Welcome back geology fans! If atoms are the letters that make up the language
the minerals are the words those alphabetical atoms form.
It is time for us to put those words together into sentences called
rocks, and just as sentences convey information,
you should stop thinking of the rocks you come across in your daily life and
travels as just some rock, or even take it for granite, or a basic basalt.
From now on, if you're willing to follow me down this path, these rocks are forever more
INFORMATION BOMBS! Rocks tell stories to those
prepared to listen. As poet Allen Ginsberg said,
"Don Carpenter has a real geologist's hammer / He can hit a rock and split open
and look inside / and utter some mantra",
to which I would like to add, "and the rock tells a story to those who have
learned their language."
And we geologists, in turn, love to tell those stories to those who wish to hear.
Like minerals, rocks are naturally occurring and solid,
but they are made of many minerals or mineral-like matter.
That mineral-like part shows us once again that nature doesn't like to
There is rarely a purely concrete definition.
In fact, concrete and asphalt aren't rocks because they don't form naturally,
so they can tell us about archaeology but not geology.
Conversely to concrete, coal is a rock even though it is made mostly
of organic matter,
and obsidian is a rock, even though it's made of natural glass.
Now we get to the meat of this episode, with an overview of the three rock types,
and general strategies for identifying them.
I'm pretty sure everyone is familiar with the rock cycle as you have all listened to
the Earth & Environmental Systems audio podcast first as was urged in the first
episode of "Earth Explorations",
but here at least I can show the rock cycle in graphics
Nature doesn't like to be pigeonholed,
but we abusive geologists have shoved the information bombs . . .
um . . . rocks . . . into igneous, sedimentary, and metamorphic.
Agni was the old Indian god of fire, and
Agni-ous - igneous rocks are those that crystallize out of molten magma.
Sedimentary is from the Latin "sedere", "to sit",
which gave "sedimentum", "to settle", and these are the rocks that settle out
in the air and water on the Earth's surface.
Metamorphic means "changed shape",
and these are rocks that have been chemically and/or physically altered,
while remaining in the solid state. But how do we know if a rock before us is
igneous, sedimentary, or metamorphic? Let's start with igneous.
If you encounter rock which seems to have tightly packed mineral crystals,
which are not generally lined up or deformed,
then we start thinking "igneous". Maybe the tightly packed crystals are too small to see,
but being that the crystals grow in interlocking patterns out of a melt,
the igneous information bombs tend to be pretty dense and strong rocks, though
there are some exceptions to this which we will meet and learn how to properly
stort along the way.
Following this episode, we will look into the igneous rocks in detail.
We've done much the heavy lifting for igneous rocks already with episode 14 on
Bowen's reaction series of silicate minerals, so it may be of use to review
that episode before proceeding to our next episode overviewing the
igneous rocks. Settling back into basic identification,
sedimentary rocks come to be through weathering, erosion,
deposition, and lithification, or precipitate out of water when dissolved
minerals reach saturation.
As settling and precipitating particles
lay themselves down, they make horizontally layered deposits.
Sedimentary rocks often have empty spaces between the mineral grains that
can fill with less dense
air or liquids, so the sedimentary rocks rocks are often a little less dense
than our other two rock groups.
The deeper sedimentary rocks get buried and compressed,
the denser they tend to get. But to begin, sedimentary rocks are formed
at or near the surface of the earth where the pressure and temperature are
lower than down below.
This is the same place most living and dying organisms inhabit.
Sedimentary rocks are the fossil bearing rocks.
Every fossil I have ever found was in sedimentary rocks
or deposits, though very odd exceptions can occur.
Made of pieces other rock, monomineralic, without being too recrystallized
often slightly less dense than metamorphic or igneous,
often forming in layers, and/or containing fossils
are the settlement requires that we will look for.
Recrystallization, when one mineral grain begins to grow into another,
happens with burial, but we can only have minimal recrystallization and stay in the
Recrystallization fully develops in the metamorphic rocks which are formed from heating,
pressuring, and reacting with hot fluids. Like the human soul,
we can start with any kind of rock material and subject it to exquisite
torment, and yet sustain throughout and not melt away,
but transform in the solid state into a new and beautiful and often more durable form.
The pressure aspect of our rock's torment tends to line up rod and plate shaped minerals,
while packing them tightly together. This is why we said
igneous could be identified mainly by tightly packed crystals that are not
lined up. When pressure lines up the plate and rod-shaped minerals,
those with one and two directions of cleavage respectively,
we say the rock is foliated metamorphic.
But as we know some minerals have three or more directions a cleavage, or no
cleavage it all.
Such minerals don't line up but they do recrystallize into what might be called
a sugary texture.
Quartz and calcite are two such minerals, and as we will see
the sedimentary rocks can supply us with fairly pure quartz formations and fairly
pure calcite formations,
so their metamorphic product is fairly monomineralic,
and does have a recrystallized appearance,
which we will use to distinguish from the monomineralic sedimentary rocks they
often come from.
I'm betting that if you're watching this, and have gotten this far,
you have a rock and mineral collection of some sort.
Bring it out, dust it off, and get ready to start up a conversation with these
Again, we have dichotomous keys for rock identification on the iTunesU notes page.
You will have to decide if the rock you have is igneous,
sedimentary, or metamorphic depending on those clues that we just gave;
igneous have tightly packed crystals not lined up and some oddball variants that
we will look at more in the next episodes;
sedimentary rocks are layered, have fossils, are made of pieces other rock or
but not too recrystalized; metamorphic can be recrystallized
monomineralic with a sugary texture when non-foliated,
or the minerals are lined up in one or more dimensions and the
foliated metamorphic rocks.
We really begin learning the language of the rocks in our next episode when we
start with the igneous rocks, and kick it off with their
general properties. So see you here next time on Earth Explorations.