0:00:00.500,0:00:02.440 We know that an[br]element is defined 0:00:02.440,0:00:04.350 by the number of protons it has. 0:00:04.350,0:00:05.401 For example, potassium. 0:00:05.401,0:00:07.150 We look at the periodic[br]table of elements. 0:00:07.150,0:00:09.700 And I have a snapshot of[br]it, of not the entire table 0:00:09.700,0:00:10.720 but part of it here. 0:00:10.720,0:00:12.920 Potassium has 19 protons. 0:00:12.920,0:00:14.360 And we could write it like this. 0:00:14.360,0:00:15.818 And this is a little[br]bit redundant. 0:00:15.818,0:00:18.280 We know that if it's potassium[br]that atom has 19 protons. 0:00:18.280,0:00:20.720 And we know if an[br]atom has 19 protons 0:00:20.720,0:00:23.260 it is going to be potassium. 0:00:23.260,0:00:28.920 Now, we also know that not all[br]of the atoms of a given element 0:00:28.920,0:00:31.260 have the same[br]number of neutrons. 0:00:31.260,0:00:33.340 And when we talk[br]about a given element, 0:00:33.340,0:00:35.070 but we have different[br]numbers of neutrons 0:00:35.070,0:00:37.640 we call them isotopes[br]of that element. 0:00:37.640,0:00:40.930 So for example,[br]potassium can come 0:00:40.930,0:00:43.980 in a form that has[br]exactly 20 neutrons. 0:00:43.980,0:00:45.230 And we call that potassium-39. 0:00:48.190,0:00:50.760 And 39, this mass[br]number, it's a count 0:00:50.760,0:00:56.710 of the 19 protons[br]plus 20 neutrons. 0:00:56.710,0:00:59.780 And this is actually the most[br]common isotope of potassium. 0:00:59.780,0:01:03.360 It accounts for, I'm[br]just rounding off, 0:01:03.360,0:01:08.760 93.3% of the potassium that[br]you would find on Earth. 0:01:08.760,0:01:12.040 Now, some of the other[br]isotopes of potassium. 0:01:12.040,0:01:14.240 You also have potassium--[br]and once again writing 0:01:14.240,0:01:16.490 the K and the 19 are a[br]little bit redundant-- 0:01:16.490,0:01:18.530 you also have potassium-41. 0:01:18.530,0:01:20.460 So this would have 22 neutrons. 0:01:20.460,0:01:22.580 22 plus 19 is 41. 0:01:22.580,0:01:28.180 This accounts for about 6.7%[br]of the potassium on the planet. 0:01:28.180,0:01:30.550 And then you have a[br]very scarce isotope 0:01:30.550,0:01:33.890 of potassium called[br]potassium-40. 0:01:33.890,0:01:38.030 Potassium-40 clearly[br]has 21 neutrons. 0:01:38.030,0:01:40.380 And it's very, very,[br]very, very scarce. 0:01:40.380,0:01:46.010 It accounts for only 0.0117%[br]of all the potassium. 0:01:46.010,0:01:48.590 But this is also the[br]isotope of potassium 0:01:48.590,0:01:51.350 that's interesting to us[br]from the point of view 0:01:51.350,0:01:56.490 of dating old, old rock, and[br]especially old volcanic rock. 0:01:56.490,0:01:59.680 And as we'll see, when you[br]can date old volcanic rock 0:01:59.680,0:02:01.530 it allows you to date[br]other types of rock 0:02:01.530,0:02:03.810 or other types of fossils[br]that might be sandwiched 0:02:03.810,0:02:06.900 in between old volcanic rock. 0:02:06.900,0:02:10.570 And so what's really interesting[br]about potassium-40 here 0:02:10.570,0:02:15.192 is that it has a half-life[br]of 1.25 billion years. 0:02:15.192,0:02:17.400 So the good thing about[br]that, as opposed to something 0:02:17.400,0:02:19.950 like carbon-14, it can[br]be used to date really, 0:02:19.950,0:02:21.520 really, really old things. 0:02:21.520,0:02:27.160 And every 1.25[br]billion years-- let 0:02:27.160,0:02:32.660 me write it like this,[br]that's its half-life-- 0:02:32.660,0:02:36.460 so 50% of any given[br]sample will have decayed. 0:02:36.460,0:02:40.785 And 11% will have[br]decayed into argon-40. 0:02:45.880,0:02:47.530 So argon is right over here. 0:02:47.530,0:02:49.690 It has 18 protons. 0:02:49.690,0:02:51.960 So when you think about[br]it decaying into argon-40, 0:02:51.960,0:02:54.050 what you see is that[br]it lost a proton, 0:02:54.050,0:02:56.060 but it has the same mass number. 0:02:56.060,0:02:59.640 So one of the protons must of[br]somehow turned into a neutron. 0:02:59.640,0:03:02.240 And it actually captures[br]one of the inner electrons, 0:03:02.240,0:03:03.827 and then it emits[br]other things, and I 0:03:03.827,0:03:05.660 won't go into all the[br]quantum physics of it, 0:03:05.660,0:03:07.380 but it turns into argon-40. 0:03:07.380,0:03:12.686 And 89% turn into calcium-40. 0:03:12.686,0:03:15.060 And you see calcium on the[br]periodic table right over here 0:03:15.060,0:03:16.480 has 20 protons. 0:03:16.480,0:03:18.760 So this is a situation[br]where one of the neutrons 0:03:18.760,0:03:20.287 turns into a proton. 0:03:20.287,0:03:22.120 This is a situation[br]where one of the protons 0:03:22.120,0:03:23.710 turns into a neutron. 0:03:23.710,0:03:25.640 And what's really[br]interesting to us 0:03:25.640,0:03:29.520 is this part right over here. 0:03:29.520,0:03:32.170 Because what's cool about argon,[br]and we study this a little bit 0:03:32.170,0:03:36.150 in the chemistry playlist, it is[br]a noble gas, it is unreactive. 0:03:36.150,0:03:39.230 And so when it is embedded[br]in something that's 0:03:39.230,0:03:42.310 in a liquid state it'll[br]kind of just bubble out. 0:03:42.310,0:03:46.160 It's not bonded to[br]anything, and so it'll just 0:03:46.160,0:03:48.950 bubble out and just go[br]out into the atmosphere. 0:03:48.950,0:03:50.990 So what's interesting[br]about this whole situation 0:03:50.990,0:03:54.950 is you can imagine what happens[br]during a volcanic eruption. 0:03:54.950,0:03:57.410 Let me draw a volcano here. 0:03:57.410,0:04:00.670 So let's say that[br]this is our volcano. 0:04:00.670,0:04:04.160 And it erupts at some[br]time in the past. 0:04:04.160,0:04:08.795 So it erupts, and you have[br]all of this lava flowing. 0:04:12.730,0:04:16.070 That lava will contain some[br]amount of potassium-40. 0:04:16.070,0:04:17.779 And actually, it'll[br]already contain 0:04:17.779,0:04:18.805 some amount of argon-40. 0:04:22.570,0:04:24.410 But what's neat[br]about argon-40 is 0:04:24.410,0:04:27.600 that while it's lava, while it's[br]in this liquid state-- so let's 0:04:27.600,0:04:30.400 imagine this lava[br]right over here. 0:04:30.400,0:04:33.890 It's a bunch of stuff[br]right over here. 0:04:37.080,0:04:39.020 I'll do the potassium-40. 0:04:39.020,0:04:41.600 And let me do it in a color[br]that I haven't used yet. 0:04:41.600,0:04:44.010 I'll do the[br]potassium-40 in magenta. 0:04:44.010,0:04:48.090 It'll have some[br]potassium-40 in it. 0:04:48.090,0:04:49.090 I'm maybe over doing it. 0:04:49.090,0:04:50.940 It's a very scarce isotope. 0:04:50.940,0:04:53.040 But it'll have some[br]potassium-40 in it. 0:04:53.040,0:05:01.280 And it might already have some[br]argon-40 in it just like that. 0:05:01.280,0:05:03.187 But argon-40 is a noble gas. 0:05:03.187,0:05:04.520 It's not going to bond anything. 0:05:04.520,0:05:06.980 And while this lava[br]is in a liquid state 0:05:06.980,0:05:10.090 it's going to be[br]able to bubble out. 0:05:10.090,0:05:11.420 It'll just float to the top. 0:05:11.420,0:05:12.640 It has no bonds. 0:05:12.640,0:05:14.687 And it'll just evaporate. 0:05:14.687,0:05:15.770 I shouldn't say evaporate. 0:05:15.770,0:05:17.720 It'll just bubble[br]out essentially, 0:05:17.720,0:05:19.460 because it's not[br]bonded to anything, 0:05:19.460,0:05:24.990 and it'll sort of just seep out[br]while we are in a liquid state. 0:05:24.990,0:05:26.820 And what's really[br]interesting about that 0:05:26.820,0:05:28.778 is that when you have[br]these volcanic eruptions, 0:05:28.778,0:05:32.900 and because this argon-40[br]is seeping out, by the time 0:05:32.900,0:05:38.490 this lava has hardened[br]into volcanic rock-- 0:05:38.490,0:05:42.230 and I'll do that volcanic[br]rock in a different color. 0:05:42.230,0:05:45.910 By the time it has[br]hardened into volcanic rock 0:05:45.910,0:05:49.360 all of the argon-40[br]will be gone. 0:05:49.360,0:05:50.960 It won't be there anymore. 0:05:50.960,0:05:53.680 And so what's neat is, this[br]volcanic event, the fact 0:05:53.680,0:05:55.800 that this rock[br]has become liquid, 0:05:55.800,0:05:58.600 it kind of resets the[br]amount of argon-40 there. 0:05:58.600,0:06:01.310 So then you're only going to[br]be left with potassium-40 here. 0:06:04.281,0:06:06.280 And that's why the argon-40[br]is more interesting, 0:06:06.280,0:06:09.317 because the calcium-40 won't[br]necessarily have seeped out. 0:06:09.317,0:06:11.400 And there might have already[br]been calcium-40 here. 0:06:11.400,0:06:12.775 So it won't[br]necessarily seep out. 0:06:12.775,0:06:15.010 But the argon-40 will seep out. 0:06:15.010,0:06:16.560 So it kind of resets it. 0:06:16.560,0:06:19.275 The volcanic event resets[br]the amount of argon-40. 0:06:21.865,0:06:23.240 So right when the[br]event happened, 0:06:23.240,0:06:27.580 you shouldn't have any argon-40[br]right when that lava actually 0:06:27.580,0:06:28.980 becomes solid. 0:06:28.980,0:06:32.850 And so if you fast forward[br]to some future date, 0:06:32.850,0:06:35.840 and if you look at the sample--[br]let me copy and paste it. 0:06:40.380,0:06:44.980 So if you fast forward to[br]some future date, and you 0:06:44.980,0:06:51.542 see that there is some[br]argon-40 there, in that sample, 0:06:51.542,0:06:54.350 you know this is[br]a volcanic rock. 0:06:54.350,0:06:57.450 You know that it was due to[br]some previous volcanic event. 0:06:57.450,0:07:02.675 You know that this argon-40 is[br]from the decayed potassium-40. 0:07:08.230,0:07:12.190 And you know that it has decayed[br]since that volcanic event, 0:07:12.190,0:07:14.709 because if it was there before[br]it would have seeped out. 0:07:14.709,0:07:17.250 So the only way that this would[br]have been able to get trapped 0:07:17.250,0:07:19.760 is, while it was liquid[br]it would seep out, 0:07:19.760,0:07:22.670 but once it's solid it can[br]get trapped inside the rock. 0:07:22.670,0:07:25.560 And so you know the only[br]way this argon-40 can 0:07:25.560,0:07:28.920 exist there is by decay[br]from that potassium-40. 0:07:28.920,0:07:30.720 So you can look at the ratio. 0:07:30.720,0:07:36.170 So you know for every[br]one of these argon-40's, 0:07:36.170,0:07:40.580 because only 11% of the decay[br]products are argon-40's, 0:07:40.580,0:07:43.650 for every one of[br]those you must have 0:07:43.650,0:07:49.150 on the order of about nine[br]calcium-40's that also decayed. 0:07:49.150,0:07:52.940 And so for every one of these[br]argon-40's you know that there 0:07:52.940,0:07:56.425 must have been 10[br]original potassium-40's. 0:07:56.425,0:07:57.800 And so what you[br]can do is you can 0:07:57.800,0:08:00.840 look at the ratio of the[br]number of potassium-40's there 0:08:00.840,0:08:03.330 are today to the number[br]that there must have been, 0:08:03.330,0:08:05.902 based on this evidence right[br]over here, to actually date it. 0:08:05.902,0:08:07.360 And in the next[br]video I'll actually 0:08:07.360,0:08:09.140 go through the[br]mathematical calculation 0:08:09.140,0:08:11.111 to show you that you[br]can actually date it. 0:08:11.111,0:08:12.610 And the reason this[br]is really useful 0:08:12.610,0:08:15.110 is, you can look[br]at those ratios. 0:08:15.110,0:08:18.179 And volcanic eruptions[br]aren't happening every day, 0:08:18.179,0:08:20.720 but if you start looking over[br]millions and millions of years, 0:08:20.720,0:08:22.220 on that time scale,[br]they're actually 0:08:22.220,0:08:25.520 happening reasonably frequent. 0:08:25.520,0:08:27.040 And so let's dig in the ground. 0:08:27.040,0:08:29.450 So let's say this is the[br]ground right over here. 0:08:29.450,0:08:33.600 And you dig enough and you[br]see a volcanic eruption, 0:08:33.600,0:08:37.115 you see some volcanic[br]rock right over there, 0:08:37.115,0:08:38.240 and then you dig even more. 0:08:38.240,0:08:42.440 There's another layer of[br]volcanic rock right over there. 0:08:42.440,0:08:44.850 So this is another[br]layer of volcanic rock. 0:08:47.820,0:08:50.460 So they're all going to have a[br]certain amount of potassium-40 0:08:50.460,0:08:51.900 in it. 0:08:51.900,0:08:55.420 This is going to have some[br]amount of potassium-40 in it. 0:08:55.420,0:08:59.060 And then let's say this one[br]over here has more argon-40. 0:08:59.060,0:09:00.411 This one has a little bit less. 0:09:00.411,0:09:02.910 And using the math that we're[br]going to do in the next video, 0:09:02.910,0:09:04.790 let's say you're[br]able to say that this 0:09:04.790,0:09:07.820 is, using the half-life, and[br]using the ratio of argon-40 0:09:07.820,0:09:12.190 that's left, or using the[br]ratio of the potassium-40 left 0:09:12.190,0:09:16.200 to what you know was there[br]before, you say that this must 0:09:16.200,0:09:20.600 have solidified 100[br]million years ago, 100 0:09:20.600,0:09:23.090 million years[br]before the present. 0:09:23.090,0:09:25.909 And you know that this layer[br]right over here solidified. 0:09:25.909,0:09:27.700 Let's say, you know it[br]solidified about 150 0:09:27.700,0:09:30.220 million years[br]before the present. 0:09:30.220,0:09:32.870 And let's say you feel pretty[br]good that this soil hasn't been 0:09:32.870,0:09:34.810 dug up and mixed or[br]anything like that. 0:09:34.810,0:09:36.810 It looks like it's been[br]pretty untouched when 0:09:36.810,0:09:39.570 you look at these soil[br]samples right over here. 0:09:39.570,0:09:45.040 And let's say you see[br]some fossils in here. 0:09:45.040,0:09:49.070 Then, even though carbon-14[br]dating is kind of useless, 0:09:49.070,0:09:51.410 really, when you get[br]beyond 50,000 years, 0:09:51.410,0:09:55.170 you see these fossils in[br]between these two periods. 0:09:55.170,0:09:56.670 It's a pretty good[br]indicator, if you 0:09:56.670,0:09:59.910 can assume that this soil hasn't[br]been dug around and mixed, 0:09:59.910,0:10:03.710 that this fossil is[br]between 100 million and 150 0:10:03.710,0:10:04.680 million years old. 0:10:04.680,0:10:06.120 This event happened. 0:10:06.120,0:10:08.730 Then you have these[br]fossils got deposited. 0:10:08.730,0:10:12.049 These animals died, or[br]they lived and they died. 0:10:12.049,0:10:13.840 And then you had this[br]other volcanic event. 0:10:13.840,0:10:17.690 So it allows you, even though[br]you're only directly dating 0:10:17.690,0:10:19.650 the volcanic rock,[br]it allows you, 0:10:19.650,0:10:22.430 when you look at the layers,[br]to relatively date things 0:10:22.430,0:10:24.000 in between those layer. 0:10:24.000,0:10:26.140 So it isn't just about[br]dating volcanic rock. 0:10:26.140,0:10:29.590 It allows us to date things[br]that are very, very, very old 0:10:29.590,0:10:34.530 and go way further back in time[br]than just carbon-14 dating.