[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.50,0:00:02.44,Default,,0000,0000,0000,,We know that an\Nelement is defined
Dialogue: 0,0:00:02.44,0:00:04.35,Default,,0000,0000,0000,,by the number of protons it has.
Dialogue: 0,0:00:04.35,0:00:05.40,Default,,0000,0000,0000,,For example, potassium.
Dialogue: 0,0:00:05.40,0:00:07.15,Default,,0000,0000,0000,,We look at the periodic\Ntable of elements.
Dialogue: 0,0:00:07.15,0:00:09.70,Default,,0000,0000,0000,,And I have a snapshot of\Nit, of not the entire table
Dialogue: 0,0:00:09.70,0:00:10.72,Default,,0000,0000,0000,,but part of it here.
Dialogue: 0,0:00:10.72,0:00:12.92,Default,,0000,0000,0000,,Potassium has 19 protons.
Dialogue: 0,0:00:12.92,0:00:14.36,Default,,0000,0000,0000,,And we could write it like this.
Dialogue: 0,0:00:14.36,0:00:15.82,Default,,0000,0000,0000,,And this is a little\Nbit redundant.
Dialogue: 0,0:00:15.82,0:00:18.28,Default,,0000,0000,0000,,We know that if it's potassium\Nthat atom has 19 protons.
Dialogue: 0,0:00:18.28,0:00:20.72,Default,,0000,0000,0000,,And we know if an\Natom has 19 protons
Dialogue: 0,0:00:20.72,0:00:23.26,Default,,0000,0000,0000,,it is going to be potassium.
Dialogue: 0,0:00:23.26,0:00:28.92,Default,,0000,0000,0000,,Now, we also know that not all\Nof the atoms of a given element
Dialogue: 0,0:00:28.92,0:00:31.26,Default,,0000,0000,0000,,have the same\Nnumber of neutrons.
Dialogue: 0,0:00:31.26,0:00:33.34,Default,,0000,0000,0000,,And when we talk\Nabout a given element,
Dialogue: 0,0:00:33.34,0:00:35.07,Default,,0000,0000,0000,,but we have different\Nnumbers of neutrons
Dialogue: 0,0:00:35.07,0:00:37.64,Default,,0000,0000,0000,,we call them isotopes\Nof that element.
Dialogue: 0,0:00:37.64,0:00:40.93,Default,,0000,0000,0000,,So for example,\Npotassium can come
Dialogue: 0,0:00:40.93,0:00:43.98,Default,,0000,0000,0000,,in a form that has\Nexactly 20 neutrons.
Dialogue: 0,0:00:43.98,0:00:45.23,Default,,0000,0000,0000,,And we call that potassium-39.
Dialogue: 0,0:00:48.19,0:00:50.76,Default,,0000,0000,0000,,And 39, this mass\Nnumber, it's a count
Dialogue: 0,0:00:50.76,0:00:56.71,Default,,0000,0000,0000,,of the 19 protons\Nplus 20 neutrons.
Dialogue: 0,0:00:56.71,0:00:59.78,Default,,0000,0000,0000,,And this is actually the most\Ncommon isotope of potassium.
Dialogue: 0,0:00:59.78,0:01:03.36,Default,,0000,0000,0000,,It accounts for, I'm\Njust rounding off,
Dialogue: 0,0:01:03.36,0:01:08.76,Default,,0000,0000,0000,,93.3% of the potassium that\Nyou would find on Earth.
Dialogue: 0,0:01:08.76,0:01:12.04,Default,,0000,0000,0000,,Now, some of the other\Nisotopes of potassium.
Dialogue: 0,0:01:12.04,0:01:14.24,Default,,0000,0000,0000,,You also have potassium--\Nand once again writing
Dialogue: 0,0:01:14.24,0:01:16.49,Default,,0000,0000,0000,,the K and the 19 are a\Nlittle bit redundant--
Dialogue: 0,0:01:16.49,0:01:18.53,Default,,0000,0000,0000,,you also have potassium-41.
Dialogue: 0,0:01:18.53,0:01:20.46,Default,,0000,0000,0000,,So this would have 22 neutrons.
Dialogue: 0,0:01:20.46,0:01:22.58,Default,,0000,0000,0000,,22 plus 19 is 41.
Dialogue: 0,0:01:22.58,0:01:28.18,Default,,0000,0000,0000,,This accounts for about 6.7%\Nof the potassium on the planet.
Dialogue: 0,0:01:28.18,0:01:30.55,Default,,0000,0000,0000,,And then you have a\Nvery scarce isotope
Dialogue: 0,0:01:30.55,0:01:33.89,Default,,0000,0000,0000,,of potassium called\Npotassium-40.
Dialogue: 0,0:01:33.89,0:01:38.03,Default,,0000,0000,0000,,Potassium-40 clearly\Nhas 21 neutrons.
Dialogue: 0,0:01:38.03,0:01:40.38,Default,,0000,0000,0000,,And it's very, very,\Nvery, very scarce.
Dialogue: 0,0:01:40.38,0:01:46.01,Default,,0000,0000,0000,,It accounts for only 0.0117%\Nof all the potassium.
Dialogue: 0,0:01:46.01,0:01:48.59,Default,,0000,0000,0000,,But this is also the\Nisotope of potassium
Dialogue: 0,0:01:48.59,0:01:51.35,Default,,0000,0000,0000,,that's interesting to us\Nfrom the point of view
Dialogue: 0,0:01:51.35,0:01:56.49,Default,,0000,0000,0000,,of dating old, old rock, and\Nespecially old volcanic rock.
Dialogue: 0,0:01:56.49,0:01:59.68,Default,,0000,0000,0000,,And as we'll see, when you\Ncan date old volcanic rock
Dialogue: 0,0:01:59.68,0:02:01.53,Default,,0000,0000,0000,,it allows you to date\Nother types of rock
Dialogue: 0,0:02:01.53,0:02:03.81,Default,,0000,0000,0000,,or other types of fossils\Nthat might be sandwiched
Dialogue: 0,0:02:03.81,0:02:06.90,Default,,0000,0000,0000,,in between old volcanic rock.
Dialogue: 0,0:02:06.90,0:02:10.57,Default,,0000,0000,0000,,And so what's really interesting\Nabout potassium-40 here
Dialogue: 0,0:02:10.57,0:02:15.19,Default,,0000,0000,0000,,is that it has a half-life\Nof 1.25 billion years.
Dialogue: 0,0:02:15.19,0:02:17.40,Default,,0000,0000,0000,,So the good thing about\Nthat, as opposed to something
Dialogue: 0,0:02:17.40,0:02:19.95,Default,,0000,0000,0000,,like carbon-14, it can\Nbe used to date really,
Dialogue: 0,0:02:19.95,0:02:21.52,Default,,0000,0000,0000,,really, really old things.
Dialogue: 0,0:02:21.52,0:02:27.16,Default,,0000,0000,0000,,And every 1.25\Nbillion years-- let
Dialogue: 0,0:02:27.16,0:02:32.66,Default,,0000,0000,0000,,me write it like this,\Nthat's its half-life--
Dialogue: 0,0:02:32.66,0:02:36.46,Default,,0000,0000,0000,,so 50% of any given\Nsample will have decayed.
Dialogue: 0,0:02:36.46,0:02:40.78,Default,,0000,0000,0000,,And 11% will have\Ndecayed into argon-40.
Dialogue: 0,0:02:45.88,0:02:47.53,Default,,0000,0000,0000,,So argon is right over here.
Dialogue: 0,0:02:47.53,0:02:49.69,Default,,0000,0000,0000,,It has 18 protons.
Dialogue: 0,0:02:49.69,0:02:51.96,Default,,0000,0000,0000,,So when you think about\Nit decaying into argon-40,
Dialogue: 0,0:02:51.96,0:02:54.05,Default,,0000,0000,0000,,what you see is that\Nit lost a proton,
Dialogue: 0,0:02:54.05,0:02:56.06,Default,,0000,0000,0000,,but it has the same mass number.
Dialogue: 0,0:02:56.06,0:02:59.64,Default,,0000,0000,0000,,So one of the protons must of\Nsomehow turned into a neutron.
Dialogue: 0,0:02:59.64,0:03:02.24,Default,,0000,0000,0000,,And it actually captures\None of the inner electrons,
Dialogue: 0,0:03:02.24,0:03:03.83,Default,,0000,0000,0000,,and then it emits\Nother things, and I
Dialogue: 0,0:03:03.83,0:03:05.66,Default,,0000,0000,0000,,won't go into all the\Nquantum physics of it,
Dialogue: 0,0:03:05.66,0:03:07.38,Default,,0000,0000,0000,,but it turns into argon-40.
Dialogue: 0,0:03:07.38,0:03:12.69,Default,,0000,0000,0000,,And 89% turn into calcium-40.
Dialogue: 0,0:03:12.69,0:03:15.06,Default,,0000,0000,0000,,And you see calcium on the\Nperiodic table right over here
Dialogue: 0,0:03:15.06,0:03:16.48,Default,,0000,0000,0000,,has 20 protons.
Dialogue: 0,0:03:16.48,0:03:18.76,Default,,0000,0000,0000,,So this is a situation\Nwhere one of the neutrons
Dialogue: 0,0:03:18.76,0:03:20.29,Default,,0000,0000,0000,,turns into a proton.
Dialogue: 0,0:03:20.29,0:03:22.12,Default,,0000,0000,0000,,This is a situation\Nwhere one of the protons
Dialogue: 0,0:03:22.12,0:03:23.71,Default,,0000,0000,0000,,turns into a neutron.
Dialogue: 0,0:03:23.71,0:03:25.64,Default,,0000,0000,0000,,And what's really\Ninteresting to us
Dialogue: 0,0:03:25.64,0:03:29.52,Default,,0000,0000,0000,,is this part right over here.
Dialogue: 0,0:03:29.52,0:03:32.17,Default,,0000,0000,0000,,Because what's cool about argon,\Nand we study this a little bit
Dialogue: 0,0:03:32.17,0:03:36.15,Default,,0000,0000,0000,,in the chemistry playlist, it is\Na noble gas, it is unreactive.
Dialogue: 0,0:03:36.15,0:03:39.23,Default,,0000,0000,0000,,And so when it is embedded\Nin something that's
Dialogue: 0,0:03:39.23,0:03:42.31,Default,,0000,0000,0000,,in a liquid state it'll\Nkind of just bubble out.
Dialogue: 0,0:03:42.31,0:03:46.16,Default,,0000,0000,0000,,It's not bonded to\Nanything, and so it'll just
Dialogue: 0,0:03:46.16,0:03:48.95,Default,,0000,0000,0000,,bubble out and just go\Nout into the atmosphere.
Dialogue: 0,0:03:48.95,0:03:50.99,Default,,0000,0000,0000,,So what's interesting\Nabout this whole situation
Dialogue: 0,0:03:50.99,0:03:54.95,Default,,0000,0000,0000,,is you can imagine what happens\Nduring a volcanic eruption.
Dialogue: 0,0:03:54.95,0:03:57.41,Default,,0000,0000,0000,,Let me draw a volcano here.
Dialogue: 0,0:03:57.41,0:04:00.67,Default,,0000,0000,0000,,So let's say that\Nthis is our volcano.
Dialogue: 0,0:04:00.67,0:04:04.16,Default,,0000,0000,0000,,And it erupts at some\Ntime in the past.
Dialogue: 0,0:04:04.16,0:04:08.80,Default,,0000,0000,0000,,So it erupts, and you have\Nall of this lava flowing.
Dialogue: 0,0:04:12.73,0:04:16.07,Default,,0000,0000,0000,,That lava will contain some\Namount of potassium-40.
Dialogue: 0,0:04:16.07,0:04:17.78,Default,,0000,0000,0000,,And actually, it'll\Nalready contain
Dialogue: 0,0:04:17.78,0:04:18.80,Default,,0000,0000,0000,,some amount of argon-40.
Dialogue: 0,0:04:22.57,0:04:24.41,Default,,0000,0000,0000,,But what's neat\Nabout argon-40 is
Dialogue: 0,0:04:24.41,0:04:27.60,Default,,0000,0000,0000,,that while it's lava, while it's\Nin this liquid state-- so let's
Dialogue: 0,0:04:27.60,0:04:30.40,Default,,0000,0000,0000,,imagine this lava\Nright over here.
Dialogue: 0,0:04:30.40,0:04:33.89,Default,,0000,0000,0000,,It's a bunch of stuff\Nright over here.
Dialogue: 0,0:04:37.08,0:04:39.02,Default,,0000,0000,0000,,I'll do the potassium-40.
Dialogue: 0,0:04:39.02,0:04:41.60,Default,,0000,0000,0000,,And let me do it in a color\Nthat I haven't used yet.
Dialogue: 0,0:04:41.60,0:04:44.01,Default,,0000,0000,0000,,I'll do the\Npotassium-40 in magenta.
Dialogue: 0,0:04:44.01,0:04:48.09,Default,,0000,0000,0000,,It'll have some\Npotassium-40 in it.
Dialogue: 0,0:04:48.09,0:04:49.09,Default,,0000,0000,0000,,I'm maybe over doing it.
Dialogue: 0,0:04:49.09,0:04:50.94,Default,,0000,0000,0000,,It's a very scarce isotope.
Dialogue: 0,0:04:50.94,0:04:53.04,Default,,0000,0000,0000,,But it'll have some\Npotassium-40 in it.
Dialogue: 0,0:04:53.04,0:05:01.28,Default,,0000,0000,0000,,And it might already have some\Nargon-40 in it just like that.
Dialogue: 0,0:05:01.28,0:05:03.19,Default,,0000,0000,0000,,But argon-40 is a noble gas.
Dialogue: 0,0:05:03.19,0:05:04.52,Default,,0000,0000,0000,,It's not going to bond anything.
Dialogue: 0,0:05:04.52,0:05:06.98,Default,,0000,0000,0000,,And while this lava\Nis in a liquid state
Dialogue: 0,0:05:06.98,0:05:10.09,Default,,0000,0000,0000,,it's going to be\Nable to bubble out.
Dialogue: 0,0:05:10.09,0:05:11.42,Default,,0000,0000,0000,,It'll just float to the top.
Dialogue: 0,0:05:11.42,0:05:12.64,Default,,0000,0000,0000,,It has no bonds.
Dialogue: 0,0:05:12.64,0:05:14.69,Default,,0000,0000,0000,,And it'll just evaporate.
Dialogue: 0,0:05:14.69,0:05:15.77,Default,,0000,0000,0000,,I shouldn't say evaporate.
Dialogue: 0,0:05:15.77,0:05:17.72,Default,,0000,0000,0000,,It'll just bubble\Nout essentially,
Dialogue: 0,0:05:17.72,0:05:19.46,Default,,0000,0000,0000,,because it's not\Nbonded to anything,
Dialogue: 0,0:05:19.46,0:05:24.99,Default,,0000,0000,0000,,and it'll sort of just seep out\Nwhile we are in a liquid state.
Dialogue: 0,0:05:24.99,0:05:26.82,Default,,0000,0000,0000,,And what's really\Ninteresting about that
Dialogue: 0,0:05:26.82,0:05:28.78,Default,,0000,0000,0000,,is that when you have\Nthese volcanic eruptions,
Dialogue: 0,0:05:28.78,0:05:32.90,Default,,0000,0000,0000,,and because this argon-40\Nis seeping out, by the time
Dialogue: 0,0:05:32.90,0:05:38.49,Default,,0000,0000,0000,,this lava has hardened\Ninto volcanic rock--
Dialogue: 0,0:05:38.49,0:05:42.23,Default,,0000,0000,0000,,and I'll do that volcanic\Nrock in a different color.
Dialogue: 0,0:05:42.23,0:05:45.91,Default,,0000,0000,0000,,By the time it has\Nhardened into volcanic rock
Dialogue: 0,0:05:45.91,0:05:49.36,Default,,0000,0000,0000,,all of the argon-40\Nwill be gone.
Dialogue: 0,0:05:49.36,0:05:50.96,Default,,0000,0000,0000,,It won't be there anymore.
Dialogue: 0,0:05:50.96,0:05:53.68,Default,,0000,0000,0000,,And so what's neat is, this\Nvolcanic event, the fact
Dialogue: 0,0:05:53.68,0:05:55.80,Default,,0000,0000,0000,,that this rock\Nhas become liquid,
Dialogue: 0,0:05:55.80,0:05:58.60,Default,,0000,0000,0000,,it kind of resets the\Namount of argon-40 there.
Dialogue: 0,0:05:58.60,0:06:01.31,Default,,0000,0000,0000,,So then you're only going to\Nbe left with potassium-40 here.
Dialogue: 0,0:06:04.28,0:06:06.28,Default,,0000,0000,0000,,And that's why the argon-40\Nis more interesting,
Dialogue: 0,0:06:06.28,0:06:09.32,Default,,0000,0000,0000,,because the calcium-40 won't\Nnecessarily have seeped out.
Dialogue: 0,0:06:09.32,0:06:11.40,Default,,0000,0000,0000,,And there might have already\Nbeen calcium-40 here.
Dialogue: 0,0:06:11.40,0:06:12.78,Default,,0000,0000,0000,,So it won't\Nnecessarily seep out.
Dialogue: 0,0:06:12.78,0:06:15.01,Default,,0000,0000,0000,,But the argon-40 will seep out.
Dialogue: 0,0:06:15.01,0:06:16.56,Default,,0000,0000,0000,,So it kind of resets it.
Dialogue: 0,0:06:16.56,0:06:19.28,Default,,0000,0000,0000,,The volcanic event resets\Nthe amount of argon-40.
Dialogue: 0,0:06:21.86,0:06:23.24,Default,,0000,0000,0000,,So right when the\Nevent happened,
Dialogue: 0,0:06:23.24,0:06:27.58,Default,,0000,0000,0000,,you shouldn't have any argon-40\Nright when that lava actually
Dialogue: 0,0:06:27.58,0:06:28.98,Default,,0000,0000,0000,,becomes solid.
Dialogue: 0,0:06:28.98,0:06:32.85,Default,,0000,0000,0000,,And so if you fast forward\Nto some future date,
Dialogue: 0,0:06:32.85,0:06:35.84,Default,,0000,0000,0000,,and if you look at the sample--\Nlet me copy and paste it.
Dialogue: 0,0:06:40.38,0:06:44.98,Default,,0000,0000,0000,,So if you fast forward to\Nsome future date, and you
Dialogue: 0,0:06:44.98,0:06:51.54,Default,,0000,0000,0000,,see that there is some\Nargon-40 there, in that sample,
Dialogue: 0,0:06:51.54,0:06:54.35,Default,,0000,0000,0000,,you know this is\Na volcanic rock.
Dialogue: 0,0:06:54.35,0:06:57.45,Default,,0000,0000,0000,,You know that it was due to\Nsome previous volcanic event.
Dialogue: 0,0:06:57.45,0:07:02.68,Default,,0000,0000,0000,,You know that this argon-40 is\Nfrom the decayed potassium-40.
Dialogue: 0,0:07:08.23,0:07:12.19,Default,,0000,0000,0000,,And you know that it has decayed\Nsince that volcanic event,
Dialogue: 0,0:07:12.19,0:07:14.71,Default,,0000,0000,0000,,because if it was there before\Nit would have seeped out.
Dialogue: 0,0:07:14.71,0:07:17.25,Default,,0000,0000,0000,,So the only way that this would\Nhave been able to get trapped
Dialogue: 0,0:07:17.25,0:07:19.76,Default,,0000,0000,0000,,is, while it was liquid\Nit would seep out,
Dialogue: 0,0:07:19.76,0:07:22.67,Default,,0000,0000,0000,,but once it's solid it can\Nget trapped inside the rock.
Dialogue: 0,0:07:22.67,0:07:25.56,Default,,0000,0000,0000,,And so you know the only\Nway this argon-40 can
Dialogue: 0,0:07:25.56,0:07:28.92,Default,,0000,0000,0000,,exist there is by decay\Nfrom that potassium-40.
Dialogue: 0,0:07:28.92,0:07:30.72,Default,,0000,0000,0000,,So you can look at the ratio.
Dialogue: 0,0:07:30.72,0:07:36.17,Default,,0000,0000,0000,,So you know for every\None of these argon-40's,
Dialogue: 0,0:07:36.17,0:07:40.58,Default,,0000,0000,0000,,because only 11% of the decay\Nproducts are argon-40's,
Dialogue: 0,0:07:40.58,0:07:43.65,Default,,0000,0000,0000,,for every one of\Nthose you must have
Dialogue: 0,0:07:43.65,0:07:49.15,Default,,0000,0000,0000,,on the order of about nine\Ncalcium-40's that also decayed.
Dialogue: 0,0:07:49.15,0:07:52.94,Default,,0000,0000,0000,,And so for every one of these\Nargon-40's you know that there
Dialogue: 0,0:07:52.94,0:07:56.42,Default,,0000,0000,0000,,must have been 10\Noriginal potassium-40's.
Dialogue: 0,0:07:56.42,0:07:57.80,Default,,0000,0000,0000,,And so what you\Ncan do is you can
Dialogue: 0,0:07:57.80,0:08:00.84,Default,,0000,0000,0000,,look at the ratio of the\Nnumber of potassium-40's there
Dialogue: 0,0:08:00.84,0:08:03.33,Default,,0000,0000,0000,,are today to the number\Nthat there must have been,
Dialogue: 0,0:08:03.33,0:08:05.90,Default,,0000,0000,0000,,based on this evidence right\Nover here, to actually date it.
Dialogue: 0,0:08:05.90,0:08:07.36,Default,,0000,0000,0000,,And in the next\Nvideo I'll actually
Dialogue: 0,0:08:07.36,0:08:09.14,Default,,0000,0000,0000,,go through the\Nmathematical calculation
Dialogue: 0,0:08:09.14,0:08:11.11,Default,,0000,0000,0000,,to show you that you\Ncan actually date it.
Dialogue: 0,0:08:11.11,0:08:12.61,Default,,0000,0000,0000,,And the reason this\Nis really useful
Dialogue: 0,0:08:12.61,0:08:15.11,Default,,0000,0000,0000,,is, you can look\Nat those ratios.
Dialogue: 0,0:08:15.11,0:08:18.18,Default,,0000,0000,0000,,And volcanic eruptions\Naren't happening every day,
Dialogue: 0,0:08:18.18,0:08:20.72,Default,,0000,0000,0000,,but if you start looking over\Nmillions and millions of years,
Dialogue: 0,0:08:20.72,0:08:22.22,Default,,0000,0000,0000,,on that time scale,\Nthey're actually
Dialogue: 0,0:08:22.22,0:08:25.52,Default,,0000,0000,0000,,happening reasonably frequent.
Dialogue: 0,0:08:25.52,0:08:27.04,Default,,0000,0000,0000,,And so let's dig in the ground.
Dialogue: 0,0:08:27.04,0:08:29.45,Default,,0000,0000,0000,,So let's say this is the\Nground right over here.
Dialogue: 0,0:08:29.45,0:08:33.60,Default,,0000,0000,0000,,And you dig enough and you\Nsee a volcanic eruption,
Dialogue: 0,0:08:33.60,0:08:37.12,Default,,0000,0000,0000,,you see some volcanic\Nrock right over there,
Dialogue: 0,0:08:37.12,0:08:38.24,Default,,0000,0000,0000,,and then you dig even more.
Dialogue: 0,0:08:38.24,0:08:42.44,Default,,0000,0000,0000,,There's another layer of\Nvolcanic rock right over there.
Dialogue: 0,0:08:42.44,0:08:44.85,Default,,0000,0000,0000,,So this is another\Nlayer of volcanic rock.
Dialogue: 0,0:08:47.82,0:08:50.46,Default,,0000,0000,0000,,So they're all going to have a\Ncertain amount of potassium-40
Dialogue: 0,0:08:50.46,0:08:51.90,Default,,0000,0000,0000,,in it.
Dialogue: 0,0:08:51.90,0:08:55.42,Default,,0000,0000,0000,,This is going to have some\Namount of potassium-40 in it.
Dialogue: 0,0:08:55.42,0:08:59.06,Default,,0000,0000,0000,,And then let's say this one\Nover here has more argon-40.
Dialogue: 0,0:08:59.06,0:09:00.41,Default,,0000,0000,0000,,This one has a little bit less.
Dialogue: 0,0:09:00.41,0:09:02.91,Default,,0000,0000,0000,,And using the math that we're\Ngoing to do in the next video,
Dialogue: 0,0:09:02.91,0:09:04.79,Default,,0000,0000,0000,,let's say you're\Nable to say that this
Dialogue: 0,0:09:04.79,0:09:07.82,Default,,0000,0000,0000,,is, using the half-life, and\Nusing the ratio of argon-40
Dialogue: 0,0:09:07.82,0:09:12.19,Default,,0000,0000,0000,,that's left, or using the\Nratio of the potassium-40 left
Dialogue: 0,0:09:12.19,0:09:16.20,Default,,0000,0000,0000,,to what you know was there\Nbefore, you say that this must
Dialogue: 0,0:09:16.20,0:09:20.60,Default,,0000,0000,0000,,have solidified 100\Nmillion years ago, 100
Dialogue: 0,0:09:20.60,0:09:23.09,Default,,0000,0000,0000,,million years\Nbefore the present.
Dialogue: 0,0:09:23.09,0:09:25.91,Default,,0000,0000,0000,,And you know that this layer\Nright over here solidified.
Dialogue: 0,0:09:25.91,0:09:27.70,Default,,0000,0000,0000,,Let's say, you know it\Nsolidified about 150
Dialogue: 0,0:09:27.70,0:09:30.22,Default,,0000,0000,0000,,million years\Nbefore the present.
Dialogue: 0,0:09:30.22,0:09:32.87,Default,,0000,0000,0000,,And let's say you feel pretty\Ngood that this soil hasn't been
Dialogue: 0,0:09:32.87,0:09:34.81,Default,,0000,0000,0000,,dug up and mixed or\Nanything like that.
Dialogue: 0,0:09:34.81,0:09:36.81,Default,,0000,0000,0000,,It looks like it's been\Npretty untouched when
Dialogue: 0,0:09:36.81,0:09:39.57,Default,,0000,0000,0000,,you look at these soil\Nsamples right over here.
Dialogue: 0,0:09:39.57,0:09:45.04,Default,,0000,0000,0000,,And let's say you see\Nsome fossils in here.
Dialogue: 0,0:09:45.04,0:09:49.07,Default,,0000,0000,0000,,Then, even though carbon-14\Ndating is kind of useless,
Dialogue: 0,0:09:49.07,0:09:51.41,Default,,0000,0000,0000,,really, when you get\Nbeyond 50,000 years,
Dialogue: 0,0:09:51.41,0:09:55.17,Default,,0000,0000,0000,,you see these fossils in\Nbetween these two periods.
Dialogue: 0,0:09:55.17,0:09:56.67,Default,,0000,0000,0000,,It's a pretty good\Nindicator, if you
Dialogue: 0,0:09:56.67,0:09:59.91,Default,,0000,0000,0000,,can assume that this soil hasn't\Nbeen dug around and mixed,
Dialogue: 0,0:09:59.91,0:10:03.71,Default,,0000,0000,0000,,that this fossil is\Nbetween 100 million and 150
Dialogue: 0,0:10:03.71,0:10:04.68,Default,,0000,0000,0000,,million years old.
Dialogue: 0,0:10:04.68,0:10:06.12,Default,,0000,0000,0000,,This event happened.
Dialogue: 0,0:10:06.12,0:10:08.73,Default,,0000,0000,0000,,Then you have these\Nfossils got deposited.
Dialogue: 0,0:10:08.73,0:10:12.05,Default,,0000,0000,0000,,These animals died, or\Nthey lived and they died.
Dialogue: 0,0:10:12.05,0:10:13.84,Default,,0000,0000,0000,,And then you had this\Nother volcanic event.
Dialogue: 0,0:10:13.84,0:10:17.69,Default,,0000,0000,0000,,So it allows you, even though\Nyou're only directly dating
Dialogue: 0,0:10:17.69,0:10:19.65,Default,,0000,0000,0000,,the volcanic rock,\Nit allows you,
Dialogue: 0,0:10:19.65,0:10:22.43,Default,,0000,0000,0000,,when you look at the layers,\Nto relatively date things
Dialogue: 0,0:10:22.43,0:10:24.00,Default,,0000,0000,0000,,in between those layer.
Dialogue: 0,0:10:24.00,0:10:26.14,Default,,0000,0000,0000,,So it isn't just about\Ndating volcanic rock.
Dialogue: 0,0:10:26.14,0:10:29.59,Default,,0000,0000,0000,,It allows us to date things\Nthat are very, very, very old
Dialogue: 0,0:10:29.59,0:10:34.53,Default,,0000,0000,0000,,and go way further back in time\Nthan just carbon-14 dating.