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

Summary

Explore Earth's dynamic geology in this Crash Course Geography episode with Alizé Carrère. Journey from Earth's solid exterior to its core, learning about rock formation and the rock cycle. Discover the differences between continental and oceanic crust, the processes of magma cooling, and the creation of igneous, sedimentary, and metamorphic rocks. Understand how geological activities shape landforms like the Himalayas, and gain insight into the rock cycle's continuous transformations. This episode also acknowledges the traditional Indigenous and Aboriginal lands and encourages viewers to engage with local histories.

Takeaways

• 🌏 The Earth's solid exterior is composed of a variety of rocks, some of which are still being formed by volcanic activity.
• 🔍 Geologists, rock climbers, and enthusiasts recognize the unique characteristics of each rock, emphasizing their distinct nature.
• 🌌 The Earth formed 4.5 billion years ago from a nebula of dust and gas, developing into layers including the atmosphere, crust, mantle, and core.
• 🏔 The Earth's crust is thin and less dense, consisting of two types: continental crust made of lighter rocks and oceanic crust made of heavier rocks.
• 🌋 Igneous rocks, formed from cooling magma, make up about 90% of the Earth's crust and can be either intrusive like granite or extrusive like basalt.
• 🏞 The formation of the Himalayas is attributed to the collision of the Indian and Eurasian plates, leading to volcanic activity and the creation of granite.
• 📏 The texture of rocks is influenced by the rate of magma cooling; slower cooling results in larger crystals, as seen in granite.
• 🌊 Rocks at the surface are subject to weathering and erosion, which can break them down and transport sediments to new locations.
• 🏜 Sedimentary rocks are formed from compacted and cemented sediments, such as sandstone from sand particles and limestone from organic remains.
• 🧱 Metamorphic rocks result from the transformation of existing rocks under intense temperature and pressure, without melting, like gneiss from granite.
• 🔁 The rock cycle illustrates the continuous alteration of rocks from one type to another through geological processes over millions of years.
• 🌱 Crash Course Geography acknowledges the importance of recognizing and respecting the history and relationship of Indigenous peoples with the land.

Q & A

What is the Earth's solid exterior made of?

The Earth's solid exterior is made of a variety of rocks, some of which are still being formed by active volcanoes as lava hardens upon contact with the atmosphere or ocean.

How old are most of the Earth's rocks?

Most of the Earth's rocks are extremely old, with histories that can span millions of years.

What are the two types of crust on Earth?

The two types of crust on Earth are continental crust, which makes up the major landmasses and is rich in silicon and aluminum, and oceanic crust, which forms the ocean floors and is rich in iron, silicon, and magnesium.

What is the composition of the Earth's mantle?

The mantle is rich in elements like iron, magnesium compounds, and silicates, and its density gradually changes as one goes deeper into the Earth due to increasing pressure.

What are the core layers of the Earth made of?

The Earth's core is made of iron and nickel; the outer core is molten due to high temperatures, while the inner core remains solid due to tremendous pressure.

How do scientists study the Earth's interior?

Scientists study the Earth's interior by analyzing how seismic waves from earthquakes travel through the planet, which helps them model the interior structure.

What is the significance of studying the Earth's interior?

Studying the Earth's interior helps us understand phenomena such as earthquakes, volcanic eruptions, the formation of continents, and even the origin of the planet itself.

What is the relationship between magma and igneous rocks?

Magma, when it cools and solidifies, forms igneous rocks. The type of igneous rock depends on whether the magma cools above or below the Earth's surface.

How does the cooling of magma affect the texture of igneous rocks?

The cooling rate of magma determines the texture of the resulting igneous rock. Slow cooling produces larger mineral crystals, as seen in granite, while rapid cooling results in fine-grained textures, as in basalt.

What is the process of rock formation called when sediments are compacted and transformed into rock?

The process is called sedimentation, and it leads to the formation of sedimentary rocks, which often retain the original layers or strata.

What causes existing rocks to change into metamorphic rocks?

Existing rocks change into metamorphic rocks through the process of metamorphism, which involves recrystallization of minerals due to intense temperature and pressure changes without the rock melting.

How does the rock cycle illustrate the continuous transformation of rocks?

The rock cycle shows that rocks can transform from one type to another over millions of years, such as from igneous to sedimentary to metamorphic, and potentially back to igneous again.

What is the importance of acknowledging Indigenous and Aboriginal peoples in geographical discussions?

Acknowledging Indigenous and Aboriginal peoples in geographical discussions is important as it recognizes their traditional and ongoing relationship with the land and its elements, and respects their history and contributions.

Outlines

00:00

🌏 Earth's Geological Formation and Composition

This paragraph introduces the diverse nature of Earth's rocks, highlighting their dynamic history and the geological processes that shape them. It explains the formation of Earth from a nebula and the development of its layered structure, including the atmosphere, crust, mantle, and core. The crust is differentiated into continental and oceanic types, each with distinct mineral compositions and densities. The paragraph also discusses the role of minerals in forming rocks and the significance of the rock cycle, which includes the transformation of rocks through various geological processes.

05:01

🔥 The Rock Cycle and Geological Processes

This section delves into the rock cycle, focusing on the formation of igneous rocks such as granite through the cooling of magma. It describes how the rate of cooling affects the rock's texture and the formation of intrusive and extrusive igneous rocks. The paragraph also covers the weathering and erosion processes that expose rocks to new environmental conditions, leading to their transformation into sedimentary rocks through compaction and cementation of sediments. Metamorphism is also discussed, where existing rocks are altered into new types without melting, resulting in harder and more compact rocks like gneiss. The journey of a granite boulder from its formation to its exposure on the surface illustrates the continuous changes rocks undergo over geological time.

10:01

🏔 Acknowledgement of Indigenous Peoples and Geography

The final paragraph shifts focus from geological processes to the acknowledgment of Indigenous and Aboriginal peoples' relationships with the land. It addresses the issue of modern geopolitical divisions and place names that often disregard the original inhabitants' perspectives. The paragraph encourages viewers to learn about the history of their local area through resources like native-land.ca and to engage with Indigenous and Aboriginal communities. The paragraph concludes with a note on the production of the Crash Course Geography series and an invitation for viewers to support the project on Patreon.

Keywords

💡Rocks

Rocks are naturally occurring solid aggregates of minerals. In the context of the video, rocks are the Earth's solid exterior, varying in age and formation processes. They are central to the theme as they represent the Earth's geology and the processes that shape it, such as volcanic activity and sedimentation.

💡Geologists

Geologists are scientists who study the Earth, its materials, and the processes that shape it. They play a key role in understanding the history and formation of rocks, as mentioned in the script when discussing the formation of the Earth's crust and the various types of rocks.

💡Crust

The Earth's crust is the outermost layer, composed of minerals that solidified from the original gas and dust. It is divided into continental and oceanic crust, with different densities and compositions, which is crucial to understanding the Earth's structure and the formation of landmasses.

💡Mantle

The mantle is the thick layer beneath the Earth's crust, rich in iron, magnesium, and silicates. It is significant to the video's theme as it discusses the mantle's role in the formation of magma and its gradual change in density with depth, affecting geological processes.

💡Core

The Earth's core is the innermost layer, composed of iron and nickel, with a solid inner core and a molten outer core. It is relevant to the video's theme as it explains the dynamics of the Earth's interior, which influences seismic activity and the planet's overall structure.

💡Seismic Waves

Seismic waves are vibrations that travel through the Earth, generated by events like earthquakes. They are vital to the video's narrative as they are used by scientists to study the Earth's interior, contributing to our understanding of its structure and the processes occurring within.

💡Igneous Rocks

Igneous rocks form from the cooling and solidification of magma or lava. They are the most abundant type of rock in the Earth's crust and are central to the video's theme, illustrating the processes of rock formation and the history of the Earth's surface.

💡Sedimentary Rocks

Sedimentary rocks are formed from the compaction and cementation of sediments. The video discusses their formation from the accumulation of sediments like those from the Himalayas, highlighting the continuous transformation of the Earth's surface materials.

💡Metamorphic Rocks

Metamorphic rocks result from the transformation of existing rock types under intense heat and pressure. The video uses gneiss as an example, showing how rocks can change through metamorphism, which is a key concept in the rock cycle discussed in the script.

💡Plate Tectonics

Plate tectonics is the theory that describes the movement of the Earth's lithosphere into plates. It is integral to the video's theme as it explains the formation of mountain ranges like the Himalayas and the processes that shape the Earth's surface.

💡Rock Cycle

The rock cycle is the continuous process by which rocks are formed, broken down, and reformed. It is a central concept in the video, illustrating the dynamic nature of the Earth's geology and the interconnected processes that create and transform rocks.

Highlights

Earth's solid exterior is composed of a vast variety of rocks, some of which are still forming today.

Rocks have a history that can span millions of years, constantly changing form.

Geologists and rock climbers recognize the unique characteristics of each rock.

The Earth solidified 4.5 billion years ago from a nebula of dust and gas.

The Earth's structure consists of different shell-layers including the atmosphere and crust.

Minerals are inorganic, naturally occurring chemical compounds with a crystalline structure.

There are two types of crust on Earth: continental and oceanic, with distinct compositions.

The mantle, rich in iron and magnesium, changes density with depth.

The Earth's core is composed of iron and nickel, with a molten outer core and a solid inner core.

Seismic waves from earthquakes help scientists model the Earth's interior.

Igneous rocks make up about 90 percent of the Earth's crust and form from cooling magma.

The formation of the Himalayas is linked to the collision of the Indian and Eurasian plates.

Rock texture is influenced by the rate of magma cooling and the depth within the crust.

Sedimentary rocks are formed from compacted and cemented sediments.

Metamorphic rocks result from the recrystallization of minerals under intense pressure and temperature changes.

The rock cycle illustrates the continuous transformation of rocks from one type to another over millions of years.

The story of a granite boulder exemplifies the complex journey of rocks through geological processes.

Acknowledgment of Indigenous and Aboriginal peoples' relationship with the land is important in geographical discussions.

Transcripts

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.