IGNEOUS ROCKS & MAGMA

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44 Terms

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  1. Association

Usually found together.

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  1. What is a Mineraloid?

A naturally occurring earth material that lacks one or more of the essential requirements to be classified as a mineral. Example: volcanic rock = glass = obsidian (no fixed internal atomic arrangement of atoms/ions).

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  1. Felsic rocks are characterized by…

Light color and silica-rich composition.

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  1. Mafic rocks are characterized by…

Dark color and richness in magnesium (Mg) and iron (Fe).

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  1. Ultramafic composition means…

Olivine-rich (>90 %) and very dark.

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  1. What does “Pegmatitic” texture mean?

Igneous rocks with very large crystals, often several cm or larger.

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  1. Granite – classification?

Igneous (intrusive) + Felsic + Phaneritic texture = light-colored coarse-grained rock.

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  1. Aphanitic texture means…

Fine-grained; crystals too small to see without magnification.

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  1. Basalt – classification?

Igneous (extrusive) + Mafic + Aphanitic texture.

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  1. Phaneritic texture means…

Coarse-grained texture formed by slow cooling.

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  1. Gabbro – classification?

Igneous (intrusive) + Mafic + Phaneritic texture = dark coarse-grained rock.

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  1. Rhyolite – classification?

Igneous (extrusive) + Felsic + Aphanitic texture.

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  1. Basalt (vesicular variant) – classification?

Igneous + Mafic + few vesicles (texture).

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  1. What is a Vesicle?

A hole in volcanic rock left by a gas bubble; when many merge the texture becomes frothy.

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  1. Scoria – definition?

Mafic igneous rock with frothy vesicular texture; bubbles touching each other.

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  1. Pumice – definition?

Felsic, extrusive, frothy rock light enough to float on water.

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  1. Peridotite – definition?

Igneous (intrusive) + Ultramafic + Phaneritic texture; main mantle rock.

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  1. Obsidian – texture?

Glassy igneous rock with conchoidal fracture.

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  1. Porphyritic texture means…

Two distinct grain sizes (large phenocrysts in fine groundmass); name based on groundmass (e.g., Porphyritic Basalt).

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  1. What controls magma viscosity?

Composition + temperature + dissolved gases.

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  1. Low-viscosity magma behaves how?

Flows easily like water = Mafic.

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  1. High-viscosity magma behaves how?

Resists flow like syrup = Felsic.

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  1. How do gas contents affect eruptions?

Low gas → coherent lava flows; High gas → explosive eruptions.

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  1. Summary: composition + temperature + gas → viscosity relationship.

Felsic + cool + gas-rich = thick / explosive; Mafic + hot + low gas = runny / gentle.

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  1. What does Bowen’s Reaction Series show?

Sequence in which minerals crystallize from cooling magma (high-T mafic → low-T felsic).

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  1. Which minerals form early in Bowen’s Series?

Olivine, Pyroxene, and Ca-plagioclase (mafic, high-temperature).

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  1. Which minerals form late in Bowen’s Series?

Orthoclase, Muscovite, Quartz (felsic, low-temperature).

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  1. “Higher on the arrow” in Bowen’s Series means…

Crystallizes early → likely phenocryst in later felsic magma.

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  1. What are Hydrous minerals?

Minerals containing OH⁻ ions that form late as magma cools and water concentrates.

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  1. Main magma types (Bowen’s Series)?

Ultramafic → Mafic → Intermediate → Felsic (dark → light).

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  1. Differentiation of Magma – definition?

Process where magma changes composition as minerals crystallize out at different temperatures; early mafic minerals remove Fe & Mg → remaining melt more felsic.

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  1. How does crystal separation affect magma?

Early-formed mafic crystals settle out → magma becomes more silica-rich (felsic).

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  1. P–T Phase Diagram shows…

How pressure & temperature control solid / partial-melt / liquid states of rock.

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  1. Solidus line – definition?

Temperature where melting begins; below = solid rock.

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  1. Liquidus line – definition?

Temperature where rock is completely molten (100 % liquid).

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  1. Partial-melting zone – definition?

Region between solidus & liquidus with both crystals and magma.

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  1. Geothermal Gradient – definition?

Rate of temperature increase with depth (≈ 25–30 °C/km); explains melting potential at depth.

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  1. Hot Spot – definition?

Localized region where temperature exceeds normal geothermal gradient → magma rises → shield volcano formation (e.g., Hawaii).

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  1. Peridotite (Mantle Composition)?

Rising dry peridotite undergoes decompression melting at divergent boundaries producing basaltic magma/oceanic crust.

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  1. Decompression Melting – definition?

Melting due to pressure drop faster than temperature decrease in rising mantle rock; no extra heat required.

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  1. Flux Melting – definition?

Melting caused by addition of water/volatiles lowering the melting point; typical above subduction zones.

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  1. Magma Generation below Cascade Volcanic Chain?

Water from subducting Juan de Fuca plate → flux melting of mantle → rising magma → volcanoes in WA/OR/CA.

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  1. Mid-Ocean Ridge – process?

Divergent boundary where rising mantle experiences decompression melting → mafic basaltic crust formation.

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  1. Magma Evolution Summary?

Three magma generation processes: decompression, flux, and heat transfer.