclusion that the Earth was immeasurably older than suggested by the Bible.

HMS Beagle in the Galapagos (painted by John Chancellor) - Credit: hmsbeagleproject.otg
HMS Beagle in the Galapagos Islands, painted by John Chancellor. Credit: hmsbeagleproject.otg

In the early 19th century, the mining industry and Industrial Revolution stimulated the rapid development of the concept of the stratigraphic column – that rock formations were arranged according to their order of formation in time. Concurrently, geologists and natural scientists began to understand that the age of fossils could be determined geologically (i.e. that the deeper the layer they were found in was from the surface, the older they were).

During the imperial period of the 19th century, European scientists also had the opportunity to conduct research in distant lands. One such individual was Charles Darwin, who had been recruited by Captain FitzRoy of the HMS Beagle to study the coastal land of South America and give geological advice.

Darwin’s discovery of giant fossils during the voyage helped to establish his reputation as a geologist, and his theorizing about the causes of their extinction led to his theory of evolution by natural selection, published in On the Origin of Species in 1859.

During the 19th century, the governments of several countries including Canada, Australia, Great Britain and the United States began funding geological surveys that would produce geological maps of vast areas of the countries. Thought largely motivated by territorial ambitions and resource exploitation, they did benefit the study of geology.

The Earth's Tectonic Plates. Credit: msnucleus.org
The Earth’s Tectonic Plates. Credit: msnucleus.org

By this time, the scientific consensus established the age of the Earth in terms of millions of years, and the increase in funding and the development of improved methods and technology helped geology to move farther away from dogmatic notions of the Earth’s age and structure.

By the early 20th century, the development of radiometric dating (which is used to determine the age of minerals and rocks), provided the necessary the data to begin getting a sense of the Earth’s true age. By the turn of the century, geologists now believed the Earth to be 2 billion years old, which opened doors for theories of continental movement during this vast amount of time.

In 1912, Alfred Wegener proposed the theory of Continental Drift, which suggested that the continents were joined together at a certain time in the past and formed a single landmass known as Pangaea. In accordance with this theory, the shapes of continents and matching coastline geology between some continents indicated they were once attached together.

The super-continent Pangea during the Permian period (300 - 250 million years ago). Credit: NAU Geology/Ron Blakey
The super-continent Pangea during the Permian period (300 – 250 million years ago). Credit: NAU Geology/Ron Blakey

Research into the ocean floor also led directly to the theory of Plate Tectonics, which provided the mechanism for Continental Drift. Geophysical evidence suggested lateral motion of continents and that oceanic crust is younger than continental crust. This geophysical evidence also spurred the hypothesis of paleomagnetism, the record of the orientation of the Earth’s magnetic field recorded in magnetic minerals.

Then there was the development of seismology, the study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies, in the early 20th century. By measuring the time of travel of refracted and reflected seismic waves, scientists were able to gradually infer how the Earth was layered and what lay deeper at its core.

For example, in 1910, Harry Fielding Ried put forward the “elastic rebound theory”, based on his studies of the 1906 San Fransisco earthquake. This theory, which stated that earthquakes occur when accumulated energy is released along a fault line, was the first scientific explanation for why earthquakes happen, and remains the foundation for modern tectonic studies.

Earth viewed from the Moon by the Apollo 11 spacecraft. Credit: NASA
Earth viewed from the Moon by the Apollo 11 spacecraft. Credit: NASA

Then in 1926, English scientist Harold Jeffreys claimed that below the crust, the core of the Earth is liquid, based on his study of earthquake waves. And then in 1937, Danish seismologist Inge Lehmann went a step further and determined that within the earth’s liquid outer core, there is a solid inner core.

By the latter half of the 20th century, scientists developed a comprehensive theory of the Earth’s structure and dynamics had formed. As the century played out, perspectives shifted to a more integrative approach, where geology and Earth sciences began to include the study of the Earth’s internal structure, atmosphere, biosphere and hydrosphere into one.

This was assisted by the development of space flight, which allowed for Earth’s atmosphere to be studied in detail, as well as photographs taken of Earth from space. In 1972, the Landsat Program, a series of satellite missions jointly managed by NASA and the U.S. Geological Survey, began supplying satellite images that provided geologically detailed maps, and have been used to predict natural disasters and plate shifts.

Earth’s Layers:

The Earth can be divided into one of two ways – mechanically or chemically. Mechanically – or rheologically, meaning the study of liquid states – it can be divided into the lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. But chemically, which is the more popular of the two, it can be divided into the crust, the mantle (which can be subdivided into the upper and lower mantle), and the core – which can also be subdivided into the outer core, and inner core.

The inner core is solid, the outer core is liquid, and the mantle is solid/plastic. This is due to the relative melting points of the different layers (nickel–iron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases. At the surface, the nickel-iron alloys and silicates are cool enough to be solid. In the upper mantle, the silicates are generally solid but localized regions of melt exist, leading to limited viscosity.

In contrast, the lower mantle is under tremendous pressure and therefore has a lower viscosity than the upper mantle. The metallic nickel–iron outer core is liquid because of the high temperature. However, the intense pressure, which increases towards the inner core, dramatically changes the melting point of the nickel–iron, making it solid.

The differentiation between these layers is due to processes that took place during the early stages of Earth’s formation (ca. 4.5 billion years ago). At this time, melting would have caused denser substances to sink toward the center while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron, along with nickel and some lighter elements, whereas less dense elements migrated to the surface along with silicate rock.

Earth’s Crust:

The crust is the outermost layer of the planet, the cooled and hardened part of the Earth that ranges in depth from approximately 5-70 km (~3-44 miles). This layer makes up only 1% of the entire volume of the Earth, though it makes up the entire surface (the continents and the ocean floor).

The Earth's layers (strata) shown to scale. Credit: pubs.usgs.gov
The Earth’s layers (strata) shown to scale. Credit: pubs.usgs.gov

The thinner parts are the oceanic crust, which underlies the ocean basins at a depth of 5-10 km (~3-6 miles), while the thicker crust is the continental crust. Whereas the oceanic crust is composed of dense material such as iron magnesium silicate igneous rocks (like basalt), the continental crust is less dense and composed of sodium potassium aluminum silicate rocks, like granite.

The uppermost section of the mantle (see below), together with the crust, constitutes the lithosphere – an irregular layer with a maximum thickness of perhaps 200 km (120 mi). Many rocks now making up Earth’s crust formed less than 100 million (1×108) years ago. However, the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that Earth has had a solid crust for at least that long.

Upper Mantle:

The mantle, which makes up about 84% of Earth’s volume, is predominantly solid, but behaves as a very viscous fluid in geological time. The upper mantle, which starts at the “Mohorovicic Discontinuity” (aka. the “Moho” – the base of the crust) extends from a depth of 7 to 35 km (4.3 to 21.7 mi) downwards to a depth of 410 km (250 mi). The uppermost mantle and the overlying crust form the lithosphere, which is relatively rigid at the top but becomes noticeably more plastic beneath.

Compared to other strata, much is known about the upper mantle, thanks to seismic studies and direct investigations using mineralogical and geological surveys. Movement in the mantle (i.e. convection) is expressed at the surface through the motions of tectonic plates. Driven by heat from deeper in the interior, this process is responsible for Continental Drift, earthquakes, the formation of mountain chains, and a number of other geological processes.

Computer simulation of the Earth's field in a period of normal polarity between reversals.[1] The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core
Computer simulation of the Earth’s field in a period of normal polarity between reversals.  Credit: science.nasa.gov
The mantle is also chemically distinct from the crust, in addition to being different in terms of rock types and seismic characteristics. This is due in large part to the fact that the crust is made up of solidified products derived from the mantle, where the mantle material is partially melted and viscous. This causes incompatible elements to separate from the mantle, with less dense material floating upward and solidifying at the surface.

The crystallized melt products near the surface, upon which we live, are typically known to have a lower magnesium to iron ratio and a higher proportion of silicon and aluminum. These changes in mineralogy may influence mantle convection, as they result in density changes and as they may absorb or release latent heat as well.

In the upper mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F). Between the upper and lower mantle, there is also what is known as the transition zone, which ranges in depth from 410-660 km (250-410 miles).

Lower Mantle:

The lower mantle lies between 660-2,891 km (410-1,796 miles) in depth. Temperatures in this region of the planet can reach over 4,000 °C (7,230 °F) at the boundary with the core, vastly exceeding the melting points of mantle rocks. However, due to the enormous pressure exerted on the mantle, viscosity and melting are very limited compared to the upper mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous.

The internal structure of Earth. Credit: Wikipedia Commons/Kelvinsong
The internal structure of Earth. Credit: Wikipedia Commons/Kelvinsong

Outer Core:

The outer core, which has been confirmed to be liquid (based on seismic investigations), is 2300 km thick, extending to a radius of ~3,400 km. In this region, the density is estimated to be much higher than the mantle or crust, ranging between 9,900 and 12,200 kg/m3. The outer core is believed to be composed of 80% iron, along with nickel and some other lighter elements.

Denser elements, like lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. The temperature of the outer core ranges from 4,300 K (4,030 °C; 7,280 °F) in the outer regions to 6,000 K (5,730 °C; 10,340 °F) closest to the inner core.

Because of its high temperature, the outer core exists in a low viscosity fluid-state that undergoes turbulent convection and rotates faster than the rest of the planet. This causes eddy currents to form in the fluid core, which in turn creates a dynamo effect that is believed to influence Earth’s magnetic field. The average magnetic field strength in Earth’s outer core is estimated to be 25 Gauss (2.5 mT), which is 50 times the strength of the magnetic field measured on Earth’s surface.

Inner Core:

Like the outer core, the inner core is composed primarily of iron and nickel and has a radius of ~1,220 km. Density in the core ranges between 12,600-13,000 kg/m³, which suggests that there must also be a great deal of heavy elements there as well – such as gold, platinum, palladium, silver and tungsten.

Artist’s illustration of Earht's core via Huff Post Science
Artist’s illustration of Earth’s core, inner core, and inner-inner core. Credit: Huff Post Science

The temperature of the inner core is estimated to be about 5,700 K (~5,400 °C; 9,800 °F). The only reason why iron and other heavy metals can be solid at such high temperatures is because their melting temperatures dramatically increase at the pressures present there, which ranges from about 330 to 360 gigapascals.

Because the inner core is not rigidly connected to the Earth’s solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been considered. By observing changes in seismic waves as they passed through the core over the course of many decades, scientists estimate that the inner core rotates at a rate of one degree faster than the surface. More recent geophysical estimates place the rate of rotation between 0.3 to 0.5 degrees per year relative to the surface.

Recent discoveries also suggest that the solid inner core itself is composed of layers, separated by a transition zone about 250 to 400 km thick. This new view of the inner core, which contains an inner-inner core, posits that the innermost layer of the core measures 1,180 km (733 miles) in diameter, making it less than half the size of the inner core. It has been further speculated that while the core is composed of iron, it may be in a different crystalline structure that the rest of the inner core.

What’s more, recent studies have led geologists to conjecture that the dynamics of deep interior is driving the Earth’s inner core to expand at the rate of about 1 millimeter a year. This occurs mostly because the inner core cannot dissolve the same amount of light elements as the outer core.

The freezing of liquid iron into crystalline form at the inner core boundary produces residual liquid that contains more light elements than the overlying liquid. This in turn is believed to cause the liquid elements to become buoyant, helping to drive convection in the outer core. This growth is therefore likely to play an important role in the generation of Earth’s magnetic field by dynamo action in the liquid outer core. It also means that the Earth’s inner core, and the processes that drive it, are far more complex than previously thought!

Yes indeed, the Earth is a strange and mysteries place, titanic in scale as well as the amount of heat and energy that went into making it many billions of years ago. And like all bodies in our universe, the Earth is not a finished product, but a dynamic entity that is subject to constant change. And what we know about our world is still subject to theory and guesswork, given that we can’t examine its interior up close.

As the Earth’s tectonic plates continue to drift and collide, its interior continues to undergo convection, and its core continues to grow, who knows what it will look like eons from now? After all, the Earth was here long before we were, and will likely continue to be long after we are gone.

We have written many articles about Earth for Universe Today. Here’s are some Interesting Facts about Earth, and here’s one about the Earth’s inner inner core, and another about how minerals stop transferring heat at the core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Earth. Listen here, Episode 51: Earth.