What Are Rocks and How Do They Form? Crash Course Geography #18 - Free Educational videos for Students in K-12

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

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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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