Final answer:
Rocks formed by contact metamorphism aren't necessarily denser than their original forms because the process involves changes in mineral composition and structure that can alter the rock's density. The formation of new minerals with different packing structures or the manner of recrystallization can affect the overall density of the metamorphic rock.
Step-by-step explanation:
Explaining why rocks formed by contact metamorphism aren't necessarily denser than the rocks they formed from involves understanding the geologic processes that transform these rocks. Metamorphism occurs when pre-existing rocks are subjected to high temperatures and pressures for long periods. Heat from magma intrusion or the geothermal gradient, and pressure from the overlying rock weight or tectonic forces, can lead to the rearrangement of minerals or the formation of new minerals within the rock.
During contact metamorphism, the mineral composition and structure of rocks are altered due to the proximity to hot magma. However, this does not automatically increase density. The original rock, or protolith, can experience a volume change through re-crystallization and mineral transformation that affects its density. The new minerals may have different crystal structures and may not pack together as tightly as in the original rock, leading to a metamorphic rock that is not necessarily denser.
Additionally, factors such as the amount in which the rock melts or the type of new minerals formed can also impact the final density of the metamorphic rock. Not all metamorphic rocks will exhibit an increase in density; in some cases, they can even be less dense than the protolith, depending on the specific conditions and mineral reactions during metamorphism.
Thus, while metamorphic rocks formed through contact metamorphism undergo significant physical and chemical changes, those changes do not always lead to an increase in density compared to the rocks they originated from.