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  • Have you ever heard the term,

  • "tip of the iceberg"?

  • You know that icebergs are mostly underwater,

  • their immense bulk hidden beneath the water.

  • But why is that so?

  • Well, the density of pure ice

  • is less than that of sea water.

  • Usually only 1/9 of the volume of an iceberg

  • is above the water.

  • The shape of the underwater portion

  • is difficult to discern

  • by looking at the above-surface portion.

  • This has led to the expression,

  • "tip of the iceberg."

  • Here are some thing you might not know

  • about the icy islands.

  • The life of an iceberg

  • begins many thousands of years

  • before it reaches the ocean.

  • Unlike sea ice or pack ice,

  • which form when the ocean freezes,

  • glaciers are made of fresh water.

  • For thousands of years,

  • these glaciers build layer upon layer of ice,

  • constantly compressing,

  • moving,

  • adding snow,

  • compressing,

  • and moving again

  • as they inch along like a frozen river.

  • It is the force of gravity

  • that pulls them towards the sea,

  • where a glacier may calve off to become an iceberg

  • or continue to spread up as an ice shelf

  • or an ice tongue.

  • Once an iceberg breaks away

  • from the glacier or ice shelf,

  • it will usually live for three to six years,

  • floating around, carried by the currents

  • and tidal movements of the ocean.

  • As it floats along,

  • it is battered by waves,

  • melts,

  • and smashes into land

  • and sometimes other icebergs.

  • Some icebergs are so unstable

  • that they have more dramatic ends,

  • heaving up,

  • collapsing,

  • and sometimes even exploding.

  • And as they fall apart,

  • many icebergs make all sorts of strange sounds.

  • When a piece of iceberg melts,

  • it makes a fizzing sound,

  • called Bergie Seltzer.

  • This sound is made when the water-ice interface

  • reaches compressed air bubbles trapped in the ice.

  • As this happens, each bubble bursts,

  • making a popping sound.

  • There are six official size classifications for icebergs.

  • The smallest icebergs are called growlers.

  • They can be up the size of your car

  • and are very dangerous for ships and boats

  • because usually they sit just at the waterline

  • where they are not easy to spot.

  • Next are the bergy bits

  • - yes, that is their scientific name -

  • which can be up to the size of your home.

  • The other four sizes are small,

  • medium,

  • large,

  • and very large.

  • So just how big is a very large iceberg?

  • Officially, any iceberg looming larger

  • than 270 feet high above sea level

  • and 670 feet long

  • is considered very large.

  • That's 27 stories of looming, blue ice.

  • And how do icebergs get that blue color anyways?

  • When snow on the glacier

  • is compressed over many hundreds of years,

  • the weight of the snow

  • forces the air bubbles out of the ice,

  • creating pure ice with very little air trapped inside.

  • This compression is seen

  • when the glacier calves,

  • creating a blue iceberg.

  • An iceberg that has not experienced

  • as much compression

  • and has a large amount of air and surface edges

  • reflects light as white.

  • Although they form in far northern or southern areas,

  • icebergs can float thousands of miles.

  • An iceberg from the Arctic floated

  • as far south as Bermuda.

  • Antarctic icebergs are mostly trapped

  • in the Circumpolar Current,

  • never giving them a chance to float north.

  • However, they have been known to interrupt

  • shipping lanes between Australia,

  • South America,

  • and South Africa.

  • For all their travelling,

  • many people think

  • that these slabs of ice are barren of life,

  • but these seemingly sterile ice slabs

  • also harbor their own complex ecosystems

  • and they shape the ecosystems

  • that they pass through.

  • They become mobile, floating ecosystems.

  • Even in the coldest seas,

  • icebergs are always melting,

  • at least a little bit.

  • This melting has a major impact

  • on the ocean around an iceberg.

  • The fresh water from the berg

  • creates a pool of fresh water

  • that can extend a nautical mile away from the iceberg.

  • This water is colder than the surrounding sea water,

  • and the temperature variation creates thermal currents

  • in the vicinity of the iceberg.

  • Life thrives on and around an iceberg.

  • Young icefish hide in small ice holes to avoid predators,

  • while a variety of invertebrates,

  • like jellyfish and siphonophores,

  • congregate in the area.

  • Many of them come to feed on krill,

  • tiny shrimp-like creatures.

  • Snow petrels nest on the icebergs

  • and feed on the sea life nearby.

  • Whales and seals and penguins seem to like them too.

  • And even now that you know all this,

  • we're still at the tip of the iceberg.

  • There are all sorts of things

  • we don't know about icebergs.

  • Perhaps you'll be the one to see a little deeper.

Have you ever heard the term,

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【TED-Ed】What's below the tip of the iceberg? - Camille Seaman

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    Bing-Je   に公開 2013 年 12 月 14 日
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