Questions: Igneous Rock Texture and Cooling History

Question 1

A geologist finds a rock with large (5 mm) interlocking crystals of quartz and feldspar. Which interpretation is most consistent with this texture?

• A. Magma erupted rapidly and quenched against cold ocean water
• B. Magma cooled slowly deep within the crust over millions of years
• C. The rock originally had fine crystals that recrystallized under high pressure
• D. The magma had very low silica content, which allows fast crystallization

Answer: (B)

Large, interlocking crystals (phaneritic texture) form only when magma cools slowly, giving few nuclei time to grow large by incorporating atoms from the surrounding melt. This requires the insulation of deep crustal emplacement. Rapid quenching (option A) would produce aphanitic or glassy texture. Recrystallization under pressure produces metamorphic texture (foliation, etc.), not igneous texture. Composition (silica content) affects mineral type but not crystal size directly.

Question 2

A volcanic rock shows large plagioclase crystals (phenocrysts) embedded in a fine-grained dark matrix. What cooling history does this porphyritic texture record?

• A. The rock cooled entirely at the surface, with denser crystals settling and growing first
• B. The large crystals formed slowly at depth, then the magma ascended and the remaining melt cooled rapidly
• C. The rock was subjected to heat metamorphism that grew large crystals after initial cooling
• D. The fine-grained matrix formed first, and the large crystals grew later through hydrothermal fluid infiltration

Answer: (B)

Porphyritic texture is the signature of two-stage cooling. The phenocrysts nucleated and grew over extended time while the magma resided at depth — slow cooling allowed them to grow large. When the magma ascended (by eruption or shallower emplacement), the remaining liquid cooled quickly, producing many nuclei that never had time to grow large — the fine-grained groundmass. The size contrast between phenocrysts and groundmass directly records the magnitude of the cooling-rate change.

Question 3

A rock with glassy texture (like obsidian) contains no mineral crystals because it cooled so slowly that no nucleation occurred.

• T. True
• F. False

Answer: False

Glassy texture forms from *extremely rapid* cooling, not slow cooling. When lava is quenched (erupted into water or air), atoms in the melt freeze in place before they can organize into crystal lattices. There is no time for nucleation or growth — the result is an amorphous solid with no crystalline structure. Slow cooling produces the opposite: large, well-formed crystals (phaneritic texture). Obsidian's conchoidal fracture and lack of grain boundaries are consequences of its non-crystalline structure.

Question 4

Two rocks with identical mineral compositions is expected to have formed under the same conditions.

• T. True
• F. False

Answer: False

Composition and texture are largely independent. Granite and rhyolite have very similar mineral compositions (both are silicic), but granite is phaneritic (slow-cooled, intrusive) and rhyolite is aphanitic (fast-cooled, extrusive). Similarly, gabbro and basalt share a mafic composition but have coarse and fine textures respectively. Texture reveals cooling rate and emplacement depth; composition reveals source magma chemistry. Both must be examined to fully characterize an igneous rock.

Question 5

Explain why pegmatitic rocks can have crystals exceeding a meter in length, while normal slow-cooling magmas produce crystals only centimeters long.

Model answer: Pegmatitic texture forms from volatile-rich late-stage melts where water and other dissolved gases dramatically lower viscosity and enhance ion diffusion. Lower viscosity allows atoms to move more freely through the melt, traveling greater distances to reach growing crystal faces. Higher diffusion rates let crystals incorporate material from a wider volume of melt. The high volatile content also concentrates rare elements and suppresses competition from many nucleation sites. The combined effect is extraordinary crystal growth even when cooling rates are comparable to normal plutonic settings.

This contrasts with normal magmas where even slow cooling is viscosity-limited — atoms cannot move fast enough to build very large crystals regardless of time available. The volatile content is the key variable distinguishing pegmatite formation from normal plutonic rock formation.