How do archaeologists determine the age of a find?

Question

or what are the methods, tools, or techniques that archaeologists use to determine the age of a find?

Answer 1

Interesting question, and there's lots of answers!

Method1. Radiocarbon Dating. Types of carbon only found in living things change over time after they die, very slowly but very precisely. If you have some carbon like material in the find (anything organic such as paper or cloth or bone or charcoal or wood - or even ink) you can pin a date within a couple of hundred years (usually). Radiocarbon dating will take you back about 60,000 years (although the accuracy at that range is plus and minus thousands of years, not hundreds).

Method2. Man-made Telltales. The presence of something in the site that can be dated from other sources. So a coin with a head of a ruler on it will tell us that the find is not BEFORE that ruler's reign (but it might be long AFTER!). Interestingly most coins don't have dates on them! Sometimes a 'style' of pottery, or clothing, or weapon, or even hair style will allow experts to pin the date down very precisely (although some people go in for 'old fashioned' styles when they select things to be buried with). You'd also add in here any language or script, as styles of words and writing change over time, even the style of how people are 'laid out' in graves changes over time.

Method3. Tree ring dating, properly called dendrochronology. If you cut through a tree you notice it has 'rings', each representing a years worth of growth. An old tree will have hundreds of them. A ring created in a 'good' year will be thicker than one created during a year of drought etc, so if you counted back the tree rings on two separate trees you'd notice the 'skinny' rings appearing in the same 'place' in the sequence. When wood is cut down and turned into beams used in building and bridges etc the tree rings can still be 'read'. For hundreds of locations around the world scientists have followed the tree-ring story backwards - first looking at tree rings in trees that are still alive today, and then looking at matching rings in progressively older (and older) pieces of timber. If you have a piece of timber from a tree that lived from 1000AD to 1100AD, you'll find an matching set of rings in the last part of the record of a tree that lived from 900AD to 1050AD. Again it doesn't prove that a piece of already dead and very old timber wasn't used in the construction of something you've just dug up, but it means it can't be 'older' than the tree it came from. Oh, and of course this only works for wooden objects! Tree rings in Germany have been followed back through pieces of timber from overlapping periods as far back as 8,000BC. Because the 'pattern' of good and bad years (and thick and skinny tree rings) is quite random, when you find a piece of timber you just have to compare its pattern with the aready known tree ring sequence to find the point where the pattern 'matches'. Just like a fingerprint. The problem is, though, that climate varies from place to place, so the tree ring sequence from Germany isn't going to help me match a piece of timber from North America (well not easily...).

Method4: Relativism. As you dig things up in places where there has been a long record of continuous occupation, you have evidence of other times above and below the place where you are digging. Lower down equals older - usually. Sometimes you know a lot more about the layers above and below the place where you are currently digging, so you 'bracket' your find between the already known dates for those other finds. Oh, which raises the question, what covers each layer up - do people deliberately 'bury' older civilizations and build on top of them? Well it's partly that, they didn't have bulldozers and trucks to take the rubble from demolished (or burnt out) buildings away, so they simply flattened them and built on top. And there was also a lot of rubbish (and animal manure) accumulating around buildings (no rubbish collection service in most places...). What can be deceptive is when someone BURIES a stash of treasure (often containing coins) in a deep hole for security, or drops stuff down a well - so it's never as simple as it seems!

Method5. Natural Telltales. Tree rings are part of this, but a range of more haphazard methods include evidence of fires and earthquakes, and ash deposits from volcanoes and (in the very long term) meteorite collisions with the earth. These get spread over huge areas, and some of those events have been dated from other sources, so when you find them at 'your' site, you have a 'date-marker'. Same story with sea-level changes, some of those are known, so if you find them at your site you have a rough date. Plant pollen comes in here as well, it is incredibly tough stuff, and the plant it came from can usually be identified. Some plants grow only in certain conditions which we already know happened at certain times (after fire or flood), so that can give a date.

There's a few other chemical methods that are similar to radiocarbon dating which work on objects not made of carbon (obviously useful if you don't have any carbon in your 'find'). One thing that all of this points out is that your ability to 'date' a find very often relies on someone else's expertise (a laboratory for the chemical dating methods), or someone else's work (all of those other methods, where people have already established dates for styles, or tree rings, or the names and reign-dates of rulers who appear on coins etc). Archeology is very much a 'shared' achievement, and builds on (sometimes) very small steps taken 'one at a time'.

Answer 2

a classic is carbon dating- you measure the amount of radioactive carbon in an organic compound (no intake after death) and calculate backwards. Not very accurate, but works within a century or so.
Sometimes they get lucky and find something with a date on it- like coins- so it's a case of "not later than".
Sometimes the layer is between two (or more) layers (eg. Troy has 8 or 9 "civilisation" layers) so there is a rough time frame

For real old finds- like dinosaur bones- you can calculate the age of the whole rock formation- eg. knowing how long it takes for sand or mud to become rock

Answer 3

They can use carbon dating, and they have to analyze the carbon, they could use tree-ring dating, which is for counting the number of rings in a wooden object and comparing it to tree-ring samply things. They can also date it by the layer of sediment the object was found in.

Answer 4

Carbon dating.

http://science.howstuffworks.com/carbon-14.htm
http://en.wikipedia.org/wiki/Carbon_dating