Final answer:
True, igneous rocks are ideal for radiometric dating because they form in a single geological event and capture the isotopes used for dating at the time of crystallization. The difference in cooling rates between plutonic and volcanic rocks affects how their radiometric ages are interpreted. Geologists depend on the assumption that rock remains a closed system after formation for accurate age determinations.
Step-by-step explanation:
Igneous rocks are indeed ideal for radiometric dating because they crystallize from molten material in a single geological event. This means the rocks can often be dated to provide an accurate age of formation. The true cooling age of the rock is represented by the point at which the newly formed minerals lock in the isotopes used for dating.
When we consider plutonic and volcanic rocks separately, it is largely due to their different cooling histories. Plutonic rocks, which are intrusive, cool slowly beneath the Earth's surface and have a coarse-grained texture. Volcanic rocks, which are extrusive, cool quickly at the Earth's surface and tend to have a fine-grained texture. These differing cooling rates can affect the outcome of radiometric dating, as it determines when the isotopic clocks start ticking.
Furthermore, geologists use the assumption that the rock system remained closed since its formation to ensure the parent-daughter ratio in isotopes has not been altered, which would affect any radiometric age calculated. This method typically provides an accurate depiction of the rock's formation age, assuming the system truly remained closed and no isotopes have been gained or lost since the rock's crystallization.