1 00:00:00,000 --> 00:00:06,000 Hi, Dr. D. Hi, guys. What's up? 2 00:00:06,000 --> 00:00:09,000 We felt our treehouse shake this morning and wanted to know what happened. 3 00:00:09,000 --> 00:00:11,000 Yeah, I felt something, too. 4 00:00:11,000 --> 00:00:15,000 We think that it might have been an earthquake, but we can't prove it yet. 5 00:00:15,000 --> 00:00:17,000 Have you ever been in an earthquake? 6 00:00:17,000 --> 00:00:21,000 Yes, I have, but it didn't feel the same as what I felt this morning. 7 00:00:21,000 --> 00:00:24,000 See, I told you it wasn't an earthquake. 8 00:00:24,000 --> 00:00:26,000 Wait a minute. All earthquakes don't feel the same. 9 00:00:26,000 --> 00:00:31,000 To begin understanding earthquakes, you have to know something about the structure of the Earth. 10 00:00:31,000 --> 00:00:33,000 That makes sense. What can you tell us? 11 00:00:33,000 --> 00:00:35,000 Come on, let me show you. 12 00:00:35,000 --> 00:00:37,000 Okay. 13 00:00:37,000 --> 00:00:39,000 Let me cut this peach in half. 14 00:00:39,000 --> 00:00:40,000 Okay, be careful. 15 00:00:40,000 --> 00:00:42,000 I will, thanks. 16 00:00:42,000 --> 00:00:44,000 This will be our Earth. 17 00:00:44,000 --> 00:00:46,000 Only, it's a lot smaller. 18 00:00:46,000 --> 00:00:48,000 It's called a scale model. 19 00:00:48,000 --> 00:00:50,000 The pit is like the core of the Earth. 20 00:00:50,000 --> 00:00:52,000 The Earth's core is about half of its diameter. 21 00:00:52,000 --> 00:00:53,000 What's it made of? 22 00:00:53,000 --> 00:00:56,000 We've never been there, but we think it's made of iron and nickel. 23 00:00:56,000 --> 00:01:00,000 The inner part of the core is solid, and the outer part is liquid. 24 00:01:00,000 --> 00:01:06,000 It is so dense, if we had a gallon jug full of the Earth's core material, it would weigh over 100 pounds. 25 00:01:06,000 --> 00:01:08,000 Wait, Dr. D, you lost me. 26 00:01:08,000 --> 00:01:12,000 How do we know how big the core is and what it's made of if we've never been there? 27 00:01:12,000 --> 00:01:14,000 Well, it's just a guess. 28 00:01:14,000 --> 00:01:16,000 You're kidding. 29 00:01:16,000 --> 00:01:19,000 Well, actually, it's a pretty good guess, based upon a lot of evidence. 30 00:01:19,000 --> 00:01:24,000 One of the biggest clues we have to the structure of the Earth comes from our study of earthquakes. 31 00:01:24,000 --> 00:01:28,000 So we're using earthquakes to help us understand earthquakes? 32 00:01:28,000 --> 00:01:29,000 That's right. 33 00:01:29,000 --> 00:01:32,000 The part of the peach that you eat is called the Earth's mantle. 34 00:01:32,000 --> 00:01:36,000 It's not as dense as the core, but it's more dense than what's on the outside. 35 00:01:36,000 --> 00:01:38,000 What's the outside called? 36 00:01:38,000 --> 00:01:39,000 It's called the crust. 37 00:01:39,000 --> 00:01:41,000 Just like the crust of a bread. 38 00:01:41,000 --> 00:01:42,000 Very good. 39 00:01:42,000 --> 00:01:45,000 Like the skin of a peach, the Earth's crust is very thin. 40 00:01:45,000 --> 00:01:49,000 The crust under the oceans is typically between 5 and 10 kilometers thick. 41 00:01:49,000 --> 00:01:52,000 It's made of a dark, dense rock called basalt. 42 00:01:52,000 --> 00:01:56,000 The continental crust is between 30 and 70 kilometers thick. 43 00:01:56,000 --> 00:01:59,000 It's made of a lighter, less dense rock, like this granite. 44 00:01:59,000 --> 00:02:02,000 Is there a reason why the Earth is layered like this? 45 00:02:02,000 --> 00:02:04,000 It's because of density. 46 00:02:04,000 --> 00:02:06,000 Watch us experiment over here. 47 00:02:07,000 --> 00:02:17,000 I'm going to put this green-tinted water and this salad oil into this graduated cylinder and shake them up. 48 00:02:17,000 --> 00:02:20,000 Let's see what happens. 49 00:02:29,000 --> 00:02:32,000 The salad oil is on top, and then the water. 50 00:02:32,000 --> 00:02:35,000 Let's put in some motor oil. 51 00:02:36,000 --> 00:02:42,000 It turns out, the least dense, which is motor oil, floats on the top. 52 00:02:42,000 --> 00:02:45,000 And the most dense, water, sinks to the bottom. 53 00:02:45,000 --> 00:02:48,000 It's just like the early history of the Earth, when it was all liquid. 54 00:02:48,000 --> 00:02:51,000 Earth, liquid, strange. 55 00:02:51,000 --> 00:02:53,000 But what does this have to do with earthquakes? 56 00:02:53,000 --> 00:02:57,000 When the Earth cooled, the crust's upper mantle became rigid and brittle. 57 00:02:57,000 --> 00:03:00,000 They broke into about 12 segments, which we call plates. 58 00:03:00,000 --> 00:03:04,000 These plates float on the more dense, but flexible, mantle below. 59 00:03:04,000 --> 00:03:06,000 Kind of like this egg white. 60 00:03:06,000 --> 00:03:08,000 These plates are in continual movement. 61 00:03:08,000 --> 00:03:10,000 Wow, this is too weird. 62 00:03:10,000 --> 00:03:12,000 Why don't we feel them moving? 63 00:03:12,000 --> 00:03:16,000 They move about as fast as your fingernails grow, which is only a few centimeters per year. 64 00:03:16,000 --> 00:03:20,000 But we have fossil evidence of this movement, which we call plate tectonics. 65 00:03:20,000 --> 00:03:23,000 Are fossils also clues to understanding earthquakes? 66 00:03:23,000 --> 00:03:25,000 Yes. 67 00:03:25,000 --> 00:03:28,000 But Dr. D, aren't there other things that could cause vibrations? 68 00:03:28,000 --> 00:03:30,000 Well, yes, that's a good question. 69 00:03:30,000 --> 00:03:33,000 Anything that makes a very loud sound can make things shake a little. 70 00:03:33,000 --> 00:03:38,000 I watched fireworks on the 4th, and some were so big, I did feel a vibration. 71 00:03:38,000 --> 00:03:40,000 Oh no, I'm late for an appointment. 72 00:03:40,000 --> 00:03:43,000 Why don't you take a little and discuss this amongst yourselves?