from towering mountains to the gravel
and pebbles along a river
earth's solid exterior is made of a huge
variety of rocks
some are even being formed this very
moment as active volcanoes spew lava
that hardens as it hits the atmosphere
or ocean
but most of the earth's rocks are
extremely old each rock is a
shapeshifter
shifting form over time with a history
that can span millions of years
and here's what geologists and rock
climbers and your ant with a collection
of heart-shaped rocks
know that lots of us overlook one rock
is
not just like any other i'm alize
carrere and this is crash course
geography
[Music]
way back 4.5 billion years ago when the
solar system was forming
the earth solidified as a swirling
nebula of dust and gas that collapsed
under its own gravity
then as gravity kept pulling on
different molecules the earth formed its
spheroid shape made up of different
shell layers
in fact even though we sometimes think
of it as being separate from the earth
the atmosphere is really the first and
lightest shell with its own set of
layers
at the bottom of the atmosphere things
start to feel more solid and we hit
earth's crust
compared to the rest of the planet the
crust is extremely thin and has a low
density which is how tightly packed the
molecules are that make up something
particles in the original gas and dust
that ended up in the earth's crust
became the minerals or
inorganic naturally occurring chemical
compounds with a crystalline structure
and rocks solid collections of minerals
that we find on the planet today
there are actually two types of crust on
earth continental crust and oceanic
crust
continental crust makes up the major
land masses on earth that are exposed to
the atmosphere
it's made of light-colored and
lightweight rocks rich in silicon and
aluminum
which help make it the least dense layer
besides the atmosphere but not the
thinnest
that would be the oceanic crust which is
what forms the vast ocean floors
oceanic crust is made of heavy dark
colored iron-rich rocks that also have a
lot of silicon and magnesium
it's denser than the continental crust
but only a few kilometers thick
beneath the crust is the much thicker
mantle it stretches for roughly 2 900
kilometers and is rich in elements like
iron
magnesium compounds and combinations of
silicon and oxygen
called silicates the mantle is so thick
it actually gradually changes density as
we go deeper into the earth
the lower mantle is closer to the center
where pressure is higher so it's denser
as everything is pushed together more
the last layer in our journey to the
center of the earth is the core made of
iron and nickel
the 2400 kilometer thick outer core is
so hot
all that iron becomes molten and turns
to liquid but the hot dense inner core
of iron with a radius of 960 kilometers
is always solid because of the
tremendous pressure no one has been to
the center of the earth but scientists
study how seismic waves from earthquakes
travel through the planet to model the
earth's interior
and learning about what earth is like on
the inside helps us learn about
earthquakes volcanic eruptions
how continents formed and even about the
origin of the planet itself
some of the elements show up a lot but
each layer has a distinct chemical
composition and temperature
and each one in its own way helps give
us the rocks and landforms we see on the
surface
like here high in the himalayas where a
large chunk of granite is newly exposed
on the surface
during the day its grains glint in the
sun and as night falls the rock blends
into the darkness
an occasional goat clambers on its
rounded dome searching for a tuft of
grass
it seems innocuous enough but seeing
granite here means that at some point in
time
eons ago volcanic activity was
transforming the surface
within the earth's crust and beneath the
surface is magma or molten rock
that can cool and solidify into igneous
rock igneous rocks make up about 90
of the earth's crust though you might
not notice because they're often covered
by other types of rocks soil or ocean
we actually end up with different types
of igneous rocks depending on whether
magma cools above or below earth's
surface
when magma cools and solidifies beneath
the earth's surface it forms
intrusive igneous rock and granite is an
intrusive igneous rock but when magma
erupts onto the surface we call it
lava and after it cools and solidifies
it becomes extrusive igneous rock there
aren't any volcanoes in the himalayas
but
60 million years ago in the initial
himalayan mountain building phase
volcanic activity like magma churning
beneath the surface would have been
common
from measuring the magnetism of rocks
dating plant and animal fossils in the
rock
and studying the changes in how land
moves we know the himalayan mountain
ranges formed when the indian and
eurasian plates or chunks of crust
floating independently over the mantle
collided and this process still
continues today
around 60 million years ago the indian
plate was about 6 400 kilometers south
of the eurasian plate
as it moved north an ancient ocean
called the tethy sea
was dragged down beneath the eurasian
plate into the earth's interior
the oceanic crust and all the tiny
sediment particles that used to be on
the shore of the sea
were also dragged down where they melted
into magma eventually the magma moved
into cracks and fissures deep inside the
earth
where it solidified into our granite if
we brush off some of the dirt and grass
and ask that goat to move along
we can get a better look at our rock and
its texture rocks contain minerals that
form crystals which is when molecules or
atoms are arranged in a regular
repeating pattern
how fast magma cools affects
crystallization and the texture of a
rock
intrusive rocks like granite cool slowly
so they have more time for larger
mineral crystals to form
which is why granite looks
coarse-grained and we can even see the
crystals without a microscope
magma can also occur at different depths
within the crust and mantle
which means it's exposed to different
temperature and pressure conditions too
heavier minerals deeper down will
crystallize first and be denser and
darker
while minerals that form closer to the
surface are less dense and lighter in
color
so our granite is felsic which means
it's rich in light-colored
lighter weight minerals especially
silicon and aluminum and the magma that
it came from was closer to the surface
on the other hand lava cools very
quickly when it hits earth's surface
which limits how crystals grow extrusive
rocks like basalt end up with small
individual minerals and a fine grain
texture that looks much more seamless
and basalt is mafic which means it's
rich in darker heavier minerals like
compounds of magnesium and iron
even though it formed from lava on the
surface the original magma was deep in
the earth's crust or mantle
yet somehow our chunk of granite made
its way to the surface
like maybe it was uplifted as the indian
plate pushed further north and as the
himalayas rose
at the surface rocks have to deal with
different temperatures and pressures
than where they formed deep within the
crust
not to mention weathering and erosion or
being broken down by the earth's
atmosphere
water and living things water with its
ability to dissolve practically anything
can especially alter disintegrate and
decompose rocks
the pieces can then be picked up and
deposited elsewhere so once the extra
rocks and soil are removed by weathering
and erosion
our granite is exposed to a totally new
surface environment
and it might seem like the granite
outcrop is just sitting there doing
nothing
but unseen processes are operating like
the pressure is different out here on
the surface so the outer few centimeters
of the rock might expand outward and
crack
then the loose outer layers of rock can
slough off like a snake shedding its
skin
or temperature differences can also
cause the rock to expand or contract
this leads to granular disintegration or
when individual mineral grains break
free from iraq
which is how over thousands or millions
of years tons of little rock dust pieces
accumulated around the base of this
granite boulder
so as clouds gather over the mountain
top and a steady rain begins
the little mineral grains can get washed
into a stream and may eventually be
dropped along the channel banks during a
flood
or they'll bounce along with the water
and travel all the way to where the
river empties into the sea
and the grains become part of the ocean
bottom grains like these are sediments
centuries of monsoons and soil erosion
have blanketed the floor of the bay of
bengal in up to 20 kilometers of
sediment from the himalayas
so part of our granite boulder is
actually lying on the bottom of the
ocean
if we could slice into all the sediment
lying on the floor of the bay of bengal
we'd likely see horizontal layers or
strata from different times when large
amounts of sediments were deposited
over time the pressure from the weight
of the material above compacts
cements and transforms the sediments
into sedimentary rock
which still shows some of the original
layers so sedimentary rock like
sandstone is made of cemented sand-sized
particles of quartz and other minerals
it has very visible grains lots of tiny
little holes and is very resistant to
weathering
other sedimentary rocks like limestone
are formed when the remains of organisms
like shellfish
corals and plankton sink to the ocean
floor coal is another one of these
organic sedimentary rocks that's created
when organic matter accumulates and
compacts in swampy environments over
millions of years
at the bottom of the ancient tethy sea
which disappeared about 20 million years
ago
sedimentary rocks would have formed from
sediments brought down by rivers but as
the indian plate pushed northward
the gap between the indian plate and the
eurasian plate narrowed
as the plates collided and the himalayas
formed the sediment on the seafloor was
compressed
and crumpled on top of being squished
and crumpled the rocks also have to go
through intense temperature and pressure
changes
all this action causes the existing rock
to go through metamorphism and change
into a completely new rock type all the
minerals from the original rock
recrystallize without having to melt
down into molten rock the new
metamorphic rocks are typically harder
more compact and more resistant to
weathering so if any sediment from our
chunk of granite got caught up as the
tethy sea was sucked under
it would probably change into nice nice
has alternate bands of light and dark
minerals and can form from a variety of
different rocks
it's also very hard and resistant to
weathering and erosion so
our granite boulder started life as
igneous rock but as pieces broke
off they could have been compacted into
sedimentary rock or changed into
metamorphic rock
it seems like it sat there for all of
time but rocks like our chunk of granite
are continuously altered over millions
of years from one rock type to another
as a part of the rock cycle
but the story of our granite is not the
story of all rocks
there are many pathways through the
cycle like igneous rocks could skip
being sedimentary rocks and go directly
to being a metamorphic rock
or even re-melt and recrystallize to
make new igneous rock
whether scaling a 3 000 foot high
granite monolith or kicking a pebble
down the road
each piece of rock has a story that may
be millions of years old
etched in the stone by processes both on
the surface and deep within the earth
next time we'll tell the stories of
another kind of shapeshifter
continents and how plate tectonics have
created the earth we know today
many maps and borders represent modern
geopolitical divisions that have often
been decided without the consultation
permission or recognition of the land's
original inhabitants
many geographical place names also don't
reflect the indigenous or aboriginal
people's languages
so we at crash course want to
acknowledge these people's traditional
and ongoing relationship with that land
and all the physical and human
geographical elements of it
we encourage you to learn more about the
history of the place you call home
through resources like nativeland.ca
and by engaging with your local
indigenous and aboriginal nations
through the websites and resources they
provide
thanks for watching this episode of
crash course geography which was filmed
at the team sandoval peer studio
and was made with the help of all these
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