Questions: How Metamorphic Rocks Form

Question 1

What is the key difference between how metamorphic rocks form and how igneous rocks form?

• A. Metamorphic rocks form at the surface; igneous rocks form underground
• B. Metamorphic rocks form from changes caused by heat and pressure without melting; igneous rocks form from melted rock that cools
• C. Metamorphic rocks are always harder than igneous rocks
• D. There is no difference — they form the same way

Answer: (B)

The critical difference is melting. Metamorphic rocks change while still solid — minerals rearrange without the rock becoming liquid. Igneous rocks require melting first and then cooling. If a metamorphic rock gets hot enough to actually melt, the resulting rock would be igneous, not metamorphic.

Question 2

Limestone that is metamorphosed becomes marble. What changed and what stayed the same?

Model answer: The mineral composition stays essentially the same (both are mainly calcium carbonate), but the texture changes. The small, dull grains of limestone recrystallize into larger, interlocking crystals, giving marble its smooth texture and ability to be polished. The chemical ingredients are the same, but heat and pressure reorganized them.

This example shows that metamorphism is about reorganization, not new ingredients. The calcium carbonate crystals in limestone grow larger and lock together more tightly under heat and pressure, producing the distinctive look and feel of marble.

Question 3

A rock should melt in order to become a metamorphic rock.

• T. True
• F. False

Answer: False

Metamorphism specifically means change without melting. Heat and pressure cause minerals to recrystallize and rearrange while the rock stays solid. If the rock melts, the process has crossed into igneous territory — the cooled result would be an igneous rock, not metamorphic.