I want to get back to the questions in my mind about radiometric dating methods which scientists use to date fossils. I'm not satisfied that anyone has adequately answered those questions.
Just for gits and shiggles, let me give it a try.
First, you need to understand what radiometric dating is actually measuring. What it measures is how long a mineral has been closed. That is, how long the isotopes in question have been locked in place. For example, in liquid rock (i.e. magma) gasses, like argon, escape quite easily. When the rock goes from liquid to solid this isn't the case anymore. With respect to argon, the rock becomes closed when it solidifies. Any argon produced by the decay of potassium-40 is now trapped in the rock. Therefore, by measuring the amount of potassium and argon in the rock you can tell how long ago it went from liquid to solid.
So what happens if that solid rock gets weathered and turned into a sediment further down the stream? Well, dating the chunks from that weathered rock can not tell you how old that sediment is. It can only tell you how long ago the source rock formed. This is why sediments are not generally dated. Instead, igneous rocks that solidified in their current position above and below the sediments are used to give a date range for the formation of that sedimentary layer.
Which raises more questions. What forces cause the lava to rise to the extent that it raises the plates?
The same forces that cause hot water to move from the bottom of a pot to the top. It is convection currents.
How far above the planet's mantel does the magma rise before it becomes cooled enough to harden into intrusive igneous rock?
Ever been to the Hawaiian Islands?
If it is the cooled magma rock that is tested by the radiometric dating, does it date differently than the rising magma and if so, why?
For K/Ar dating, most assuredly. Liquid magma does not hold on to argon gas. It escapes into the atmosphere. Magma can only hold onto argon produced by the decay of potassium-40 after the magma has solidified.
What you are seeing is a flood basalt that solidified on top of a lot of sediment. It is called Lizard Butte because . . . well . . . it looks like a lizard resting on a hill. Anyway, the flood basalt on top of the sediments can be dated, and that gives a minimum age for the sediments below. If you were to dig below the sediments and find another basalt or igneous rock layer this would allow you to construct an age range for the sediments between the igneous layers.