BBC 6 Minute English | THE EARTH CORE | English CC | Daily Listening

source: Daily Listening    2016年10月9日

0:06 Now, have you read any books by Jules Verne?
0:08 Yes, I have.
0:10 Journey to the Centre of the Earth was my favourite book as a child!
0:13 A German Professor and his two companions climb down a volcano in search of the Earth’s
0:18 centre – or core.
0:21 They visit strange lands inhabited by dinosaurs and giant prehistoric humans, and sail across
0:26 an underground ocean.
0:28 Hmm.
0:29 Very exciting but it doesn’t sound very realistic.
0:33 How do they get out again?
0:34 Well, they shoot to the surface from the mouth of Mount Etna during a volcanic eruption.
0:39 Wow!
0:40 That sounds very uncomfortable!
0:41 Well, on today's show we're going to discuss what scientists really know about the Earth's
0:46 core.
0:47 Yes.
0:48 The Earth has a dense inner core surrounded by a fluid outer core.
0:52 Dense, by the way, means heavy in relation to its size.
0:57 But, Rob, I've got a question for you as usual: how big do you think the inner core is?
1:05 Is it the size of … a) the Moon?
1:08 b) Jupiter? or c) Mars?
1:12 Right!
1:13 Well, I haven't a clue to be honest so I'm going to take a guess and say c) Mars.
1:19 Well, we'll find out later on in the show if you're right.
1:23 But before we get there, let's find out a bit more about what the Earth is made of.
1:28 Well, the Earth has layers, a bit like an onion.
1:31 I like your technical language, Rob!
1:33 But I'm trying to keep things simple for you, Alice!
1:35 Thanks.
1:36 It has a thin outer layer or crust where we live.
1:40 And this includes our continents and the ocean floors.
1:43 Then beneath that there's another layer called the mantle.
1:47 And beneath that, is the Earth's core – over 6000km below the surface.
1:52 Right.
1:53 But what's the Earth made of, Rob?
1:54 It's a good question.
1:55 And it depends on which layer you're talking about!
1:59 The crust and mantle are rock and contain a lot of silicate – which is the same stuff
2:05 that glass is made of.
2:07 But the outer and inner parts of the core mainly consist of iron.
2:11 And the core is very hot.
2:13 Am I right?
2:14 You are indeed.
2:15 The professor and his companions wouldn't have survived very long down there!
2:19 The outer core is a swirling mass of molten – or liquid – metal and it’s as hot
2:24 as the surface of the Sun!
2:26 Wow!
2:27 That must be so hot!
2:28 Right.
2:29 Let's listen now to Simon Redfern talking about the inner core and what's happening
2:33 in there.
2:36 And so over time, the planet has started to cool.
2:40 And as it cools, eventually at the centre of the Earth, the highest pressure point,
2:46 we pass over the crystallization temperature – the freezing temperature of iron – and
2:50 iron starts to freeze at the centre of the Earth.
2:53 And you get a crystal of iron right in the middle that starts to grow.
2:58 I'm a bit worried that the Earth is freezing in the middle!
3:01 Don't worry, Alice!
3:03 In this case, because of the incredibly high pressure in the core, the freezing point of
3:08 iron is actually about 6000 degrees!
3:12 And the iron has been cooling down and crystallizing for a billion years – and at a rate of just
3:17 half a millimetre every year.
3:19 Ah well, yes, that sounds like slow progress.
3:22 Certainly.
3:23 Now moving on, we should also talk about the fact that it's the liquid iron outer core
3:28 that generates magnetic fields – and it's thanks to these magnetic fields that life
3:33 on Earth is possible.
3:36 Let's hear more about this.
3:37 Deuss: Well, the magnetic field is very important because it protects us against cosmic radiation
3:43 so that's one really...
3:44 Bragg: How does it do that?
3:46 Deuss: It just creates a shield, which will just deflect the cosmic rays from the Sun
3:51 to actually reach us at the surface.
3:52 So it protects us.
3:54 Bragg: So it goes up there…
3:56 Deuss: Yeah, so you would see that the radiation kind of goes right into the Earth and not
4:00 actually reach us.
4:04 So there's a magnetic field round the Earth that protects us from the Sun's cosmic rays.
4:10 I'd like a magnetic field round me.
4:13 It could be my superpower – like in X Men!
4:16 Calm down, Magneto.
4:18 Now the magnetosphere is the area around the Earth in which the Earth's magnetic field
4:22 is felt.
4:24 It protects us from the Sun's radiation and the flow of particles, which would otherwise
4:28 strip away – or remove – the Earth's atmosphere.
4:31 Right, I see.
4:33 And what does 'radiation' mean?
4:35 Well, radiation means heat or energy or particles in the form of rays – in this case, the
4:41 Sun's rays.
4:42 OK.
4:43 And 'deflect'?
4:44 To deflect means to make something change direction.
4:47 Right, I see.
4:49 Thank you.
4:50 Now, Rob, I asked you, do you remember, at the beginning of the show, how big is the
4:55 Earth's inner core?
4:57 Is it the size of … a) the Moon?
5:00 b) Jupiter? or c) Mars?
5:04 Yes, and I had a wild guess and I said c) Mars.
5:07 Yes.
5:08 And I'm afraid that's wrong, Rob.
5:11 The answer is a) the Moon.
5:14 Would you like to shape up and tell us which words we learned on the show today?
5:18 Of course.
5:19 Good idea.
5:20 We heard: core
5:24 dense crust
5:30 mantle silicate
5:34 molten magnetosphere
5:40 strip away radiation
5:46 deflect Yes.
5:47 Thank you, Rob