What Are Rocks and How Do They Form? Crash Course Geography #18 - By CrashCourse
Transcript
00:0-1 | from towering mountains to the gravel and pebbles along the | |
00:02 | river earth . Solid exterior is made of a huge | |
00:05 | variety of rocks . Some are even being formed this | |
00:08 | very moment as active volcanoes spew lava that hardens as | |
00:11 | it hits the atmosphere or ocean . But most of | |
00:13 | the Earth's rocks are extremely old . Each rock is | |
00:16 | a shape shifter shifting form over time with a history | |
00:19 | that can spend millions of years . And here's what | |
00:21 | geologists and rock climbers and your aunt with a collection | |
00:24 | of heart shaped rocks know that lots of us overlook | |
00:27 | one rock is not just like any other . I'm | |
00:30 | al is a career and this is crash course geography | |
00:34 | . Yeah . Yeah . Film Way back 4.5 billion | |
00:42 | years ago when the solar system was forming the earth | |
00:45 | solidified as a swirling nebula of dust and gas that | |
00:48 | collapsed under its own gravity . Then as gravity kept | |
00:51 | pulling on different molecules the earth formed . It's spheroid | |
00:54 | shape made up of different shell layers . In fact | |
00:56 | , even though we sometimes think of it as being | |
00:58 | separate from the earth , the atmosphere is really the | |
01:00 | first and lightest shell with its own set of layers | |
01:03 | at the bottom of the atmosphere , things start to | |
01:05 | feel more solid and we hit Earth's crust compared to | |
01:08 | the rest of the planet . The crust is extremely | |
01:10 | thin and has a low density , which is how | |
01:12 | tightly packed the molecules are , that make up something | |
01:15 | particles in the original gas and dust that ended up | |
01:17 | in the earth's crust became the minerals or inorganic , | |
01:20 | naturally occurring chemical compounds with a crystalline structure and rocks | |
01:23 | . Solid collections of minerals that we find on the | |
01:26 | planet today , there are actually two types of crust | |
01:28 | on earth , continental crust and oceanic crust . Continental | |
01:32 | crust makes up the major landmasses on earth that are | |
01:34 | exposed to the atmosphere . It's made of light colored | |
01:37 | and lightweight rocks rich in silicon and aluminum , which | |
01:40 | helped make it the least dense layer besides the atmosphere | |
01:43 | . But not the thinnest that would be the oceanic | |
01:45 | crust , which is what forms the vast ocean floors | |
01:49 | . Oceanic crust is made of heavy , dark colored | |
01:51 | iron rich rocks that also have a lot of silicon | |
01:53 | and magnesium . It's denser than the continental crust , | |
01:56 | but only a few kilometers thick beneath the crust is | |
01:59 | the much thicker mantle . It stretches for roughly 2900 | |
02:02 | kilometers and is rich in elements like iron , magnesium | |
02:05 | compounds and combinations of silicon and oxygen called silicates . | |
02:08 | The mantle is so thick , it actually gradually changes | |
02:11 | density as we go deeper into the earth . The | |
02:13 | lower mantle is closer to the center where pressure is | |
02:16 | higher , so it's denser as everything is pushed together | |
02:18 | more . The last layer in our journey to the | |
02:20 | center of the earth is the core made of iron | |
02:23 | and nickel . The 2400 kilometer thick outer core is | |
02:26 | so hot all that iron becomes molten and turns to | |
02:29 | liquid . But the hot , dense inner core of | |
02:31 | iron with a radius of 960 kilometers , is always | |
02:35 | solid because of the tremendous pressure . No one has | |
02:37 | been to the center of the earth , but scientists | |
02:39 | study how seismic waves from earthquakes traveled through the planet | |
02:42 | to model the earth's interior . And learning about what | |
02:44 | earth is like on the inside helps us learn about | |
02:47 | earthquakes , volcanic eruptions , how continents formed , and | |
02:50 | even about the origin of the planet itself . Some | |
02:53 | of the elements show up a lot , but each | |
02:55 | layer has a distinct chemical composition and temperature , and | |
02:58 | each one in its own way helps give us the | |
03:00 | rocks and landforms we see on the surface like here | |
03:02 | , high in the Himalayas , where a large chunk | |
03:04 | of granite is newly exposed on the surface during the | |
03:07 | day it's grains glint in the sun and as night | |
03:10 | falls , the rock blends into the darkness , an | |
03:12 | occasional goat clambers on its rounded dome , searching for | |
03:15 | a tuft of grass . It seems innocuous enough , | |
03:17 | but seeing granite here means that at some point in | |
03:20 | time eons ago , volcanic activity was transforming the surface | |
03:24 | within the earth's crust and beneath the surface is magma | |
03:26 | or molten rock that can cool and solidify into igneous | |
03:30 | rock . Igneous rocks make up about 90% of the | |
03:32 | earth's crust . Though you might not notice because they're | |
03:34 | often covered by other types of rocks , soil or | |
03:37 | ocean . We actually end up with different types of | |
03:39 | igneous rocks depending on whether magma cools above or below | |
03:42 | earth's surface . When magma cools and solidifies beneath the | |
03:45 | earth's surface , it forms intrusive igneous rock and granite | |
03:49 | is an intrusive igneous rock , but when magma erupts | |
03:52 | onto the surface , we call it lava , and | |
03:54 | after it cools and solidifies , it becomes exclusive igneous | |
03:57 | rock . There aren't any volcanoes in the Himalayas , | |
03:59 | but 60 million years ago . In the initial Himalayan | |
04:02 | mountain building phase , volcanic activity like magma churning beneath | |
04:05 | the surface would have been comin from measuring the magnetism | |
04:08 | of rocks , dating , plant and animal fossils in | |
04:10 | the rock and studying the changes in how land moves | |
04:13 | . We know the Himalayan mountain ranges formed when the | |
04:15 | indian and Eurasian plates , or chunks of crust floating | |
04:18 | independently over the mantle collided , and this process still | |
04:22 | continues today around 60 million years ago , the Indian | |
04:25 | plate was about 6400 kilometers south of the Eurasian plate | |
04:29 | . As it moved north . An ancient ocean called | |
04:31 | the Tethys sea was dragged down beneath the Eurasian plate | |
04:34 | into the earth's interior . The oceanic crust and all | |
04:37 | the tiny sediment particles that used to be on the | |
04:39 | shore of the sea were also dragged down where they | |
04:42 | melted into magma . Eventually the magma moved into cracks | |
04:45 | and fissures deep inside the earth where it's solidified into | |
04:48 | our granite . If we brush off some of the | |
04:50 | dirt and grass and ask that go to move along | |
04:53 | , we can get a better look at our rock | |
04:54 | and its texture . Rocks contain minerals that form crystals | |
04:58 | , which is when molecules or atoms are arranged in | |
05:00 | a regular repeating patterns . How fast magma cools , | |
05:03 | affects crystallization and the texture of Iraq intrusive rocks like | |
05:06 | granite cools slowly , so they have more time for | |
05:09 | larger mineral crystals to form . Which is why granite | |
05:12 | looks coarse grained . And we can even see the | |
05:14 | crystals without a microscope . Magma can also occur at | |
05:16 | different depths within the crust and mantle , which means | |
05:19 | it's exposed to different temperature and pressure conditions . Too | |
05:22 | heavier minerals deeper down will crystallize first and be denser | |
05:26 | and darker while minerals that form closer to the surface | |
05:28 | are less dense and lighter in color . So our | |
05:31 | granite is fell sick , which means it's rich in | |
05:33 | light colored , lighter weight minerals , especially silicon and | |
05:36 | aluminum . And the magma that it came from was | |
05:38 | closer to the surface . On the other hand , | |
05:40 | lava cools very quickly when it hits earth's surface , | |
05:43 | which limits how crystals grow exclusive . Rocks like basalt | |
05:46 | end up with small individual minerals and a fine grained | |
05:49 | texture that looks much more seamless . And assault is | |
05:51 | Math IQ , which means it's rich in darker , | |
05:53 | heavier minerals like compounds of magnesium and iron , even | |
05:57 | though it formed from lava on the surface , the | |
05:59 | original magma was deep in the earth's crust or mantle | |
06:02 | . Yet somehow our chunk of granite made its way | |
06:04 | to the surface , like maybe it was uplifted as | |
06:06 | the Indian plate pushed further north , and as the | |
06:08 | Himalayas rose at the surface , rocks have to deal | |
06:11 | with different temperatures and pressures than where they formed deep | |
06:13 | within the crust , not to mention weathering and erosion | |
06:16 | , or being broken down by the earth's atmosphere . | |
06:18 | Water and living things . Water with its ability to | |
06:21 | dissolve practically anything , can especially alter disintegrate and decomposed | |
06:25 | rocks . The pieces can then be picked up and | |
06:27 | deposited elsewhere , so once the extra rocks and soil | |
06:30 | are removed by weathering and erosion , our granite is | |
06:33 | exposed to a totally new surface environment , and it | |
06:36 | might seem like the granite outcrop is just sitting there | |
06:38 | , doing nothing . But unseen processes are operating like | |
06:41 | the pressure is different out here on the surface , | |
06:43 | so the outer few centimetres of the rock might expand | |
06:45 | outward and crack . Then the loose outer layers of | |
06:48 | rock can slough off like a snake , shedding its | |
06:50 | skin . Or temperature differences can also cause the rock | |
06:53 | to expand or contract . This leads to granular disintegration | |
06:57 | , or when individual mineral grains break free from Iraq | |
06:59 | , which is how over thousands or millions of years | |
07:02 | tons of little rock dust pieces accumulated around the base | |
07:05 | of this granite boulder , so as clouds gather over | |
07:08 | the mountain top and a steady rain begins . The | |
07:10 | little mineral grains can get washed into a stream and | |
07:13 | may eventually be dropped along the channel banks during a | |
07:15 | flood , or they'll bounce along with the water and | |
07:17 | travel all the way to where the river empties into | |
07:19 | the sea and the grains become part of the ocean | |
07:22 | bottom grains like these are sediments . Centuries of monsoons | |
07:25 | and soil erosion have blanketed the floor of the Bay | |
07:27 | of Bengal in up to 20 kilometers of sediment from | |
07:30 | the Himalayas . So part of our granite boulder is | |
07:33 | actually lying on the bottom of the ocean . If | |
07:35 | we could slice into all the sediment lying on the | |
07:37 | floor of the Bay of Bengal , we'd likely see | |
07:39 | horizontal layers or strata from different times when large amounts | |
07:42 | of sediments were deposited over time . The pressure from | |
07:45 | the weight of the material above compacts cement and transforms | |
07:48 | the sediments into sedimentary rock , which still shows some | |
07:51 | of the original layers . So sedimentary rocks like sandstone | |
07:54 | is made of cemented sand sized particles of courts and | |
07:57 | other minerals . It has very visible grains , lots | |
08:00 | of tiny little holes and is very resistant to weathering | |
08:02 | . Other sedimentary rocks like limestone are formed when the | |
08:05 | remains of organisms like shellfish , corals and plankton sink | |
08:08 | to the ocean floor . Coal is another one of | |
08:11 | these organic sedimentary rocks that's created when organic matter accumulates | |
08:14 | and compacts in swampy environments over millions of years . | |
08:18 | At the bottom of the ancient TTC , which disappeared | |
08:21 | about 20 million years ago , sedimentary rocks would have | |
08:24 | formed from sediments brought down by rivers , but as | |
08:26 | the Indian plate pushed northward , the gap between the | |
08:29 | indian plate and the Eurasian plate narrowed as the plates | |
08:32 | collided and the Himalayas formed . The sediment on the | |
08:35 | sea floor was compressed and crumpled on top of being | |
08:38 | squished and crumpled . The rocks also have to go | |
08:40 | through intense temperature and pressure changes . All this action | |
08:44 | causes the existing rock to go through metamorphosis , um | |
08:46 | and change into a completely new rock type . All | |
08:48 | the minerals from the original rock , re crystallized without | |
08:51 | having to melt down into molten rock . The new | |
08:53 | metamorphic rocks are typically harder , more compact and more | |
08:57 | resistant to weathering , so if any sediment from our | |
08:59 | chunk of granite got caught up as the TTC was | |
09:01 | sucked under , it would probably changing . Nice , | |
09:04 | nice , has alternate bands of light and dark minerals | |
09:07 | and conformed from a variety of different rocks . It's | |
09:09 | also very hard and resistant to weathering and erosion . | |
09:12 | So our granite boulder started life as igneous rock , | |
09:15 | but as pieces broke off , they could have been | |
09:17 | compacted into sedimentary rock or changed into metamorphic rock . | |
09:21 | It seems like it sat there for all of time | |
09:23 | . But rocks like our chunk of granite are continuously | |
09:26 | altered over millions of years from one rock type to | |
09:28 | another as a part of the rock cycle . But | |
09:30 | the story of our granite is not the story of | |
09:32 | all rocks . There are many pathways through the cycle | |
09:35 | . Like igneous rocks could skip being sedimentary rocks and | |
09:38 | go directly to being a metamorphic rock or even re | |
09:41 | melt and re crystallized to make new igneous rock , | |
09:44 | whether scaling a 3000 ft high granite monolith or kicking | |
09:47 | a pebble down the road . Each piece of rock | |
09:49 | has a story that maybe millions of years old , | |
09:52 | etched in the stone by processes both on the surface | |
09:54 | and deep within the earth . Next time we'll tell | |
09:57 | the stories of another kind of shapeshifter continents and how | |
10:01 | plate tectonics have created the earth . We know today | |
10:04 | many maps and borders represent modern geopolitical divisions that have | |
10:07 | often been decided without the consultation permission or recognition of | |
10:11 | the land's original inhabitants . Many geographical place names also | |
10:14 | don't reflect the indigenous or aboriginal peoples languages . So | |
10:17 | we at crash course want to acknowledge these people's traditional | |
10:20 | and ongoing relationship with that land and all the physical | |
10:23 | and human geographical elements of it . We encourage you | |
10:25 | to learn more about the history of the place you | |
10:27 | call home through resources like native land dot C . | |
10:29 | A . And by engaging with your local indigenous and | |
10:32 | aboriginal nations through the websites and resources . They provide | |
10:35 | thanks for watching this episode of Crash Course Geography , | |
10:38 | which was filmed at the Team Sandoval Peers studio and | |
10:40 | was made with the help of all these nice people | |
10:43 | . If you want to help keep Crash Course free | |
10:44 | for everyone forever , you can join our community on | |
10:47 | Patreon . |
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