Which is needed to turn an igneous rock or sedimentary rock into a metamorphic rock?

A. Erosion and deposition
B. Heat and pressure
C. Melting and cooling
D. Freezing and cementation

B. Heat and pressure

To turn an igneous rock or sedimentary rock into a metamorphic rock, the correct answer is B. Heat and pressure.

Explanation:
Metamorphic rocks are formed when existing rocks go through changes in conditions that alter their mineral composition, texture, and structure. This transformation is primarily driven by heat and pressure. Here's how it happens:

1. Heat: Increase in temperature helps in the recrystallization process, where the minerals within the rock rearrange and form new crystal shapes. Heat can be generated by the Earth's internal heat, tectonic activity, or the proximity to a magma chamber.

2. Pressure: An increase in pressure occurs due to the weight of overlying rocks or tectonic forces squeezing the rock. Pressure can lead to the formation of preferred orientation of minerals, resulting in a foliated texture (e.g., visible layers or banding) commonly found in metamorphic rocks. High pressure can also increase the stability of certain minerals, causing new minerals to form.

Therefore, the combination of heat and pressure is essential for the formation of metamorphic rocks from pre-existing rocks like igneous or sedimentary rocks.