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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