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  • I grew up in Ithaca, New York in the US where there's a huge variety of landscapes.

  • There are farms and forests and mountains and lakes and massive waterfalls all within

  • a pretty small region.

  • But swimming in the Finger Lakes right in my backyard in the summer, or visiting New

  • York City on field trips, I had no idea that the land I was walking on was actually shaped by glaciers.

  • We'll come back to New York later.

  • But it won't be our only example because actually many of the Earth's landscapes

  • were shaped by glaciers in some way or another millions of years ago.

  • And today, they're like icy, 21st century canaries in the coal mine warning us about

  • the massive changes to our world that are coming as climates change and sea levels rise.

  • These behemoth globs of compressed ice and snow moving across the land created fertile

  • soils and physical features while also serving as frozen time capsules.

  • They recorded both Earth's climatic history over several million years and contain clues

  • to its climatic future.

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

  • INTRO

  • Our journey through physical geography is almost over, and by now we definitely know

  • the Earth is ever-changing.

  • And glaciers have often been a crucial part of that change.

  • Basically, a glacier is a large mass of ice that formed from compressed snow that moves

  • thanks to its own weight and the pull of gravity.

  • And there are actually two main types.

  • When we find a glacier in a mountain range, it's called an alpine or mountain glacier.

  • And on a larger scale, a continuous mass of unconfined ice bigger than 50,000 square kilometers

  • is called a continental glacier or ice sheet.

  • In 2021, about 10 percent of the Earth's land surface is ice.

  • Most of that's found in the Greenland and Antarctic ice sheets .

  • And glaciers have a big influence on our global climate.

  • Because glaciers covered in snow are so dazzlingly white, they reflect a lot of the solar radiation

  • they get, keeping the Earth cool.

  • And we saw in our weather episodes how the enormous temperature difference between the

  • polar regions and the warm equatorial regions drives the system of heat transport around the world.

  • And like we learned in our last episode on groundwater, only 2.8 % of the Earth's water

  • is freshwater.

  • A tiny bit of that is in lakes and rivers, and almost a third is groundwater.

  • But all the rest is locked in glaciers.

  • So if all the world's glaciers were to melt, sea levels would rise about 70 meters changing

  • ocean circulation patterns, which would alter weather and climate in the mid-latitudes.

  • Not to mention create political, economic, and social upheaval, dramatically reshaping

  • coastlines and the lives of the over 2.4 billion people who live within 100 kilometers of a coast.

  • Looking to the past, it was glaciers themselves that provided the first inkling there had

  • been big changes in global climates throughout Earth's history.

  • Many naturalists -- and even a poet! -- during the early 19th century were struck by the

  • fact that the British Isles and the North German Plains -- which are areas far from

  • glaciers even today -- had deeply eroded U-shaped valleys, massiveerraticboulders far

  • from where they formed, and bedrock that was smooth like it'd been polished by abrasion.

  • These features were similar to what was seen in the Alps where there were still glaciers,

  • and they concluded that it was evidence the British Isles and North German Plains also

  • had icy pasts.

  • Today we know the most recent ice age is one of several ice ages that Earth has experienced

  • in its long history.

  • And it doesn't mean the Earth has to be entirely covered in ice.

  • An ice age alternates between glaciations, or a period when temperatures drop and ice

  • sheets grow and spread outwards over vast areas, and interglaciations when the climate is milder.

  • And this back and forth between glaciations and interglaciations means an ice age can

  • last for millions of years.

  • Right now we're actually still in an ice age.

  • We're just in the middle of an interglaciation.

  • Evidence from deep sea sediments actually shows that our ice age started when glaciers

  • started growing about 2.5 to 3.0 million years ago.

  • Both ice sheets and mountain glaciers were forming at this time, but they have some specific requirements.

  • They form above the snowline, or the lowest elevation where there's ice and snow all year round.

  • So where the amount of snow that falls each year is more than the annual ablation, or

  • how much is lost by evaporation and melting.

  • Then as layers of snow get buried and compacted into ice, the weight of the glacier reshapes

  • and realigns the crystals, making them harder and denser.

  • When the ice is about 30 meters deep, the millions of ice crystals in the bottom layers

  • change form and become plasticky and can glide over each other.

  • Which means that from this depth to its base, the glacier behaves more like silly putty

  • and sort of stretches out, carrying the brittle ice on the surface.

  • So, a glacier is not just a hard block of ice that slides down a slope.

  • Glaciers usually flow slowly.

  • Large ice sheets move a few centimeters per day and flow out in all directions, while

  • active mountain glaciers can cover several meters each day as they're pulled downhill

  • by gravity.

  • It's really weird to think about, I know.

  • But glaciers don't flow like rivers.

  • They move much slower but with tremendous energy, meaning they do a ton of work as they

  • engulf and dramatically re-shape the landscape dragging everything in their wake.

  • When a glacier stays more or less the same size, we say it's in dynamic equilibrium

  • because the amount of new snow is about equal to how much is melting.

  • But when climates warm or there's less snow, glaciers retreat and grow smaller.

  • Ultimately, when there are very few or no glaciers on the planet we consider the ice age over.

  • We're not quite to that point yet, so we're still technically in an ice age!

  • Remember I said we're in an interglacial period of this ice age, meaning there are

  • a lot fewer and smaller ice sheets and glaciers on Earth compared to their greatest extent

  • about 20,000 years ago, known as the Last Glacial Maximum.

  • At that time, much of North America, Europe and southern South America were blanketed

  • in extensive ice sheets and glaciers.

  • Which got to be more than 3 kilometers thick in parts of Canada and the US.

  • And when this sheet of frozen water began to melt and retreat quite rapidly some 15,000

  • years ago, it left behind a ridge that was basically a jumbled heap of gravel, sand,

  • silt, and clay called a terminal moraine.

  • And it still stretches more or less continuously across North America from the Puget Sound

  • in Washington state to its southern limits in Midwestern states like Missouri and Nebraska,

  • all the way to Montauk Point on New York's Long Island.

  • The ice sheet even shaped New York City as we know it today.

  • Original settlers of the area, the indigenous peoples of Algonquian-speaking languages,

  • utilized features carved by glaciers much in the same way we do now: using the waterways

  • for trade and commerce, and ridges as easy paths to walk between villages.

  • In fact, if you've ever walked down Broadway, then you've walked one of these trade paths.

  • In neighborhoods like Brooklyn, Queens, and Staten Island, terminal moraine forms the

  • high ground.

  • Initially, as the city grew these sites were ignored for homes and other buildings because

  • they were stony and inaccessible and construction was difficult.

  • Instead this land became parks, cemeteries, and golf courses.

  • Even today these remain the more densely wooded and landscaped sections of these neighborhoods

  • and are now some of the most attractive locations in these boroughs.

  • But back to the ice.

  • Like a giant bulldozer as the glacier moves forward it drags along everything in its path,

  • picking up tiny pebbles or enormous boulders often the size of a school bus.

  • When it melts, they're left strewn across the landscape as though flung...by a giant

  • playing marbles.

  • And Central Park is full of these stray boulders called erratics because, well, they're erratic.

  • They're far from where they would've originated and have different compositions from the surrounding,

  • often younger, rocks.

  • Boulders like these were what some of the naturalists were noticing over in Europe.

  • Moraines and erratics are what we call depositional features, which are features formed from the

  • debris carried by ice as glaciers melt, and they still influence how the land is used today.

  • Like we saw here, in NYC!

  • So glaciers can be like huge excavators hauling debris from one place to another, adding features

  • to the landscape as they melted and retreated.

  • But they also change the land through erosion as they advance.

  • Like the Finger Lakes in central New York.

  • Their distinct, elongated basins were eroded and deepened by ice sheets.

  • And the Great Lakes are actually former river valleys that were greatly enlarged and gouged

  • out by glacial erosion.

  • The bedrock beneath each lake is covered in thick blankets of glacial deposits, which

  • are the bits of rock fragments the ice picked up as it moved.

  • And as the ice sheet retreated north, meltwater filled in the depressions, forming the Great Lakes.

  • Mountain glaciers have also sculpted majestic landscapes in regions like the Himalayas,

  • Sierra Nevadas, Rockies, Andes, and Alps.

  • Like the Matterhorn on the Swiss-Italian border is the iconic image of the Alps.

  • Its symmetrical spire with precipitous rock walls is a siren call for climbers everywhere

  • -- and is the work of glaciers.

  • To start out, we have a mountain glacier, slowly advancing down the mountain slope.

  • As it moves, blocks of rock that have been loosened by weathering get removed orplucked

  • out as part of glacial plucking.

  • Partnered with abrasive rock debris sandpapering the landscape, this forms an armchair-shaped

  • hollow in the side of the mountain called a cirque.

  • When the ice in two adjacent cirques eats away at the ridge between them, it forms a

  • sharp, often serrated ridge called an arête.

  • And when three or more cirques carve away a mountain mass from different sides we eventually

  • get a horn or pyramidal peak like on the Matterhorn.

  • But the work of the mountain glacier isn't done.

  • As the ice grinds over uneven bedrock and the glacier is stretched as it flows over

  • a convex slope, deep vertical cracks called crevasses open up on the brittle ice on the

  • surface as a response to the stress.

  • Debris that gets left behind can form ridges along the side of the glacier, which we call

  • a lateral moraine.

  • And where two glaciers join, their lateral moraines merge to form a medial moraine.

  • Often moraines dam the foot of a cirque encasing small shimmering mountain lakes called tarns

  • which are among the most popular destinations for backcountry hikers and campers.

  • Further down, a glacier moving through a valley will usually erode the walls, and deepen and

  • broaden the valley from a V-shape into a U-shaped profile.

  • These glacial landscapes that have been left behind after the last warming are home to

  • millions of people in Eurasia and North America.

  • They've been ploughed to produce crops, dug into for sand and gravel, and paved over

  • by concrete and tarmac.

  • Lately when we hear about glaciers shaping the Earth, it's because they're breaking.

  • Like in 2021 data from uncrewed submarines showed that warm water may be weakening critical

  • stability points of the Thwaites Glacier -- which is also part of the West Antarctic Ice Sheet

  • and known as thedoomsday glacier.”

  • When it breaks it could raise global sea levels by over half a meter and potentially collapse

  • the entire West Antarctic Ice Sheet which would flood lowlands and islands across the world.

  • So the story of ice seems to be closely tied to our human story.

  • Ice has advanced and retreated over time, but when the climate stabilized after the

  • ice started melting 12 to 15000 years ago, agriculture flourished.

  • And many nomadic groups shifted to more settled lifestyles, setting the scene for many of

  • our societies today.

  • And as the ice melts further, our lives could shift again.

  • So as we contemplate that future, join us next time for a look at the geography of natural hazards.

  • 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.

I grew up in Ithaca, New York in the US where there's a huge variety of landscapes.

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What Are Glaciers? Crash Course Geography #26

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    王杰 に公開 2022 年 08 月 06 日
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