Saturday, September 19, 2015

Age of the Earth in a Nutshell

Q: In geology class my professor told me that the earth's age is based of off meters how does this work
Thanks for your help
- Theo


The age of the Earth was initially estimated by scientists by mapping stacks of sedimentary rocks in the UK, then measuring sedimentation rates in similar environments (lakes, rivers, seashore, etc.). In the 19th Century this initially gave startling - even shocking at the time - estimates in the hundreds of millions of years range. In the early 20th Century radioisotopes became available, and these were used to extend the age of the Earth into the billions (billion = thousand million) of years age range*. This physically meant measuring back to the point in time when the mineral hosting the radioisotopes and their daughter-products was last melted. THEN it just became a game of searching all over the Earth for the oldest date-able minerals with uranium and lead in them (for example, a zircon crystal). The oldest rocks found so far are in Greenland and western Australia, and based on these the Earth's age is estimated to be at least 4.55 thousand million years old. This means it is at LEAST that old. 

* Note that in some countries like the US, the word "billion" means a thousand million, while in other countries (e.g., the UK), the word "billion" means a million million.

Monday, September 14, 2015


Paleomagnetism = old + magnetism
It's the study of magnetic signatures in ancient rocks, and what they can tell us about the Earth long ago. This requires drilling out little core samples from rock outcrops, carefully marking them for orientation, then back in a laboratory measuring the orientation of the remnant magnetic field that comes from just the rock itself.

Q: Could you tell me what factors they looked at to determine that the north pole moved.

My book says it left markers but i couldn't find what markers it was referring to.
- Theo

A: There are magnetic minerals in most non-sedimentary rocks including, most commonly, magnetite. Each individual iron-containing mineral crystal grain has a magnetic moment - sort of like a tiny bar magnet - frozen in the orientation direction of the Earth's magnetic field at the time that the mineral solidified out of its original melt. This is called remnant magnetization. In an unweathered igneous rock, these tiny magnetic domains (if there are magnetic minerals in the rock) will all align in the same direction as the Earth's field. A fluxgate magnetometer can be used in the field to orient the sensor until it aligns with the rock's internal magnetic field. Geologists can use these to tell the difference between apparently similar volcanic flows in the field, just by the different remnant magnetic fields in the different flows.

In a sedimentary rock these magnetic domains will usually be randomly oriented, because the mineral grains have been jumble up as they were wind blown or washed down a stream. 

The geology sub-field of paleomagnetism is the study of how these orientations can depict the Earth's magnetic field at times in the past - but of course this means that you must also be able to date the rocks. After more than a half century of gathering data, paleomagnetic specialists can say definitively that there have been magnetic epochs when the Earth's magnetic poles were oriented just like it is now - and epochs when the magnetic field orientation has been reversed. Of intense interest in the geoscience world right now is refining the dating part, in order to see how *fast* the Earth's magnetic field orientation changes or flips over when it does. Does the orientation of the magnetic field flip overnight, or does it take 100,000 years? The difference is important.

During that switch-over time, our planet doesn't have the magnetic protection from energetic charged particles from the Sun like it normally does (these charged particles being deflected by the Earth's magnetic field are what creates the auroras). The implication here is that there will be a lot more radiation damage to creatures living on the Earth's surface during a magnetic pole switch-over transition time. The next implication: perhaps this equates with (a) a jump in biological diversity, and/or (b) a dying off of some species.