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[Expositions 8.1 (2014) 70–84]  Expositions (online) ISSN: 1747–5376 
 
 

We Are Stardust … Concentrated by Earth! 
 
WALTER ALVAREZ 
University of California, Berkeley 
 
 
The audacious aim of the emerging field of big history is to see all of the past as a unified field 
of study, combining insights from cosmology, astronomy, geology, paleontology, molecular 
evolution, paleoanthropology, archaeology, and all the fields of humanistic history, in order to 
achieve a whole new level of understanding of the human situation.  

Big History: Between Nothing and Everything, by David Christian, Cynthia Stokes Brown, 
and Craig Benjamin (2013), is the first textbook for this new academic field. Having taught big 
history at Berkeley and seen the enormous enthusiasm it engenders in students who crave to see 
the world in broad perspective, I am pleased that we now have a book suitable for big history 
courses, but as a geologist, my (predictable!) complaint is that it could have told more about the 
fascinating history of Earth.  

Although big history includes the regimes of Cosmos, Earth, Life, and Humanity, it is 
inherently anthropocentric, because we humans are doing the writing and because we know 
nothing about the histories that have unfolded on planets around other stars in this and other 
galaxies. The centrality of humans is clear from the Christian-Brown-Benjamin book, where 
human beings are the focus of three quarters of the book. As a result, the history of Earth, the 
only possible home for humans far into the foreseeable future, gets only a few pages. The 
information presented about our Earth—especially about the evolution of the atmosphere and 
about plate tectonics—is relevant and important to know, but it is just the beginning of the 
fascinating history of the earth that geologists have dug up. 

A short time spent perusing the recent geological literature would probably convince you that 
geology is currently by far the greatest source of new historical information coming from any 
branch of big history, and it seems a shame for big historians not to be familiar with and excited 
about this treasure house of growing understanding.  

In this essay, to convey the flavor of Earth history and its relevance to humanity, let me pick 
up on a point made strongly and correctly by Christian, Brown, and Benjamin—that almost all of 
the chemical elements important to humans were made in stars. And then let me show that this 
important point is incomplete, because it omits the role of Earth in concentrating those elements 
and making them useful. 
 
We are stardust … 
 
The astronomer Carl Sagan brought science to life for large numbers of people in his 1980 
television series Cosmos, which now looks like an early and very effective presentation of big 
history. Sagan was fond of saying that we are made of stardust. His point was that with the 



71  Alvarez 
 

 

exception of the three lightest—hydrogen, helium, and traces of lithium—all the chemical 
elements were cooked up inside stars, either slowly as by-products of the nuclear reactions that 
make the stars shine, or suddenly during supernovas, the great stellar explosions that scatter all 
the new elements into space. These chemical elements are the basis of the material world that is 
central to the human situation. 

Humanistic big historians have picked up on this critical feature in the history of everything, 
stressing the great importance to us of the synthesis of elements inside stars. Christian, Brown 
and Benjamin select the synthesis of elements in stars as one of their eight thresholds in big 
history. But to a big historian like me, coming from a background in geology, Sagan’s 
formulation is incomplete. The story does not end there, because chemical elements dispersed 
through interstellar space would be useless for building the world we know. 

Imagine a long-range spaceship on its way to a distant solar system, and having an emergency 
need for silicon to fabricate computer chips. Although the ship is passing through the diffuse gas 
derived from old supernova explosions and that gas includes atoms of silicon, there would be no 
way for the astronauts to harvest and use the silicon.1 The atoms are just too far apart. 
 
… Made useful by Earth 
 
But here on Earth it is easy. We have no trouble finding flint nodules, or sand on a beach, or 
crystals of quartz in a vein, all of them made of silicon bonded to oxygen—SiO2. Clearly the role 
of Earth has been essential in making silicon useful to human beings. The point of this 
contribution is to explore how Earth does it, in the hope that big historians will amend Carl 
Sagan’s idea that we are stardust, and recognize that, We are stardust, made useful by Earth. 

We should start by recognizing that there have been two main phases in Earth’s history of 
sorting chemical elements into useful concentrations. The first phase took place at the very 
beginning, as Earth was accreting. This phase gave us Earth’s bulk composition, dominated by 
four elements—oxygen, magnesium, silicon and iron—with only small amounts of the other 
elements, but with all of them mixed up together.  

The second phase has been going on ever since, with the progressive sorting out and 
concentrating of elements. Earth has many, many mechanisms for doing this work, so perhaps it 
is not surprising that big historians have focused on the conceptually simpler nucleosynthesis—
the synthesis of elements—in the big bang and in stars. The present essay explores a few of 
Earth’s sorting mechanisms, to give an idea of how Earth works its magic. The focus is on the 
element silicon, and on uses of silicon that are important in human history, so we might think of 
this as a “little big history” of silicon.2  
 
A Focus on Silicon 
 
The production of new elements inside stars that big historians appreciate so well is called stellar 
nucleosynthesis, and it lies in the domain of nuclear physics. The protons and neutrons in the 



We Are Stardust … Concentrated by Earth!  72 
 

   

nuclei of atoms are bound so tightly by the strong nuclear force that wholesale changes can take 
place only at the extremely high temperatures and pressures in the cores of stars. On the other 
hand, the concentrating of elements inside Earth and at its surface takes place under much lower 
pressures and temperatures, which have no effect on the nuclei of atoms but allow all kinds of 
changes in how the atoms are bonded together, so this is in the realm of geochemistry. 

If one had to identify the single most important discovery of geochemistry, it might be this: 
The solar system as a whole (like the universe as a whole) is fundamentally composed of a great 
deal of hydrogen, a moderate amount of helium, and only traces of all other elements, and this of 
course is the result of nucleosynthesis during the big bang and, later on, inside stars. But Earth, 
on the other hand, is dominantly composed of just four elements—oxygen, magnesium, silicon 
and iron (O, Mg, Si, Fe)—with traces of many other elements, among which the great 
components of the solar system, hydrogen and helium, are very minor. Somehow, Earth has 
selectively accumulated some of the rarer elements of the solar system. Let us see how this great 
geochemical discovery can be explained and how it affects our understanding of big history. 

Of those four dominant elements, let’s focus on silicon, first because it is the basis of most of 
the rocks that make up our planet and carry the record of Earth’s history, and second because it 
has been critical to human activities for a very long time, as the raw material for crucial 
technologies like stone tools, glass, and computer chips.  

Regarding the first point, it at first seems surprising that silicon is the basis of rocks while 
carbon is the basis of life. Carbon lies just above silicon in the periodic table, both make four 
chemical bonds with adjoining atoms, and this similarity has led science fiction writers to 
envision life based on silicon3, but that is not what happens. It turns out that carbon is a versatile 
element, bonding to hydrogen, oxygen, nitrogen, sulfur, and other carbon atoms, with both single 
and double bonds, easily changing the atoms to which it bonds, and this versatility makes it the 
perfect basis for the complex structures and dynamic processes of life. In contrast, silicon greatly 
prefers bonding to oxygen, making strong single bonds but no double bonds, and this is what is 
needed to make long-lasting minerals like the quartz (SiO2) that is abundant in continental crust, 
and the olivine (Mg2SiO4) that dominates in the deep-earth mantle. 

The minerals quartz and olivine, each with its characteristic formula, occur as grains that go 
together to make up rocks. A good analogy is that a rock is like a fruitcake, and minerals are like 
the fruit and nuts within the cake. Since solid minerals are quite resistant to change, so are the 
rocks made of minerals. Rocks generally record and remember aspects of the conditions under 
which they were deposited. Igneous rocks remember the temperature and pressure and chemistry 
of the molten magma from which they crystallized. Sedimentary rocks remember the processes 
that deposited them—rivers, wind, or glaciers. Metamorphic rocks remember the stresses that 
deformed them and the temperature and pressure under which they recrystallized into new 
minerals. Geologists have invented many ways of extracting this historical information from 
rocks and compiling it into an ever more sophisticated understanding of our planet’s past. So we 
have a motto: Ex libro lapidum historia mundi—“Out of the book of rocks comes the history of 
the earth.” 



73  Alvarez 
 

 

The second reason to focus on silicon is the critical role it has played in the rise of humanity 
to enormous technical prowess. The earliest human tools may have been made of wood, of which 
nothing remains, but the first tools of which we have a good record are made of silicon-based 
rocks. Moving from just the materials Nature gave us to artificial materials, a critical one has 
been glass, which basically comes from melting quartz. Finally, our modern, high-tech 
civilization is dependent on computer chips, which are made in sophisticated ways from silicon. 
Interestingly, Earth has concentrated the silicon used in each of these human artifacts in a 
different way.  

To see how Earth has concentrated silicon and made it useful, let us first look at the original 
accretion of the earth into a planet dominated by O, Mg, Si, and Fe, and then see how 
concentrations of Si have been formed on and within the earth. 
 
How to Build an O-Mg-Si-Fe Planet 
 
Returning to that major geochemical discovery—that the solar system is mostly made of 
hydrogen and helium while Earth is dominated by oxygen, magnesium, silicon, and iron—how 
can this be explained? What processes could account for this wholesale geochemical alteration?4  

Evidently we are looking for processes that acted very early in the history of the solar system, 
because today there aren’t any large amounts of material entering or leaving Earth, nor is there 
any sign of this happening in the four billion years for which we have a rock record. The 
occasional impact of an asteroid or comet, even a large one, is much too minor to change Earth’s 
composition, and leakage off the top of the atmosphere to space affects only a few gaseous 
elements. At the beginning of the solar system, however, 4.567 billion years ago, Earth was 
growing rapidly, as objects from dust size to planet size accreted and stuck together. There were 
certainly opportunities during the formation of the earth for selecting some elements to 
accumulate in the earth, and excluding others.  

In their textbook Christian, Brown, and Benjamin show a periodic table of the elements, 
marked up to indicate how and when the various elements were created, in the big bang and in 
stars. To supplement that figure, here is a partial periodic table that shows the fates of elements 
in the earth, singling out the big four: 



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75  Alvarez 
 

 

Silicate grains can thus become arbitrarily large, unless they are in an aggregate of grains—a 
rock—and run into another growing grain. 

So the big-four elements—silicon, oxygen, magnesium, and iron—were all incorporated in 
the silicate minerals that make up the rocky part of Earth, with the leftover iron forming the 
metallic core. And thus it is silicon, whose four bonds make those minerals possible, that we can 
think of as Earth’s favorite element. 

Now let’s go on and see how silicon, which was initially mixed up in our planet with oxygen, 
magnesium, iron, and some minor components, was sorted and concentrated by Earth into forms 
humans can use. That usage is quite amazing! A number of characteristics distinguish humans 
from all other animals, and among those features are complex tools, artificial materials, and 
computers. Tools let us do things our natural hands and bodies cannot do, artificial materials let 
us do things impossible with natural materials, and computers let us do things our natural brains 
cannot do. 

In the rest of this contribution we will look at one example from each of those three 
categories—stone tools, glass, and computer chips. Each is based on silicon, which is not just 
one of Earth’s favorite elements, but one of ours as well. Conveniently, each of those 
technologies uses silicon that was concentrated in a different way, so we can get some feeling for 
the variety of remarkable processes through which Earth has produced the mineral deposits we 
depend on. 
 
Silicon and stone tools 
 
Think of the variety of tools we use! From simple knives, hammers, saws and screwdrivers, to 
more complicated tools like violins and pianos, drill presses, agricultural harvesting machines, 
and industrial looms, to really sophisticated, high-tech tools like interplanetary spacecraft, laser 
ranging devices, global-positioning-system receivers, and computer-controlled three-dimensional 
printers—the list is almost endless and portrays a species with a technological virtuosity that 
would have been inconceivable even a million years ago. Primates have been seen to use and 
sometimes make very simple tools, and some birds and invertebrates may use found objects as 
primitive tools, but the complexity of tool manufacture and use by human beings is quite unique. 

How and when did this virtuosity begin? We are unlikely ever to find examples of objects at 
the transition from found objects to purposely-constructed tools, because wood, the most likely 
material, is so perishable. But the record for stone tools is abundant. Archaeologists, who like to 
divide history into three parts, have long divided the human past into three ages—Stone Age, 
Bronze Age, and Iron Age—based on the kinds of tools found in excavations.  

The earliest recognized stone tools are found in East Africa, dating from about 2.5 million 
years ago, and seem to have been made by Homo habilis, whose species name reflects this tool-
making ability. These are simply pebbles with random chips knocked off, giving sharp edges that 
could be used for cutting. They have been called the Oldowan Industry, after Olduvai Gorge in 
Tanzania. Subsequent stone-tool industries, like the Acheulean hand axes, display increasing 



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We Are Stardust … Concentrated by Earth!  78 
 

   

The famous Stone Age monument of Stonehenge, in southern England, was built and 
improved over about 800 years in a place where high-quality chert nodules were particularly 
abundant. Stonehenge helps us realize that silicon was as critical an element in Stone Age times 
as it is today, and perhaps we could think of Stonehenge as the first Silicon Valley. 

All of this history of manufacture and use of stone tools made from chert, and its probable 
role in our expanding brains and growing intelligence, was made possible because Earth has 
biological and then chemical mechanisms for separating silica from the other major elements—
iron and magnesium—and from all the other minor elements in our planet, and forming beautiful 
chert nodules of pure SiO2 from which tools can be made. 

 
Silicon and the manufacture of glass 
 
Chert is not the only material for making stone tools with sharp, hard edges—they have also 
been made from obsidian, a natural glass of volcanic origin. Obsidian forms when molten rock 
cools so rapidly that there is no time for crystals to grow. Although humans have long used 
naturally occurring glass, eventually they learned to make artificial glass themselves. Glass 
technology seems to have begun around 3500 BCE in Mesopotamia or Egypt, about the same 
time and place people learned to make bronze by mixing copper and tin. So glass and bronze 
appear to mark the beginning of a history in which humans have learned to make and use an ever 
more varied array of remarkable artificial materials.  

Think for a moment about what glass does for us. Glass windows allow us to have light inside 
our houses, cars, and airplanes during the day, and light bulbs illuminate them at night. Glass 
gives us waterproof pitchers, bottles and drinking glasses, and it allows us to see in mirrors and 
through corrective lenses. In telescopes, microscopes and myriad other scientific devices it has 
allowed us to understand our world. It insulates high-voltage power lines, provides fiber-optic 
cables, and gives us monitors and touch screens for computers. In mosaics and stained-glass 
windows it has been a medium for great art. 

Almost all glass made today is based on melting and then rapidly cooling silica, SiO2, with 
smaller amounts of other elements added to give it desirable properties. The point here is not the 
details of how glass is made, but rather the question how Earth has concentrated the silica we use 
for glass manufacture. This will lead us deeper into understanding Earth’s virtuosic ability to 
separate and concentrate chemical elements, because the silica used in glass making was 
concentrated in a completely different way from the silica used for making stone tools. 
Fortunately we are not dependent on chert nodules to make glass, for chert is not so abundant. 
On the contrary, there are enormous supplies of silica in that most common of geological 
deposits—plain old sand. 

Most sand is composed largely or almost entirely of little grains of quartz, and is familiar to 
anyone who ever walks on a beach or through the dunes. But it is surprising at first, when you 
think about how Earth concentrates elements, because during the original accretion of Earth there 
was no mechanism to produce quartz. As we have seen, there were mechanisms to eliminate 



79 
 

 

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81  Alvarez 
 

 

A sand grain’s story 
 
The accumulation of quartz sand on the earth’s surface can involve intricate and fascinating 
stories. One example is a remarkable discovery by William Dickinson and George Gehrels of the 
University of Arizona (Dickinson and Gehrels 2009). Anyone who has visited the Colorado 
Plateau of southeastern Utah will remember the great cliffs of Arches and Canyonlands National 
Parks, marking thick sheets of ancient sand that was deposited as dunes, in a former landscape 
much like the modern Sahara Desert. Sloping layers in the sandstone mark ancient faces of 
dunes, telling the wind direction when they were deposited, so geologists have long known that 
the ancient wind was blowing dominantly from the north and that the sand grains in these dunes 
were blown from Wyoming south to Utah. 

Almost all of the sand grains in these ancient dune deposits are quartz, but a very small 
fraction of the grains are the mineral zircon. Like quartz, zircon is very resistant to chemical 
alteration, so it lasts indefinitely, but unlike quartz, zircon can be dated, because it contains a 
little uranium which decays radioactively to lead. This in turn gives the age of the granitic source 
rocks from which the zircon grains, as well as the quartz, were derived. Dickinson and Gehrels 
dated more than 1600 zircon grains from sandstones of the Colorado Plateau, and to their 
surprise the ages did not correspond to any source rocks in Wyoming, or to any known rocks in 
western North America. The only source rocks of the right age are in the eastern United States, 
in the Appalachian Mountains!  

But how could quartz from the Appalachians get into sand dunes on the Colorado Plateau? It 
could not happen today, because sand carried westward from the Appalachians by rivers would 
end up in the Mississippi and then be carried to the Gulf of Mexico. Prior to that, in the 
Cretaceous, the interior of North America was submerged by a wide, shallow sea, and sand 
grains reaching that sea would stay there, on the sea floor. But in the Jurassic, about 150–200 
million years ago, the situation was different, as Dickinson and Gehrels pointed out. Rather than 
the present geography, with the low Appalachian Mountains in the east, the Mississippi River, in 
the middle, and the high Rocky Mountains in the west, Jurassic North America had a long, 
gradual, gentle slope from high topography in the east, uplifted because Africa was rifting away 
from North America, gradually descending to sea level where the Rocky Mountains are now. 
Before there ever was a Mississippi, a great west-flowing river, whose exact route we can only 
guess, must have carried vast numbers of Appalachian sand grains all the way to the shoreline in 
Wyoming, and from there they were blown by the wind to their final resting place in Utah. Who 
would have imagined, while gazing at a sandstone cliff in Utah, that they were seeing the 
Appalachian Mountains, disassembled and carried 2,000 miles across the continent by a long-
vanished river? 

The ability to reconstruct a great river of long ago, of which every trace has been removed by 
erosion, is an example of the wonderful historical discoveries that geologists are contributing to 
big history. And it gives a hint of the array of remarkable processes and surprising episodes by 



We Are Stardust … Concentrated by Earth!  82 
 

   

which Earth has concentrated the kinds of valuable materials that has made human civilization 
and technology possible. 
 
Silicon and computers 
 
In the early twenty-first century we live in a new silicon age, in which this element, the critical 
ingredient for our Earth itself and once the basis for Stone Age tools, is now again of critical 
importance, as the foundation for the computer chips that pervade almost every aspect of our 
lives, our technology, and our civilization. The ancient Silicon Valley of Stonehenge has been 
replaced by the new Silicon Valley in California, and similar centers of high technology 
elsewhere. 

How has Earth concentrated the silicon used for computer chips? Curiously, this has happened 
by a process completely different from those that have concentrated the silicon used in stone 
tools and in glass, and this helps us appreciate the range of Earth’s geochemical skills. 

At the earth’s surface, SiO2 is extremely insoluble in water, which is why those grains of 
quartz sand last indefinitely. But that is not the case deep in the earth, where water chemistry is 
different and the temperatures are much higher. Under those conditions, SiO2 can be dissolved 
out of the abundant silicate minerals of the crust, carried upward by the hot, silica-rich waters as 
they rise, and deposited in open fractures near the surface when conditions have reached the 
point where silica is no longer soluble. Fractures filled with SiO2 are called quartz veins, and 
they are very abundant in areas that once had hot rocks at depth, resulting in hydrothermal, or 
hot-water, circulation. 

The exquisite quartz crystals and the beautiful clusters of purple amethyst crystals, also 
quartz, on display in rock shops were deposited by hot water in open fractures that became 
quartz veins, with the perfect pyramidal crystal terminations formed as the crystal grew into an 
open cavity filled with silica-rich water that came up from depth. 

Quartz veins have long been of extreme economic importance, because gold, if present at 
depth, gets carried along with SiO2 in hydrothermal systems and deposited in veins near the 
surface. Many of the world’s great gold-producing regions have this origin, including the 
deposits of northwestern Spain that supplied the gold of the Roman Empire, and the Sierra 
Nevada foothills of California, where the richest gold concentrations are in a great system of 
quartz veins called the Mother Lode.  

Now, however, because of the great purity of SiO2 in quartz veins, these have become the 
preferred source of silicon metal, refined from the quartz, which is the basis for making 
computer chips. Of course the process of converting quartz crystals into silicon metal and adding 
just the right impurities is extremely complex and difficult, but again the point here is that Earth 
has concentrated the silica in a most interesting way. Perhaps we should conclude that 
technology is the result of human ingenuity, building on Earth’s ingenuity. 
 
 



83  Alvarez 
 

 

The Lesson for Big Historians 
 
Recognizing Earth’s three different ways of accumulating the silica we use for making stone 
tools, glass, and computer chips barely scratches the surface of the rich array of mechanisms by 
which our planet has produced the concentrations of the elements we use. Almost every chemical 
element gets enriched by Earth in one or more ways. Hydrogen and oxygen end up in the ocean 
water; oxygen and nitrogen in the atmosphere; sodium, potassium and chlorine in evaporated salt 
deposits; iron in the deposits of ancient seas whose geochemistry was unlike those of today: 
copper in hydrothermal vent chimneys on the mid-ocean spreading ridges; uranium in ancient 
river channels; calcium in limestones; carbon in limestone, coal, and hydrocarbons. 

Perhaps it is the enormous variety of these different mechanisms and the subtle geochemistry 
some of them involve that has kept them unfamiliar to most people. But surely big historians 
need to recognize that Carl Sagan’s wonderful insight—that we are made of stardust—is 
incomplete without also understanding that our planet has done the complicated, sophisticated 
job of separating that stardust into its constituent elements, which makes them useful for 
constructing rocks, animals, plants, human beings, and our technological world. 
 
 

Endnotes 
 

1. Three terms used in this essay may at first be confusing: “Silicon” means element 14, Si. 
“Silica” is the combination SiO2, which is the formula of the mineral quartz. “Silicate” 
refers to a mineral containing silicon and oxygen, such as olivine (Mg2SiO4), with a 
structure based on the “silicate tetrahedron”—a single small silicon atom surrounded by 
four large oxygen atoms in a tetrahedral arrangement. In addition, “silicone” is a 
synthetic material made of silicon, oxygen, carbon, and hydrogen; it is not an Earth 
material and does not enter our story. 

 
2. Big history is a vast topic, so Fred Spier has suggested the writing of “little big histories,” 

exploring all the history that lies behind some feature of our world. So there could be 
little big histories of cats, of a language, or of silicon, as in this essay. 

 
3. An example of the silicon-based-life concept is the Star Trek episode, “The Devil in the 

Dark.” 
 

4. This question is answered in detail in an excellent little book: Broecker 1988, 291. 
 

5. Nick and Kathy’s remarkable Big History museum exhibit, From the Big Bang to the 
World Wide Web, is available online at http://www.bigbangtowww.org/ 

 



We Are Stardust … Concentrated by Earth!  84 
 

   

6. A huge quarry in Illinois that extracts Ordovician sand, for use in glass making, from 
deposits about 455–460 million years old, can be seen on Bing Maps or Google Earth at 
41° 20.529' N, 88° 52.636' W. 

 
 

Works Cited 
 

Broecker, Wallace. 1988. How to Build a Habitable Planet. New York: Eldigio Press. 
 
Christian, David, Cynthia Stokes Brown, and Craig Benjamin. 2013. Big History: Between 

Nothing and Everything. Princeton: McGraw-Hill. 
 
Dickinson, William R., and George E. Gehrels. 2009. “U-Pb ages of detrital zircons in 

Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for 
transcontinental dispersal and intraregional recycling of sediment.” Geological Society of 
America Bulletin 121.3–4: 408–433. 


