In geology, which of the following must be true for metamorphism of a rock to occur?

Introduction / Context:
Metamorphism is the process by which existing rocks are transformed into new types of rocks through heat, pressure and chemically active fluids without the rock completely melting. This question examines your understanding of the basic conditions that must be satisfied for metamorphic changes to occur in a rock.

Given Data / Assumptions:

• A parent rock (protolith) undergoes changes in texture and mineral composition.
• The temperature and pressure conditions change compared to the original environment.
• The rock largely remains solid and does not melt into magma.
• The question asks which statements are required conditions for metamorphism.

Concept / Approach:
By definition, metamorphic rocks form when pre-existing rocks are subjected to elevated temperatures and pressures, usually deep in the Earth's crust, but not high enough to cause complete melting. The rock must remain essentially solid, so that new minerals and textures develop in the solid state. High pressure, often combined with high temperature, drives recrystallisation and alignment of minerals. Although some metamorphism can occur at relatively shallow depths (for example, near intruding magma), most significant metamorphism happens underground rather than at the surface. Therefore, all three conditions listed are typically satisfied for classical metamorphism, and the best answer is "All of the above".

Step-by-Step Solution:
Step 1: Check condition A: For metamorphism, the parent rock must not melt completely; it changes while remaining largely solid. This is true and central to the definition. Step 2: Check condition B: Metamorphism is driven by high pressures and temperatures, often described as "tons of pressure" acting over long geological times, so this statement is essentially correct. Step 3: Check condition C: Most metamorphic reactions occur below the Earth's surface, where pressure and temperature are sufficient for the minerals to rearrange; this is also true. Step 4: Since each individual statement describes a real requirement or typical condition of metamorphism, the combined option "All of the above" is correct.

Verification / Alternative check:
Standard geology texts define metamorphism as changes in solid rocks in response to temperature, pressure and fluids, with the proviso that the rock does not cross the melting boundary. Metamorphic rocks such as gneiss, schist and marble are known to form deep in the crust, where pressure is high due to the weight of overlying layers. The presence of foliations and banded textures confirms deformation under pressure, again supporting the conditions in the options. Surface processes like weathering and erosion are not considered metamorphic.

Why Other Options Are Wrong:
The parent rock must remain in solid form: True but incomplete by itself, since pressure and depth also matter.

High pressure must be applied over long periods: True for most metamorphism, but temperature and solid state are also important.
The process must take place below the Earth's surface: Generally true, but metamorphism also requires solid state and pressure; this alone is not sufficient.

Common Pitfalls:
Learners sometimes think that any high temperature change produces metamorphic rock, even if melting occurs. If a rock melts and later solidifies, the product is igneous, not metamorphic. Others may overlook the importance of pressure, assuming heat alone is enough. Remember that metamorphism is a solid-state transformation driven by heat, pressure and fluids, mostly in subsurface conditions, so all three conditions in the question work together.

Final Answer:
For metamorphism to occur, All of the above conditions are typically true.