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  • Over two thousand years ago, the great  Carthaginian military leader Hannibal crossed  

  • the Alps leading thousands of soldiers, horsesand the infamous elephants to victory over Rome.  

  • It was a tremendous undertaking, and knowing  the lay of the land or gaining strategic  

  • control of mountain passes and rivers  has often been key to empire building.

  • But we'll leave the plot of  that story for world history.  

  • Today we'll focus instead on the settingthe landscapes over which they travelled.

  • The topography, or shape of a region or land, has  influenced so much -- like lifestyles, economic  

  • activities, and transportation. We've even  gotten different cultures and languages thanks  

  • to isolated communities flourishing in places  that are hard to access by the outside world.

  • So as physical geographers, we want to know how  the scene -- the Earth -- was set. Like what  

  • shaped the surface of the Earth and led to so  many different landforms, or features. And have  

  • they always existed, or has the land changed over  time? And why is that volcano here and not there

  • We already know the Earth is the result of many  processes -- and its topography is no different.

  • Some processes operate deep within the Earth's  interior and some stay at the surface. But just as  

  • the atmosphere and biosphere are always changing  with the flow of energy and cycle of nutrients,  

  • these processes in the lithosphere  change the very ground beneath our feet.

  • I'm Alizé Carrère, and this  is Crash Course Geography.

  • INTRO

  • Whether we live in a city, a suburb, or the  countryside, we're embedded in particular  

  • landscapes. We see these hills or ponds or  lakes every day, and they become ordinary to us.

  • So we visit impressive landscapeslike the places we now call Half Dome  

  • in Yosemite National Park in the US, Uluru  in Australia, or the Serengeti Plains in  

  • Tanzania -- all natural environments that  fill us with a sense of grandeur and awe.

  • But no matter where we are, the land  is made up of physical features,  

  • like hills, valleys, and plateaus. And we can study all these physical  

  • features -- even the ordinary ones. Geomorphology  is the scientific study of landforms,  

  • the processes that make them, why they varyand their significance at different scales.

  • Towering mountains or flat plains  that stretch as far as the eye can see  

  • can seem timeless, but they're  actually always changing thanks  

  • to a network of intertwined systems  that recycle and re-shape the Earth.

  • Like the rock cycle moves  minerals through igneous,  

  • sedimentary, and metamorphic phases  that shape much of the Earth's crust,  

  • and relies on processes both deep  inside and on the surface of the Earth.

  • And where those rock cycle processes  are forming and destroying rocks  

  • follows a pattern we call the tectonic cycleThe tectonic cycle works on a huge scale and  

  • moves vast sections of Earth's crust called  plates around the globe, which creates major  

  • geographic features like mountain rangesocean basins, and continental shields.

  • And there's the hydrological cycle which  explains how water is continually transferred  

  • between being liquid, gas, or solid as it  circulates into and out of the atmosphere,  

  • lithosphere, hydrosphere, and biosphere -- each of  which is its own system with numerous sub-systems.

  • The rock, tectonic, and hydrological  cycles and their many subsystems  

  • make up the geological cycle. They also  showcase the give and take between two  

  • overarching internal and external systems  that shape the topography where earth,  

  • atmosphere, and ocean all come togetherendogenic and exogenic processes.

  • Endogenic processes are forces that  originate from within the Earth.  

  • Like igneous processes which eject fresh  rock from the interior onto the surface  

  • and tectonic processes which  work to raise or lower the land.

  • They produce initial landforms like continental  landmasses, ocean basins, and mountain ranges  

  • that span entire continents. So it's kind of like  making the initial broad strokes in a painting.

  • Like continental landmasses, ocean basins, mountain ranges that span across entire continents.

  • The fine details will come later.

  • To create landforms from  igneous and tectonic activity,  

  • the rock cycle, the tectonic cycle, and  the hydrological cycle trigger each other  

  • above and below ground in a variety  of ways that change the topography.

  • The powerful forces unleashed by tectonic  activity as plates shift around the globe  

  • and cause earthquakes and tsunamis put  rocks under tremendous stress. And rocks  

  • respond to stress differently  depending on whether they're on  

  • the surface or buried deep down. Rocks at the surface are brittle,  

  • and when tensional forces from moving plates  pull the crust in different directions,  

  • they come under stress or pressure and break  or fracture in a process called faulting.

  • These cracks or faults in the Earth's crust  show how the rock has moved on either side.  

  • The rock might've moved horizontally or vertically  as little as a centimeter or up to 15 meters over  

  • years, decades, or even centuries. Like  in September 2005 a series of earthquakes  

  • occurred along a segment of what's called the Afar  Depression, creating an opening eight meters wide.

  • Though this was nothing new for the  region, because the Afar Depression  

  • is an area of open fissures and faults at  the northern tip of the Great Rift Valley.  

  • And the Great Rift Valley is a primary  branch of the East African Rift System,  

  • which extends from present-day Jordan  in the north, through the Dead Sea and  

  • the Red Sea and along the length of East  Africa to the mouth of the Zambezi River.

  • Kind of like how the volcanoes of the Ring of Fire  mark the plate boundaries of the Pacific Ocean,  

  • the East Africa Rift system lies along the  boundaries of three tectonic platesthe Arabian,  

  • African-Nubian and the African-Somalian  plates. It's a huge set of faults that  

  • started to form when the plates began  to diverge about 25 million years ago.

  • The sudden rift in 2005 was just a small  step in the long process currently tearing  

  • the northeast of Ethiopia and Eritrea  from the rest of Africa. Eventually,  

  • it will create a new ocean -- but  that will take millions of years.

  • In particular, some of the faults of the  East African Rift system are normal faults,  

  • a common type of fault created where  the crust is moving apart vertically.  

  • On a grand scale, when pairs of  faults work together, they produce  

  • block or fault-block mountain  ranges like the Vosges in France.

  • Though faults and fissures -- and  the volcanic and earthquake activity  

  • that comes along with them -- haven't  prevented humans from living on the edge.

  • Tectonic clashes in the Great Rift Valley  have carried on for tens of millions of years.  

  • But just a few million years ago  the journey of human evolution was  

  • recorded in the sediments and fossils  throughout the region that some call  

  • thecradle of humanity,” as our early pre-human  ancestors walked and climbed across these valleys.

  • Today, the fertile slopes of the many volcanoes  that dot the length of the rift and the deep  

  • elongated lakes on the valley floor are  home not only to wildebeest, giraffes,  

  • zebras, impalas, and elephants, but  also to some of Africa's biggest cities.  

  • They're also important agricultural production  zones exporting coffee, tea, and sisal.

  • But faulting isn't the only way the tectonic  and rock cycles combine to create landforms.  

  • Unlike brittle surface rocks that break when  stretched or extended, deeply buried rocks  

  • that are heated and compressed over a long  period can bend in a process called folding.

  • Remember, sedimentary rock forms horizontal  layers or strata. And when they're squeezed or  

  • compressed -- like when two continental plates  collide at converging plate boundaries -- they  

  • bend into arches called anticlines which  are separated by troughs called synclines.

  • It's like if we lay out a piece of thick  fabric and then suddenly bunch it together  

  • from both ends to get folds. Doing that  with Earth's surface gives us fold belts,  

  • like the Jura Mountains of France and Switzerland.

  • Both folding and faulting produce large  scale landforms and shape the landscape  

  • in really dramatic ways. Like both can  be part of the mountain building process  

  • called orogenesis, which literally  meansthe birth of mountains.”

  • And really, orogenesis is just more ways the  rock cycle interacts with the tectonic cycle.  

  • Mountains are just rock masses that  have been elevated high above their  

  • surroundings by tectonic processes  along plate margins as plates move.

  • Like when the Indian plate buckled into  the Eurasian plate, vast folded and faulted  

  • mountain belts rose, and rocks in the crust  were thickened, deformed, and metamorphosed.  

  • The collision created the Himalayas, which  are really more like three broadly parallel  

  • ranges that each mark a different  sequence in the uplift process.

  • Processes that are still happeningCurrently the Indian plate pushes  

  • northward at the breakneck speed of 5 cm a year,  

  • causing the Himalaya Mountains to continue to  grow at the rate of one centimeter each year,  

  • or 10 kilometers every million years. Which  in geological terms, is like warp speed.

  • Mountain ranges can also grow from other  endogenic processes like volcanism,  

  • which is when lots and lots of volcanic rock  builds up as magma is pushed out on the surface.

  • Like on the Kamchatka Peninsula  in the Russian Far East,  

  • which is a dramatic volcanic  landscape of immense beauty.  

  • On this relatively small bit of coastal land there  are over 300 volcanoes, of which 29 are active.

  • The volcanoes are mountains or hills  constructed from igneous processes  

  • when magma deep within the Earth erupts onto the  surface as lava, and then cools and solidifies.  

  • Most commonly, magma reaches the surface where  two plates meet, which is why many volcanoes  

  • are located above subduction zones where one  tectonic plate is being dragged under another.

  • Here in Russia, the Kamchatka Peninsula lies  to the west of the Kuril-Kamchatka trench,  

  • where the plunging Pacific plate createssubduction zone and causes volcanic activity.

  • So when we're looking at an  active volcano or any mountain,  

  • what we're really seeing is part of Earth's  tectonic cycle unleashed through endogenic  

  • processes which bring fresh rock to the  surface and move and deform the crust.

  • So our picture of the Earth is coming along  nicely. Once the land has been dramatically  

  • lifted or torn apart by tectonic forces  and the internal rumblings of the Earth,  

  • initial landforms like mountain masses  are sculpted into sequential landforms.

  • These are the details -- the  [happy little] peaks, valleys,  

  • and other features honed by external or  exogenic processes acting on the surface  

  • that remove rock materials and reduce the land. In endogenic processes, the tectonic and rock  

  • cycles are the main players shaping the  Earth's crust, but in exogenic processes,  

  • the hydrological cycle comes into its  own as it interacts with the rock cycle.

  • Water circulating from the ocean to the atmosphere  is important in the annual march of seasons, but  

  • also in the geological cycle by which  Earth renews and reproduces itself.

  • Like the vapor in the atmosphere  comes down to produce life and growth  

  • and create soil from rock through weathering or  the decay and disintegration of rocks. Over time  

  • the weathered rock is picked up and carried by  water, wind, and ice in the process of erosion  

  • where it accumulates to make sedimentary rocks  or gets subducted with the diving oceanic plate.

  • Different rocks offer different resistance  to weathering and erosion based on everything  

  • from grain size and hardness to porosity and  permeability to their mineral composition.

  • But resistance actually has a big influence  on how landforms and landscapes look.  

  • Rocks that are resistant to weathering and erosion  will stand higher than less resistant rocks.  

  • And weathered rock is more easily picked  up, moved, and deposited somewhere else.

  • Like when folds are deeply eroded like in the  Appalachian Mountains along the eastern part  

  • of the US, they end up as ridges and valleysAs weaker rock like shale and limestone eroded,  

  • it left hard sandstone and quartzite to  stand as long narrow parallel ridges.

  • And the ridges have been cut through  and further eroded by rivers.  

  • These breaks influenced migration, settlement  patterns, and even cultural traits in the US  

  • in the 1700s as the flow of people, goodsand ideas were guided by this topography.

  • Weathering and erosion have also left  behind some of our most stunning landscapes.  

  • Like in the remote, dry interior of  Australia's Outback stands Uluru,  

  • a sandstone formation called an inselberg  that we think is 550 million years old.

  • It looks like a 348 meter tall rock that's 9.4  kilometers around was just dropped onto the land.  

  • But Uluru is an erosional remnant and  all that remains from an ancient plain  

  • that would've been level with the present dome.