Over two thousand years ago, the great Carthaginian military leader Hannibal crossed
the Alps leading thousands of soldiers, horses, and 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 setting: the 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 landscapes, like 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 vary, and 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 cycle. The 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 ranges, ocean 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 together: endogenic 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 plates – the 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
the “cradle 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 means “the 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 happening. Currently 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 creates a subduction 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 valleys. As 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, goods, and 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.
Centuries of erosion by wind and water have removed the weaker rock and left erosion-resistant
rock domes like this one standing above the surrounding landscape. Uluru is the most famous
and is sacred to the traditional landholders who have lived around the rock for thousands of years.
Cliffs and canyons, beaches and dunes, floodplains and river valleys,
plateaus and mountains -- these are all products of a restless Earth and greatly
influence how people live and derive meaning and a sense of place in different landscapes.
Ultimately landscapes are part of our collective human experience, our struggles, and our triumphs.
In the next few episodes we’ll talk more about endogenic and exogenic processes -- especially
weathering, erosion, and the power of water -- because of how fundamental
they are to both the ordinary and extraordinary landscapes around us.
The Earth does change beneath our feet. Sometimes infinitely slowly as with tectonic processes and
sometimes instantly when volcanoes erupt. Next time we will look at the grand fireworks show
that nature puts up to see the different types of volcanoes and different landscapes they produce.
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.