Name: What Are Rocks and How Do They Form? Crash Course Geography #18
Uploaded: 2022-07-01T01:04:03.000Z
Duration: 10 min 57 s
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From towering mountains to the gravel and pebbles along a river, Earth's solid exterior

Advanced passive voice

Some are even being formed this very moment as active volcanoes spew lava that hardens

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

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.

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

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

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

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

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

Like here, high in the Himalayas, where a large chunk of granite is newly exposed on

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

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

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

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

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

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

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

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

Extrusive rocks like basalt end up with small, individual minerals and a fine grained texture

And basalt is mafic, which means it's rich in darker, heavier minerals like compounds

Even though it formed from lava on the surface, the original magma was deep in the Earth's

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What Are Rocks and How Do They Form? Crash Course Geography #18

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