Where do rocks come from?

I was interested in rocks and mineralogy as a kid. As an adult, I became fascinated with astronomy, astrophysics and space exploration. Although I went into other fields, I followed these sciences throughout the years.

Taken for granite: It wasn't until a month or two ago that I began to wonder where rocks come from at all. With the current cosmological emphasis on black holes, neutron stars, nebulae, galactic formation and the expanding universe, it seemed that rocks were being taken for granted. Now that solid planets are being discovered in neighboring solar systems, I wondered why there was so little speculation about the actual origin of the “stuff” of which non-gas-ball planets, like our Earth, are formed.

Migawd, supposing the experts suddenly all agreed that rocks came primarily from meteors - asteroids. What does that explain? Where did they come from? I'm happy to report I learned a lot more than I expected.

As I kid, I learned some of the basic rock formation processes. Rocks, we were taught, could be classified as igneous, metamorphic and sedimentary. As an assist to my childhood memories, I consulted several web resources, including:

Cal Poly Pomona website, which offers informative discussions and examples.
British Geological Survey website, which gives more insight into rock origins.

Metamorphic rocks were formed from simpler stuff under intense heat and pressure, or perhaps by molecule-by-molecule transformation, such as fossilization and crystallization. Diamonds and coal are obvious examples of this kind of change. Others include slate, marble, and quartzite (metamorphized sandstone).

Web discussion: Metamorphic Rocks

Sedimentary rocks are mixtures of ground or crushed rock particles of any type fused together, typically, by heat and pressure, and perhaps by the glues of age, time and clay binders. Obvious examples include sandstones and clay. Opal and chalcedony are some surprising gem members of this class.

Web discussion: Sedimentary Rocks

Igneous rocks were the easiest to understand, or so it seemed. Named for the Greek word for “fire”, these are the melted rock forms spewn forth from the bowels of the earth in the form of lava and magmas of various kinds. Obviously, they are liquid mixtures of the molten remains of other rocks. Any rock or mixture of rocks can be melted into a somewhat homogenous glassy mass with sufficient heat. Obsidian was of special interest to me, because Native Americans chipped this material away to fashion razor sharp arrowheads. Obsidian is molten rock that has been allowed to cool very slowly and without violent stirring, so that a glasslike crystalline structure forms. The rate of cooling and mechanical forces help determine whether the mineral will have a fine glassy or crystalline structure, such as obsidian or granite, or a coarser structure such as basalt or “lava”.

Many sedimentary and metamorphic rocks take their present form from the forces of settlement, pressure and heat working on simple igneous silicates such as sand, particles of almost pure ground silicates.

Web discussion: Igneous Rocks

Minerals: It's worthy of mention that “minerals” are a broader classification of materials, generally solids or liquids. Salt, magnesium sulfate (“Epsom Salts”), borax, potassium nitrate, gold and crude oil are a few of tens of thousands of common mineral compounds that form the geological building block of chemistry.

Some of these occur as free elements, uncompounded with other elements (gold, sulfur).

Some appear as complex molecular chains (petroleum). Some are found in nature as very simple compounds of two elements (sodium chloride, or common table salt). Many “native” minerals are also essential to life itself.

• Web discussion: 40 Common Minerals & Their Uses
• Web discussion: Minerals

So where do rocks come from?

Mother Earth herself has a mostly iron core, a huge molten ball of liquid iron (and other very heavy elements). Atop this is a thick layer of “mantle”, molten and semi-molten silicate rock under such tremendous pressure and heat as to be otherworldly. Speaking of “otherworldly”, the stuff of the mantle is thought to be very similar to the rock structures of the moon, meteors, rocky comets and other “solid” heavenly bodies.

The mantle is often classified as “Upper”, “Middle” and “Lower”, since the kinds of silicate rocks found depend on temperature, pressure and the density of the rock found there. Lighter and hotter materials will tend to float to the lower regions of Earth's crust, giving rise to volcanism. Silicate convection is the engine that drives plate tectonics. Magma is primarily molten silicate matter that rises to the surface to form basalt. This rising or convection motion, combined with plate lifting and mountain building, brings us vast deposits of sulfur, metallic ores, minerals, and coal beds.

Web discussion: Earth's Interior & Plate Tectonics
Web discussion: Earth's Interior

Not all rocks are derived from Earth's silicate-based mantle. Sulfur, metallic salts, petroleum, coal and metallic oxides (ores) are self-evident exceptions. As anyone in prospecting, mining, oil or coal could happily confirm for us, non-silica compounds are generally the most difficult of all to extract from the earth, because you have to remove or drill through so much silicate rock to get to them. Most of the solid matter of Earth's crust, as well as its mantle, consists of silicate compounds that were mainly formed or re-formed deep beneath the surface. This includes the stuff of mountains, desert, topsoil and beach sand.

The media is currently playing up a duel in astronomy and astrophysics between two competing theories. The question: where did the planets come from? This might shed some light on our seemingly trivial question, “Where do rocks come from?”

The “gaseous accretion” theories see a prototype Solar System as a swirling disc of gas, much like pictures of spiral galaxies you have seen, such as our neighboring galaxy Andromeda, only on a smaller, solar scale. Another example cited is condensing nebula in the belt of the Orion constellation, where we can see suns being birthed out of hot foggy soups of interstellar gas.

When a cloud of and dust gas several solar systems in diameter begins to collapse inward due to gravitational attraction, the theory (that we can see in action elsewhere) is that a dense hot core will form, eventually igniting at the right mass and temperature to form a star (like our Sun). Rotation of the gas body around the core sets in for the same reason we get whirlpools in bathtub drains. Centrifugal force from the rotation eventually counterbalances inward gravitational pull, and satellite balls of gas are able to form tentative orbits.

The “core accretion” theories see the shape and orbits of a prototype Solar System as forming by somewhat the same mechanics, but more “weight” is given to the dust and rocky debris components for planet formation, and there is more discussion of what size the rocky components might be, whether “stardust” or asteroid.

“Accrete” means to grow or gather together, to adhere, as by growth. An “accretion” is an increase by natural growth or by gradual external addition. Both “gaseous” and “core” accretion theories assume naturally that larger planetary bodies would form as drifting nearby matter “fell into” them. In time, gaseous or solid leftover material in the formation of the solar system would find its way to larger bodies through accretion, or collision, or both. Either theory would help explain the more solid and orderly solar system that we see today.

“Core accretion” theories are older and do a better job (to a lay person, at least) of explaining how sensible rocky planets like Earth came into being. To explain gas giant planets like Saturn and Jupiter, one might lean toward “gaseous accretion” theory. Whether we end up preferring one theory or another to explain a particular heavenly body, it would seem as if we are debating the size and natural state of the building blocks.

Begging the question: Either theory seems to beg the question of where rocks came from. Core accretion assumes the rocks were already there, possibly as the shattered remnants of even older galactic dynasties. But nothing's cast in stone. Gaseous accretion assumes that coalescing gas clouds were responsible for formation of the planets as we know them today, so the “rocks” must have come later, if ever.

Lunar Formation

An interesting and credible theory of lunar formation suggests that our moon might have been formed from the monster collision of a proto-Earth and some other very large orbiting body, splashing a 1,000-mile radius ball of molten or vaporized rock into an Earth orbit.

As discussed later when we look at "Asteroids", it would be farfetched to suppose that Earth somehow escaped the massive bombardment that permanently pockmarked its own satellite, our Moon.

The famous photo to the left shows both heavenly bodies in the same photo, as shot from space satellite Clementine. Earth is the familiar blue ball we have come to expect, while the moon reveals a dramatic three-dimensional topography that is harder to surmise from Earth-based telescopes. (Hint: look toward the terminus, between the light and dark sides, so that sunlight will cast deep shadows showing topographical relief, because of the steep angle of the sun's rays at those points.)

The Moon, like most of the other inner planets, evolved on an entirely different path than Earth. Obviously, rocks alone are not enough. Distance from the sun, ability to form and retain an atmosphere, and an abundance of water are also all important factors.

SO, once again, we ask: where did the rocks come from? A geologist might find this an irritatingly naive question, since we know so much about the geology of terrestrial rock forms. But most of the knowledge explosion in terrestrial geology only deals directly with what happened to simple rocky matter after it got here.

For example, simpler or more elemental rocklike substances have been folded and blended into the Earth's mantle through tectonic plate subduction, and returned to the surface as new mountain ranges and volcanic activity. Silica (sand), metallic oxides (rust) and many or all of our “minerals” like sulfur would be churned through the Earth's “blender” and pressure-cooked as mixed liquids before being returned to the surface in compositions quite different than those originally plowed under. Over 4.5 billion years, these rock compounds would be more thoroughly “folded” than the best pastries or omelets!

To postulate that all of the rocky compounds in the universe resulted from convection and plate processes on like planets would strain credulity, though it would be a great ego-booster to those of who like to think that “we are not alone”!

On the other end of the scale, we have classification schemes and estimates for all of the matter in the universe. The July 2002 Astronomy Magazine has such a breakdown in the article “Moving Right Along” by Mario Livio, on the subject of “dark energy” and dark matter, and why expansion of the universe still seems to be accelerating, instead of slowing down due to gravitational attraction. The article cannot be found on the website yet, but pay a visit anyway. The breakdown:

• Dark Energy: 65%
• Heavy elements 0.03%
• Ghostly Neutrinos 0.3%
• Stars 0.5%
• Free hydrogen and helium 4%
• Dark Matter 30%

You can see right off the bat that we may still have hardly a clue what 95% of the universe is made of.

“Dark Matter” is matter we can't see, but which is presumed or inferred to exist as a counterbalance to observable motions we can see and calculate. Example might include vast clouds of dust and particulate matter too thin or too remote to diffuse light that is detectable from here. But, it might include unknown or theorized forms of matter we haven't detected yet.

“Dark energy” seems yet to be mostly a mathematicians' cosmological fudge factor to make Einstein's equations come out right. It might include the background microwave “cosmic radiation” that pervades the universe, thought to be a remnant of the “Big Bang”. But, it might include unknown or theorized forms of energy we haven't detected yet.

“Ghostly neutrinos”: you can skip this one. Whatever they are, they aren't rocks.

Stars: it's interesting how little of the universe is organized into recognizable stars, or, presumably, solar systems – one half of one percent.

Free Hydrogen and helium: the stuff stars are made of. Nice to know we have spare supplies.

From which of these cosmological categories might rocks come? Stars themselves, or possibly dark matter?

It's hard to conceive of rocks being formed from the fusion of hydrogen or helium in a solar core, or being pressed into duty from the ammonia or nitrogen soups comprising planets like Jupiter and Saturn.

We're keenly interested in the mostly poet's and musician's term "Stardust". In fact, a web search for this term finds the Hoagie Carmichael tune “Stardust” everywhere. Even here. (You can play the MIDI with the link if your browser supports it.)

"Stardust" as a whimsical term was around a long, long time before the astronomers found out just how much of it is really out there in the universe. You cannot read an article on the birth of a star, or the death of one, without seeing stardust right at the bedside. The term turns up something interesting at NASA, though:

In short, it seems that the silicates from Earth's magma have their origins in the outer shells of very old red giant and supergiant stars!

If our solar system coalesced from a huge flattened ball of swirling gases at various temperature states (“gaseous accretion”), it's curious that only the inner planets (Mercury, Venus, Earth and Mars) are “rocky” -- until we get out to oddball Pluto, the mysterious ninth planet swinging around Sol on its own eccentric orbit. Now that astronomers are learning to detect planets on distant solar systems (with the aid of Hubble and some sophisticated computers), we're finding that we're the exception, not the rule. Mostly, but not always, we're discovering the gas giant planets in close to their suns, positionally about where you might be looking for an Earth or Mars type planet.

If there are small rocky planets out there, we can't detect them yet, but we know where they aren't located in those systems.

This adds an extra element of mystery for the astronomer. It also allows us amateur rock-hounds to ask not only where the rocks came from, but, why Earth? Fear not, astronomers are asking that question too!

If we were all born of a spinning gas cloud in the beginning, it's at least plausible to conjecture that silicates, the stuff of rocks, centrifuged out into an orbital band stretching from Mercury to Mars, the inner planets.

If the solar system formed mainly by “core accretion”, it's intuitively easy to answer where rocks came from, but harder to answer, “where did the gas giant planets come from”?

Perhaps we need both models at the same time, or for explaining different but still unknown stages in the formation of the solar system. There is no reason to suppose we're logically bound to choose one theory over the other.

Either way, there are plenty of silicates here now, within the inner planets, the asteroid belt, and making up the many satellite moons of gas giant planets like Jupiter and Neptune.

If silicates are born in the outermost shells of dying red giant and red supergiant stars, how did they get here?

Comets

We've hypothesized for decades that comets contain large amounts of rocky material as well as gassy comas (tails), and now we know it (thanks to the Galileo space probe). Some scientists think that much of the rocky and metallic constituents of the Earth came from comets ... From Discovery Channel's Canada website comes an interview with Dr. Scott Tremaine, professor of astrophysics at Princeton:

The article also discusses cometary impacts with Jupiter's moon Ganymede, and the well-documented 1994 event when pieces of the celebrated comet Shoemaker-Levy 9 slammed into Jupiter itself with the force of several small atomic bombs (NASA photos and alt text above)..

So comets could be the intergalactic wagon trains hauling in the raw materials needed to build rocky, watery planets over the course of a few billion years. The gravitational “seed” could be a densely compressed ball of frozen, liquid or free gases, a slush of these plus accreted rocks, dust and asteroids, or perhaps even a protoplanet with a silicate and iron core.

Comets don't arrive with license plates saying “Orion”, “Kepler Belt” or “Land Of 1,000 Supernovae”, so of course we have no real way of knowing for sure where comets come from. The spectral signatures of visible comets give clues that suggest distributions of rare elements and gas mixtures not associated with objects in “these here parts”. Dr. Tremain's own statement strongly implies or assumes that many comets, at least, might well have come from someplace else.

Asteroids

Not surprisingly, there is renewed interest in another theory that holds that asteroid bombardment was instrumental in our planet's formation. You only have to look at the body of data cataloging known asteroids whose elliptical orbits swing them past Earth, or to watch the majestic Perseid or Leonid meteor showers in the night sky. There is a lot more visible evidence of meteors and asteroids in our neighborhood than of comets.

A pair of scientists from the University of Arizona and the University of Hawaii are advancing the theory that massive asteroid bombardment scoured the Earth so severely that any traces of earlier geological formations were erased. At about the same time, 3.9 billion years ago, the inner planets Mercury, Venus and Mars got the same treatment, and they show it. Our airless, pockmarked Moon is so close to Earth on a solar scale that it's a certainty that Earth and its satellite were subjected to the same bombardment. The moon has no erosion forces of wind and water to heal itself, and so will bear its own scars essentially forever.

There seems to be no need to choose between competing theories. The comet impact theory makes an abundance of water on our planet explainable. Asteroidal impact favors the "core accretion" theory, and has the added advantage that we can directly see evidence of it almost everywhere else in the solar system.

In summary, it seems we've been able to answer our own question “where do rocks come from?” with some degree of plausibility, even with some certainty.

Rock and rocklike matter forms most literally the foundation of Earth's crust and (with oxygen and water) provides the basis and ecological niche for all higher life forms as we know them.

If you are looking for an impossible chain of events to explain the miracle of life on this particular planet, rocks would figure prominently, but events that happened are never impossible. It is only the explanations that go begging for generations of human lifetimes.

If you are looking for the possibility of life-friendly planets in other solar systems, understanding as much as possible about this one greatly cuts to the chase.

As for our friends the rocks, all of them, from garnet to pink limestone to sheer towering cliffs of solid granite like Half Dome, geologists can already explain what forms garnet and why it's red (aluminum infused into silicates deep in the mantle). We can explain geological and epochal layers, and their distribution, and the plate tectonics and subduction that given the earth's crust such an incredibly convoluted yet beautiful geometries. We can explain how this gives rise to earthquakes, volcanism, tsunami and other great events that force their attentions upon us whether we like it or not.

And, we can explain how to get and extract coal, iron, petroleum, minerals, rare gases and elements, heat and all of the other building materials of a modern industrial society.

Ultimately, the silicate-based beginnings had their origins in the death throes of certain kinds of stars in other parts of the galaxy. The question devolves down to, “Where can you manufacture incalculable quantities of the elements silicon and oxygen, using hydrogen and helium as the starting point?” Next, “What kind of environment would be suitable to fuse those elements into molecular silicate compounds?”

The math and physics of such questions just don't allow for a tremendous latitude in the answers. No matter what the delivery system, the origin of rock (silicates) was the stars themselves. In the case of rocks, the origin wasn't just any kind of star, but the outer reaches of red giants and supergiants.

The rock delivery system could be cometary, great drifts of dust clouds light-years in diameter, or asteroidal. It could even have happened right here, born of cataclysmic stellar destruction that could have preceded the formation of our own Sun and solar system, so many (4.6) billions of years ago. Like all other matter, it started in a star.

No matter where a star of a given size is in the last stages of its death cycle, it can “blow” (a perfectly suitable lay person's term here). In some cases, a star begins to run out of hydrogen to fuel the fusion process, causing it to contract and heat up its core. This permits it to last a while longer by burning helium, a byproduct of hydrogen fusion, and fusing helium into heavier and heavier elements at the core, instead of just the corona. If the star cannot sustain energy output sufficient to keep any kind of nominal diameter, it may contract cataclysmically upon itself, into a “supernova”.

A “red giant” or “red supergiant” may collapse in upon itself too. Not all stars age gracefully into red or brown dwarf stars. The weight of gravitational pull, without energy output to keep everything at atomic distances, may collapse stellar atomic structures themselves into “neutron stars”, a state of matter so dense that an amount the size of the ball in a ball point pen would weigh as much as the battleship Missouri. When this happens in the case of stars of sufficiently great mass, like red giants and supergiants, a "black hole" results at some point. Instead of the matter collapsing into a neutron ball 9-12 miles in diameter, astronomers characterize black holes as having zero diameter. In a black hole the concepts of diameter and mass lose all meaning.

Supernovae might be one of the most efficient material delivery systems of all. One supernova sends shock waves through the entire universe. If our own sun went either supernova or red-giant in its final end, it seems that all the inner planets would be engulfed and consumed in the fireball. (If the sun were a red supergiant, its disk would extend outward to Jupiter or Saturn). As it is, with an "ordinary" supernova, the outer planets, asteroid belts and all would probably be torn asunder and ejected from the solar system, to be captured in the gravitational wells of other star systems another day.

Smithereens: No matter what class of star "goes supernova", there seems to be general agreement that only the star's outer shell is "blown away" to the rest of the universe. Very little else escapes except radiant energy. In the case of the red giant and red supergiant, the outer shell that is blasted away is specifically the outer layer of the solar atmosphere which has been manufacturing most of the silicates in the universe.

So it all begins the same way it all ends. Don't be too surprised if some of the rock on Terra is older than the Sun or the Earth. Whether as dust or chunks, rock is a durable container, known everywhere, an eminently suitable and distinguished emissary for travel and for extended stops at one location. “Rock of Ages” doesn't begin to describe the epochal journey of rocks. Mr. Sandman, bring us a dream.

Other Reading

Jet Propulsion Lab: Near-Earth Objects

Images in this article are mostly public domain (NASA, Hubble Space Telescope) -- we paid for them. In all cases where an image was downloaded from any other site, a link to that site is provided by clicking the image. Additionally, explanatory text and credits for these photos is inserted into the markup as "ALT text". It should be visible as popup commentary in most browsers, when the mouse cursor is moved over the text without clicking.

MIDI credits as follows:

Stardust - Smick and Smodoo's World - Old Codger's Midi Page
Mr. Sandman - http://members.tripod.com/~dawright/oldies.html

Alex Forbes
copyright ©June 10, 2002

Thanks: S. Wahlberg and class (April 2010)