As far as layer formation is concerned, how much does the settling of particles have to do with determining that layer's age?
The only thing that settling may be able to tell you is some very rough information about how long it may have taken that particular layer to have formed. The best example is using a very fine grained sedimentary rock. Superfine sediment does not tend to stick around on land so it almost always ends up in a large body of water as wind and water runoff carry it down. As long as the water is agitated though, the very fine particles of sediment will not settle very quickly at all. So if you have a very thick layer of very find sediment you can guess that it took quite a while for all those tini grains to settle out but you can't be totally sure. It may also just be that there was a whole lot of them. Overall settling of sediments just isn't a good lead when talking about identifying the age of rocks.
learn something new everyday--for whatever reason, I had thought
granite was sedimentary. makes much mor sense for it to be igneous.
considering how durable it is.
Not all igneous rocks are tougher than sedimentary rocks though. The previously mentioned pumice and obsidian are pretty fragile. This is an aside though.
Now to the questions.
1) Is a single layer uniform (composed of the same material)?
When you get down into the really fine details of a sedimentary rock then answer is no. The Grand Canyon is a good example where some layers are described as a "Shaley Limestone" or "Sandy Limestone". Within each layer there may be finer gradients of differing materials indicating a slight change in how things "settled" although overall it is still one layer.
As for "extrusive rocks" (I assume you mean extrusive igneous rocks) being mixed into a layer; if you all of a sudden see igneous rock showing up then you know a volcanoe has erupted somewhere nearby. It should interrupt the layer rather than be mixed into it. If the circumstances of "settling" don't change much due to the eruption then you may find that things pick up with a new layer of the same material that was deposted before the volcanic material was laid down.
2)I think I get what you mean about it being difficult to have intrusive igeous being a part of a layer. Then, can the batholiths and plutons (I think I know what they look like--quick check though--sort of like the magma chamber in a volcano?) be used to help dating.
A magma chamber is an intrusive igneous rock but plutons and batholiths are WAAAY bigger than that. Were are talking whole mountains and mountain RANGES.
Intrusive rock can help with dating because of the principle of cross cutting relationships. We know that the intrusion is younger than the "stuff" it has intruded into. If the surrounding rock wasn't there, it would not be an intrusive rock. So since we can date the intrusion absolutly, we know that the rock it intruded into is older than the date given by the intrusion.
So lets say a magma body intrudes into some sandstone. The magma body cools into granite and we come in later and absolute date it to 300 million years old. We don't know exactly how old the sandstone layer is, but we know it is older than 300 million years. The intrusion gives us an upper bound to the age of the sandstone.
3) How are extrusive (igneous) rocks used to help date? Is it just by being used as a reference point, like, you know A is before J, but not how long before? Or, is it possible to also use it for absolute dating--where you know exactly how long A is before J?
It is more than just the relative relationship. Obviously if the igneous rock is layed on top of another rock it is older. The big main point to take hope is that we have a method of assigning an absolute date to the formation of igneous rocks. We can give an igneous rock a birthday.
From there we can use that date to say the rocks above it are younger than THAT PARTICULAR AGE and the rocks below it are older than THAT PARTICULAR AGE.
So in a simplistic example:
----> Igneous Rock absolute dated at 10 MA
====> Sedimentary rock
~~~~> Igneous Rock absolute dated at 20 MA
Even though we can't date the sedimentary rock directly, we know it is between 10 and 20 MA.
The reality is of course that there are few circumstances on earth that are that easy to sandwhich between two things that we can date absolutly. Even if we just make it 1 order more difficult.
----> Igneous at 10MA
====> Sedimentary #1
~~~~> Sedimentary #2
++++> Igneous at 20MA
We can say that both Sedimentary #1 and #2 are between 10 and 20 MA but just knowing the ages of the igneous rocks does not tell us what time sedimentary #1 represents compared to sedimentary #2. That is when we have to start looking at other factors that may give us clues as to what span of time those layers represent in between the dates given by the igneous layers.
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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