Gots and Nots 3 | Additional instruction

I took a quick look through the Gots and Nots from yesterday and wanted to provide a bit of additional instruction.

Some info on numeric dating using radioactive isotopes

If you want to play around with a pre-made google sheet go here and make a copy Links to an external site.. I have set this up with some figures that will change as you change both the half-life (in yellow) and starting composition of parent isotope (currently 100%, but can think of it as atoms).

These simple calculations (see formulas in the cells) are all you need to think about taking the number of the parent radioactive isotopes in a sample to calculate an age of material.

*One important point I didn't have time to make in lecture*

Generally we chose material that in its initial chemistry only allows the parent isotope to be included in its structure. One of the best systems to think about this is probably the K/Ar system. K (potassium) is a solid and is included in many minerals (more in coming weeks) but Ar (Argon) is a gas and is generally not included in mineral structures.

Some information on the timescale

Subdivisions of geological time are generally interpreted based on some level of change in the critters (fauna) in the rock. I mentioned the two biggest mass extinctions that bracket the Paleozoic, Mesozoic, Cenozoic eras, but there are smaller mass extinctions and other distinct changes in fossils associated with the other boundaries. In Geo 110, 204, and 304 these transitions are discussed in more depth.

For this class, you might want to have a quick reference sheet to know what period and age (in years) I am referring to occasionally.

Uniformitarianism and you!

This is an important principle that underpins many of the things we talked about and will think about over the course of the semester. At its heart is the idea that the processes and rates that we see today existed in the past. For many things this is completely true (e.g. gravity, chemical reactions). For some things states changed (e.g. evolution of photosynthesis and the rise of O2).

Another aspect of this principle is the rate of change (e.g. sediment accumulating). To come back to our snow analogy from lecture Tuesday, rates of snowfall vary within a storm and across a winter. If we think about the vastness of Geological time, perhaps rates today (observed in one location) are not the same as rates in the past. To get around this, we test hypotheses about rates using multiple methods of dating events. For example you can use both isotopes of Uranium in a zircon to get an estimate of dates.

By putting together multiple different pieces of evidence we can reconstruct when uniformitarianism works and when it might break down. It is generally the starting assumption however for working through geological problems. This idea is discussed in much more depth in many other Geo classes.