In geology, which condition must be true for metamorphism of rocks to occur?

Introduction / Context:
Metamorphism is a key process in the rock cycle, transforming existing rocks into new metamorphic rocks under the influence of heat, pressure, and chemically active fluids. Unlike melting and solidification, metamorphism usually occurs without the rock becoming liquid. This question asks you to identify which condition is essential for metamorphism to occur, distinguishing it from other geological processes such as igneous activity or weathering.

Given Data / Assumptions:

• We are dealing with the geological concept of metamorphism, not melting or erosion.
• The rock that undergoes metamorphism is called the parent rock or protolith.
• Options mention grain size, being underwater, solid state, and rock type.
• We assume typical conditions of regional and contact metamorphism within the Earth crust.

Concept / Approach:
Metamorphism is defined as the mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions that differ from those under which the rocks originally formed. Crucially, the rock remains in the solid state; if it melts completely, the process becomes igneous, leading to magma and later igneous rocks. Metamorphism can affect igneous, sedimentary, or earlier metamorphic rocks, so the parent rock need not be igneous. Grain size may increase or decrease depending on conditions, and metamorphism can happen deep underground, not only underwater. Therefore, the essential condition is that the parent rock is solid during metamorphism.

Step-by-Step Solution:
Step 1: Recall the standard definition of metamorphism as the solid state transformation of rocks under changing temperature, pressure, and fluid conditions. Step 2: Note that if the rock melts, it becomes magma and any subsequent cooling produces igneous rock, not metamorphic rock. Step 3: Understand that metamorphism can affect a wide variety of parent rocks, including sedimentary rocks such as limestone and igneous rocks such as basalt or granite. Step 4: Recognise that grain size may grow through recrystallisation or, in some cases, may become finer; there is no single requirement that grains always become smaller. Step 5: Conclude that the one necessary condition is that the parent rock remains in solid form, making option about the solid state the correct answer.

Verification / Alternative check:
Geology textbooks describe regional metamorphism in mountain building zones, where enormous pressures and high temperatures act on rocks deep in the crust, but the rocks do not melt fully. Instead, minerals rearrange, and new minerals form in solid state. Contact metamorphism occurs when rocks are heated by nearby magma intrusions, again without complete melting of the host rock. Metamorphic textures such as foliation and banding are produced by solid state recrystallisation, not by crystallisation from a melt. These examples confirm that the solid state condition is central to metamorphism.

Why Other Options Are Wrong:
The grain size of the rock must always become smaller: In many metamorphic rocks such as schist and gneiss, grains actually grow larger through recrystallisation, so grain size need not always decrease. The entire process must take place underwater: Metamorphism commonly occurs deep underground under high pressure and temperature; it does not require being underwater at the surface. The parent rock must originally be an igneous rock: Sedimentary rocks like shale can metamorphose into slate or schist, showing that the parent rock can be sedimentary as well.

Common Pitfalls:
Students often confuse metamorphism with simple heating or melting. It is important to remember that metamorphism is a solid state process and that melting marks the boundary to igneous processes. Another common misconception is that only igneous rocks can undergo metamorphism, when in fact all rock types can be transformed given the right conditions. Keeping the definition of metamorphism in mind helps avoid such misunderstandings in exam questions.

Final Answer:
Metamorphism requires that the parent rock remain in the solid state during change, so the parent rock must be in solid form.