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Questions and Answers
A phaneritic texture, indicative of coarse-grained igneous rocks, suggests a ______ cooling process, allowing for larger crystal formation.
A phaneritic texture, indicative of coarse-grained igneous rocks, suggests a ______ cooling process, allowing for larger crystal formation.
slow
Considering the table, ______ is composed of potassium feldspar and quartz, with occasional biotite or hornblende, classifying it as felsic.
Considering the table, ______ is composed of potassium feldspar and quartz, with occasional biotite or hornblende, classifying it as felsic.
granite
If an igneous rock exhibits a porphyritic texture with large plagioclase feldspar phenocrysts in a fine-grained dark matrix, and is volcanic in origin, it is most likely classified as ______.
If an igneous rock exhibits a porphyritic texture with large plagioclase feldspar phenocrysts in a fine-grained dark matrix, and is volcanic in origin, it is most likely classified as ______.
basalt
Unlike granite, which also contains quartz, ______ contains more plagioclase than potassium feldspar.
Unlike granite, which also contains quartz, ______ contains more plagioclase than potassium feldspar.
Given its formation from volcanic glass and typically dark coloration, ______ is categorized as a felsic igneous rock, despite its color.
Given its formation from volcanic glass and typically dark coloration, ______ is categorized as a felsic igneous rock, despite its color.
Flashcards
Coarse-grained (Phaneritic) Texture
Coarse-grained (Phaneritic) Texture
Igneous rocks with large, visible crystals formed from slow cooling of magma.
Granite
Granite
A felsic, coarse-grained, intrusive igneous rock mainly composed of potassium feldspar and quartz.
Gabbro
Gabbro
A mafic, coarse-grained, intrusive igneous rock composed of dark plagioclase feldspar and pyroxene.
Fine-grained (Aphanitic) Texture
Fine-grained (Aphanitic) Texture
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Rhyolite
Rhyolite
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Study Notes
- Igneous rocks are classified by their mineral composition, texture, and color index.
Coarse-Grained Texture (Phaneritic)
- Granite (C-1) contains potassium feldspar and quartz, with accessory minerals like biotite and hornblende
- It is phaneritic and felsic in composition.
- It is plutonic/intrusive as it undergoes slow cooling.
- Has a few dark minerals, indicating a felsic composition
- Granodiorite (C-21) has more plagioclase than potassium feldspar, and quartz, with accessory minerals like biotite and hornblende
- It is felsic to intermediate in composition.
- Diorite (C-2) consists of plagioclase feldspar and minor quartz, with accessory minerals like hornblende, biotite, and augite
- Its overall composition is intermediate.
- Gabbro (C-3) is made up of dark plagioclase feldspar and augite, with rare accessory minerals like olivine and quartz.
- It is mafic in composition.
- Has mostly dark minerals, indicating a mafic composition
Fine-Grained Texture (Aphanitic, or Porphyritic with Fine Groundmass)
- Rhyolite (C-5) may have some quartz phenocrysts.
- Features a light matrix, indicating a felsic composition
- Dacite (C-20) may have phenocrysts of quartz, hornblende, and potassium feldspar.
- Andesite (C-6) may have phenocrysts of plagioclase feldspar and hornblende.
- Its overall composition is intermediate, and it appears dark gray/purple/green to medium dark in color
- Basalt (C-4) may have phenocrysts of plagioclase feldspar.
- Is aphanitic, vesicular, and mafic due to fast cooling as a volcanic/extrusive rock
- It has a dark matrix, indicating a mafic composition, and appears black
Other Igneous Rocks
- Obsidian (C-11) is volcanic glass, usually black or dark red.
- It is always felsic
- Volcanic Breccia (C-12) contains angular fragments of included rock in a volcanic matrix.
- It may be of any composition.
- Pumice is highly porous volcanic "foam" with pale colors.
- It tends towards the felsic end of the compositional spectrum.
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