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Here is today

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Re: Here is today

#111

Post by jurassic »

apunic wrote: post #14

I always wonder how scientists do know that something was 25, 540 million or 2.560 billion years ago.

Single crystal uranium-lead dating is one of the best all-around methods and gets used a lot for vanilla geological applications of Earth rocks; any rock that contains trace amounts of the mineral "zircon" can be dated with this technique. Zircon is special because it tends to incorporate a lot of uranium when it crystallizes while excluding lead. As time passes after the zircon crystal cools, uranium decays into radiogenic lead, and by measuring the relative abundance of these two things in the crystal we can estimate the time since that zircon crystallized (with no lead inside) and get an "age" that tells us how long it has been since that zircon crystal was formed. Zircon is common in many types of igneous rocks, which are rocks that began in a molten state. The zircon crystals in an igneous rock are resistant to weathering (compared to feldspars, another common igneous constituent), so sometimes they end up as grains in sedimentary rocks and we can use a variation on the technique called "detrital zircon geochronology" to estimate the age of sedimentary deposits.

Other systems (like samarium-neodymium) are most often used when trying to date things older than the Earth, e.g. meteorites.

http://en.wikipedia.org/wiki/Radiometric_dating

Re: Here is today

#112

Post by roc »

charlieok wrote: post #64
chestnut-tree wrote: post #55

This reminded me of something I saw at the Science Museum in London a few years ago. A round clock face is used to represent the history of the earth and a narrator tells us the geological events that happen as the clock hands travels around the clock face. At a few seconds to midnight, we're told this is when humans appear. I thought it was a clever way of illustrating how human history occupies such a tiny segment…

Rather than place the present day at midnight on the clock, how about placing the present day on the current value given by the Doomsday Clock [1]. Then, with a little arithmetic, you can label midnight with the projected date of Doomsday :) [1] http://en.wikipedia.org/wiki/Doomsday_Clock

At that scale, I don't think there'd be an appreciable difference between the two.

Re: Here is today

#113

Post by dsrguru »

bmunro wrote: post #100
dsrguru wrote: post #95

What I find most interesting is that the dawn of anatomically modern humans is actually visible on the 4.54 billion year timeline of the Earth. 200,000 years out of 4.54 billion might only be about 1/23000 of our planet's history, but that's also a whole 1/23000!

If the period of time that humans have been around was drawn to scale, it wouldn't really be visible. If we assume that the width of the bar for the life of the earth is 1500 pixels (the width changes depending on the window size), then the period we have been around will be around 0.06 pixels wide (1500/23000 = 0.065). Much closer to zero pixels than one pixel.

Good point.

Re: Here is today

#115

Post by joshschreuder »

baby wrote: post #97
joshschreuder wrote: post #89

Earlier quoted context omitted.

Out of interest, what country are you from? I'm curious at the fact you called it 'grand-grand'. Here in Australia we use 'great grand(mother etc.)'. Must be a cultural thing.

I just guessed, I'm french :)

Ah makes sense! Well I knew what you meant, I was just wondering whether other countries had this 'great-grand' thing or just Australia.

Re: Here is today

#116

Post by jurassic »

mcrittenden wrote: post #45
courtewing wrote: post #38

Earlier quoted context omitted.

Carbon dating is actually only effective up to about 50,000 years, so its really only ideal for tracking human history after we started migrating out of Africa. This is because the unstable carbon-14 isotope only has a half-life of about 5700 years. There are many other forms of radiometric dating, though. Uranium-lead dating has proven relatively accurate for periods of time between 1 million to 4.5 billion years ag…

I'm curious how it could prove accurate for that time frame? How could something like that be verified or proven?

The best way I know to "check" radiometric ages is to date a rock using more than one isotopic system (e.g. K-Ar and U-Pb) and compare the ages that you get. If you get the same age from isotope systems with different decay rates and systematics, that is strong evidence that your technique is working.

People have done this many times, and most of the error comes from different levels of uncertainty in the decay constants from one system to another. Many of the decay constants have not been updated for decades (e.g. Steiger and Jager 1977) and are suspected to be a few percent off, but the people who are qualified to make such measurements (physicists + chemists) aren't the people who want to use them in applications (geochronologists and geochemists). So the incentives aren't aligned and I doubt anyone will ever fix them.

Note: Was a geology/geophysics PhD student until recently.

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