Rocks are dynamic and constantly changing entities, formed from Earth's layered interior and transformed over millions of years through the rock cycle, holding a deep history of geological processes.
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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, changing form over time with a history that can span millions
of years.
And here’s what geologists and rock climbers and your aunt with a collection of heart shaped
rocks know that lots of us overlook: one rock is not just like any other.
I’m Alizé Carrère and this is Crash Course Geography.
INTRO
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 landmasses 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 2900 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 2,400 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 percent 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’ve
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 the 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 Tethys 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 grained 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 a rock.
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 show some of the original layers.
So a 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 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 Tethys 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 are more resistant to weathering.
So if any sediment from our chunk of granite got caught up as the Tethys Sea was sucked
under, it would probably change into gneiss.
Gneiss 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’ve been compacted into sedimentary rock or changed
into metamorphic rock.
It seems like it’s 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 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 ft high granite monolith, or kicking a pebble down the road, each piece
of rock has a story that may be million 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 shape shifter: 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 peoples languages.
So we at Crash Course want to acknowledge these peoples’ traditional and ongoing relationship
with that land and all the physical and human geographical elements of it.
We encourage you to learn about the history of the place you call home through resources
like native-land.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 is filmed at the Team
Sandoval Pierce Studio and was made with the help of all these nice people.
If you want to help keep all Crash Course free for everyone, forever, you can join our
community on Patreon.
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