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.
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.
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:
Poly Pomona website, which offers informative discussions and
Geological Survey website, which gives more insight into rock origins.
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 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 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
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
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.
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
Web discussion: Earth's
Interior & Plate Tectonics
Web discussion: Earth's
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.
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?”
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
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.
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.
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”!
Types of Matter and Energy In
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.
neutrinos”: you can skip this one. Whatever they are, they
it's interesting how little of the universe is organized into recognizable
stars, or, presumably, solar systems – one half of one percent.
and helium: the stuff stars are made of. Nice to know we have
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:
"Well...interstellar dust comes from a variety of sources,
but mostly from the atmospheres of very old stars. In the outer
layers of these red giant and red supergiant stars, the temperatures
are so low that dust grains made of silicon monoxide or graphite
can condense like rain drops directly from the stars matterial.
The dust grains are whafted into space by radiation pressure
or by some mechanical means, and over billions of years and millions
of stars, they form a persistent 'medium' of dust grains everywhere.
... Typical interstellar dust grains are about 1/2 of a micron
in diameter, but can grow up to several microns or more in the
deep dark cores of collapsing dust clouds. Eventually these dust
grains stick together to form ever larger bodies all the way
up to asteroids and planets!"
Sten Odenwald, for the NASA IMAGE/POETRY education
and Public Outreach program
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!
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
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?
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
|"We believe that comets are the building
blocks of the planets," he says. "That is the planetary
system began as a disk of dust and gas. Today, it presently consists
mostly of planets. But it's believed that comets were an intermediate
stage in the formation of planets and the comets we are seeing
are the 'sweepings from the workbench' leftover from the formation
of the planets."
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.
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.
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
There seems to be no need to choose between
competing theories. The comet impact theory makes an abundance of water
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.
Rock of Ages
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
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
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.
Fire and Ice
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”.
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.
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.
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
atmosphere which has been manufacturing most of the silicates in
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.
“Fire and Ice” - Robert Frost
... Miscellaneous Poems to 1920. 1920.
2. Fire and Ice. (From Harper’s Magazine, December 1920.)
Some say the world will end in fire,
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
is also great
and would suffice.
Jet Propulsion Lab: Near-Earth
this article are mostly public domain (NASA, Hubble Space Telescope)
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credits as follows:
and Smodoo's World - Old Codger's Midi Page
Mr. Sandman - http://members.tripod.com/~dawright/oldies.html
copyright ©June 10, 2002
and class (April 2010)