Questions: Igneous Rock Formation and Magma Differentiation

Question 1

Two igneous rock samples are collected: one has crystals several millimeters across visible to the naked eye; the other is dark and fine-grained with no visible crystals. What is the most likely explanation for their textural difference?

• A. They have different chemical compositions — the coarse-grained rock is silica-rich, the fine-grained rock is iron-rich
• B. One crystallized slowly deep underground; the other cooled rapidly at or near the surface
• C. The coarse-grained rock formed under high pressure, which forces crystals to grow larger
• D. The fine-grained rock underwent metamorphism after solidifying, grinding down its original crystals

Answer: (B)

Crystal size in igneous rocks is controlled by cooling rate, not composition. Slow cooling underground (plutonic setting) gives atoms time to migrate and attach to growing crystal faces, producing coarse-grained rock like granite or gabbro. Rapid cooling at the surface (volcanic setting) freezes atoms in place before crystals can grow, producing fine-grained rock like basalt or rhyolite. Critically, gabbro and basalt are chemically identical — the same magma composition produces both, depending only on where and how fast it solidifies. Option A states the common misconception of conflating texture with composition.

Question 2

Which process most directly explains how a single basaltic parent magma can eventually produce silica-rich granitic rocks?

• A. Assimilation: the basaltic magma melts surrounding silica-rich crust and incorporates it
• B. Metamorphism: high pressure and temperature recrystallize basalt into granite over time
• C. Fractional crystallization: early-forming iron- and magnesium-rich minerals settle out, leaving a silica-enriched residual melt
• D. Volatile exsolution: water escaping from the magma carries iron and magnesium away, concentrating silica

Answer: (C)

Fractional crystallization is the key process. As basaltic magma cools, high-temperature minerals like olivine (iron- and magnesium-rich) crystallize first. If these dense crystals settle to the magma chamber floor and are physically removed from the remaining liquid, the residual melt becomes depleted in iron and magnesium but enriched in silica, aluminum, sodium, and potassium — the ingredients of felsic minerals. Continued crystallization and removal progressively shifts the melt from basaltic to intermediate to granitic composition. Assimilation (option A) can contribute but is not the primary differentiation mechanism in most systems.

Question 3

Gabbro and basalt can have the same chemical composition even though they look completely different, because their appearance reflects cooling history rather than chemistry.

• T. True
• F. False

Answer: True

This is one of the most important insights in igneous petrology. Gabbro (coarse-grained, intrusive) and basalt (fine-grained, extrusive) occupy the same position on the chemical composition spectrum — both are mafic (iron- and magnesium-rich) with similar silica content. Their dramatically different appearances reflect only the rate at which the same magma cooled. Slow cooling underground produced centimeter-scale crystals in gabbro; rapid surface cooling produced the microcrystalline or glassy texture of basalt. The same logic connects granite (coarse) with rhyolite (fine) at the felsic end of the spectrum.

Question 4

A magma that erupts at the surface generally has a different chemical composition than a magma that solidifies underground, because the eruption process changes the chemistry.

• T. True
• F. False

Answer: False

Eruption does not change the fundamental chemical composition of magma — it only changes how fast the magma cools. The same parent melt can produce a fine-grained volcanic rock (basalt, andesite, rhyolite) if erupted, or a coarse-grained plutonic rock (gabbro, diorite, granite) if it solidifies slowly at depth. This is why gabbro-basalt and granite-rhyolite are 'twin pairs': same chemistry, contrasting texture. Chemical diversity in igneous rocks is produced by differentiation processes (fractional crystallization, magma mixing, assimilation) — not by the act of eruption.

Question 5

Why does the settling and removal of early-crystallizing minerals like olivine and pyroxene cause the remaining magma to become progressively more silica-rich over time?

Model answer: Olivine and pyroxene are mafic minerals rich in iron, magnesium, and relatively low in silica. When they crystallize from a basaltic melt and settle to the magma chamber floor (crystal settling), they remove iron and magnesium from the liquid. The elements left behind — silica, aluminum, sodium, potassium — are the building blocks of felsic minerals like feldspar and quartz. Each cycle of crystallization and removal depletes the melt in mafic components and concentrates the felsic components, gradually shifting the melt composition from basaltic toward granitic. This is fractional crystallization driving magmatic differentiation.

The key insight is that fractional crystallization is a compositional distillation: each mineral that forms and is removed takes specific elements out of the system, forcing the residual melt to evolve. The sequence of mineral crystallization (described by Bowen's Reaction Series) predicts which elements are removed at each stage and therefore how the melt composition will evolve — a quantitative prediction that can be tested against the geochemistry of real igneous suites.