Daily Archives: February 26, 2011

How old is the earth, and how do we know?

How old is the earth?

Short answer: 4,540,000,00/H30 Earth-years, plus or minus 1%.

Long answer: We don’t know exactly because direct dating of the earliest material on the surface of the Earth will only tell use a minimum age; Prior to that, the Earth’s surface was probably molten, and even after that, it may be that the earliest non-molten material has been recycled into the planet’s interior by tectonic processes. Also, the earth is a big round ball of stuff that condensed into this shape from part of a large disk-shaped blob of stuff known as the Solar Nebula. When exactly, given this, did the Earth become the Earth? Since the process took millions of years, we can’t pinpoint the age of the Earth more exactly than a certain range.

What are the oldest rocks?

The oldest rock formations on Earth are between about 3.8 and 3.9 billion years old., but there are older bits of more ancient rocks that were incorporated into these early rocks, and they date to something closer to 4.4 billion years old. These and other early materials are dated primarily using a variety of parent-daughter radiometric techniques, with the most effective for this time period being a lead-lead system.

Since rock from the time of the Earth’s formation isn’t available (because it didn’t really exist or was gobbled up in the fiery beginnings of the big round ball) the preferred method of dating the Earth is to calculate the age of meteorites. The earliest meteorites essentially date the condensation of materials in the solar system into the planets, and thus, the date of these meteorites indicates the date of the early Earth. (The Earth existed prior to this condensation in the form of whatever parts of the early solar nebula would eventually condense into this particular planet, of course.)

Meteorites from other planets?

Some meteorites are known to be fragments of Mars, so the oldest dates among these can also verify the date of accretion of material into planets in our solar system.

Rocks from the moon have not been remelted or otherwise messed up by tectonic processes and therefore would provide an excellent estimate of the age of the Earth as well. Also, since there is no real weathering of rocks on the moon, methods other than parent-daughter decay can be used, such as Fission Track dating (the older a rock, the more cosmic rays pass through it, blasting tiny little tracks in the otherwise homogeneous matrix).

Zeroing in on the age of the earth

There are hundreds of published dates of various older materials, but the following table gives a reasonable summary of some of the more important dates, culled from various sources (see list of references below):


If we chart this on a graph, we see one date that is much earlier than all the other dates, and a few that are younger.


The younger dates are simply of materials that we don’t think date the Earth’s formation, but that we know would post date it by not much. These dates verify the earlier cluster of dates that would correspond to the actual formation of the planet. The single earlier date is an obvious outlier.

Taking this series of dates, notice that the oldest (non-outlier) dates are about four and a half billion years old. As stated in the short answer.

Further information about the age of the Earth:

Dalrymple, G. Brent. 2001. The age of the Earth in the twentieth century: a problem (mostly) solved. Geological Society, London, Special Publications 2001, v. 190, p. 205-221. Click Here.

Dalrymple, G. Brent. 2006. How Old is the Earth: A Response to “Scientific” Creationism. The TalkOrigins Archive. Click Here.

Norman, M. D., Borg, L. E., Nyquist, L. E., and Bogard, D. D. (2003) Chronology, geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age, origin, structure, and impact history of the lunar crust. Meteoritics and Planetary Science, vol 38, p. 645-661.

Stassen, Chris. 2005. The Age of the Earth. The TalkOrigins Archive. Click here.

Wikipedia, Teh. 2010. Age of the Earth. Click here.

Justin Bieber’s New Haircut

Many years ago, during The Roe v. Wade Fight (Part II: The Rise of the Right Wing Yahoos) a student came to me and asked for a break because she wanted to join a bus full of Brandeis students heading down to DC to protest stuff. I told her not to worry about the quiz coming up, just go and do her civic duty. That’s when she chose to casually mention that “at least these days, we know how to do it, not like in the old days when protesters against the Viet Nam War were violent.

This caused me to whup her up side the head. Verbally, of course.

I explained that in the old old days, when the population spoke up against authority, their villages were burned and people were randomly identified as trouble makers and impaled on stakes. I talked about the early union protests, and how one workers’ village was burned to the ground with the women and children hiding in the wooden houses. I mentioned the US Amy using machines guns on veterans protesting for their rights in Washington. And so on. And, eventually I pointed out that the reason she could drag her sorry ass down to DC and have a nice peaceful protest was because of her fore-bearers, not because she and her friends had “figured something out.” Previous protesters stood in front of onrushing mounted police or railed against soldiers carrying live ammo knowing a) “I might be killed” and b) “If I am killed, it will reduce the chance that the state will be able to use these techniques on future generations of snot-nosed privileged Harvard undergrads.”

But at least, she wasn’t primarily interested in the 1980s equivalent of Justin Bieber’s New Haircut. So, now that I have your attention, here are a few current news stories:

Continue reading Justin Bieber’s New Haircut

How to communicate with your teenager

First of all, it is not “your teenager” and if that is how you view the teenager, you’ve totally lost. Second, remember the ultimate truth that you knew when you were a teenager and that “your” teenager knows now: Teenagers know things that adults don’t understand. Most adults think this is something you “grow out of” but really, it is something that is ruined by getting old. So just keep that in mind.

But that isn’t really what I wanted to blog about.

I was just sent this post on “how to speak teenage” (which should really be called teenagerese) and as an anthropologist (who studies rocks) I have decided that I can do better. So here are parts of the original posts with my corrections.

In each case, there is the phrase (the thing the teenager says) and its definition followed by the response recommended by the Yahoo site which I shall call “Yahoo-ese.” My correction are in italics.
Continue reading How to communicate with your teenager

Is the yellowstone caldera safe?

Not long after Yellowstone Park was officially created, a small group of campers were killed by Nez Perce Indians on the run from US troops1. More recently, the last time I was in the area, a ranger was killed by a Grizzly Bear (so was his horse) on the edge of the park. A quick glance at my sister’s newspaper archives (Lightning Fingers Liz a.k.a. Caldera Girl has been running newspapers in the region for nearly forty years) shows a distinctive pattern of danger in the Caldera, mainly in relation to the lack of turning lanes on highways with poor visibility and other traffic related hazards.

So, no, the Yellowstone Caldera is not especially safe, what with cars, humans, and griz everywhere. Oh, and every now and then somebody falls into a geyser. But you are probably here because you are interested in a different question: Is the Yellowstone Caldera, the volcanic feature, not the natural and cultural landscape, dangerous? In other words, is one of the largest volcanoes to exist on the earth ever gonna blow? Like this?


(Photograph from UFO Digest)

Continue reading Is the yellowstone caldera safe?