GEMS Test #3 2025 Questions and
Answers
mafic (basic) minerals - ANSWER✔✔-low SiO2 content, <50% by weight, rich in MgFe, dark
rocks, ex: olivine, hornblende, augite
felsic (acidic) minerals - ANSWER✔✔-contain feldspar and quartz minerals, >70% by weight, light
rocks, ex: orthoplase, clagioplase
intermediate minerals - ANSWER✔✔-contains biotite or hornblende minerals, 55-67% by weight,
grey colored, ex: feldspar, quartz, muscovite
intrusive (plutonic) - ANSWER✔✔-rocks that were crystallized or cooled slowly below the surface -
- large crystals, easily distinguished PHANERITIC TEXTURE
extrusive (volcanic) - ANSWER✔✔-rocks that were crystallized/cooled rapidly on the surface of the
earth -- small crystals that aren't easily distinguishable, APHANETIC TEXURE
porphyritic - ANSWER✔✔-a texture containing a mix of large and small crystals typically found in
extrusive rocks
gabbro, diorite, granite - ANSWER✔✔-Mafic intrusive, intermediate intrusive, felsic intrusive --
chemically similar but textually different
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, basalt, andisite, rhyolite - ANSWER✔✔-mafic extrusive, intermediate extrusive, felsic extrusive --
chemically similar but different textures
high SiO2 content - ANSWER✔✔-makes magma more viscous (thicker)
volatiles in magma - ANSWER✔✔-chemicals in magma in gas form include H2O, CO2 and H2S
higher viscosity - ANSWER✔✔-causes eruptions to be more explosive
pillow lavas - ANSWER✔✔-contain a chill margarine meaning they cool slower on the inside -- they
indicate there was water at the time of the eruption
composite/stratovolcanoes - ANSWER✔✔-large steep sided volcanos, explosive, contain high silica
content
pyroclastic flows - ANSWER✔✔-rapidly moving clouds of ash mixed with hot, suffocating gases
nuee ardente - ANSWER✔✔-Blast of gas associated with high viscosity
landslides - ANSWER✔✔-This happened at mount st Helen -- a massive earthquake cause the top
of the mountain to slide off which then caused it to erupt
lahaars - ANSWER✔✔-Volcanic mud flow
ash - ANSWER✔✔-Small rock particles that are shot into the atmosphere and rain down
general global ramifications - ANSWER✔✔-Ash blocks out the sun and can change the climate
FOR STUDY PURPOSES ONLY COPYRIGHT © 2025 ALL RIGHTS RESERVED 2
Answers
mafic (basic) minerals - ANSWER✔✔-low SiO2 content, <50% by weight, rich in MgFe, dark
rocks, ex: olivine, hornblende, augite
felsic (acidic) minerals - ANSWER✔✔-contain feldspar and quartz minerals, >70% by weight, light
rocks, ex: orthoplase, clagioplase
intermediate minerals - ANSWER✔✔-contains biotite or hornblende minerals, 55-67% by weight,
grey colored, ex: feldspar, quartz, muscovite
intrusive (plutonic) - ANSWER✔✔-rocks that were crystallized or cooled slowly below the surface -
- large crystals, easily distinguished PHANERITIC TEXTURE
extrusive (volcanic) - ANSWER✔✔-rocks that were crystallized/cooled rapidly on the surface of the
earth -- small crystals that aren't easily distinguishable, APHANETIC TEXURE
porphyritic - ANSWER✔✔-a texture containing a mix of large and small crystals typically found in
extrusive rocks
gabbro, diorite, granite - ANSWER✔✔-Mafic intrusive, intermediate intrusive, felsic intrusive --
chemically similar but textually different
FOR STUDY PURPOSES ONLY COPYRIGHT © 2025 ALL RIGHTS RESERVED 1
, basalt, andisite, rhyolite - ANSWER✔✔-mafic extrusive, intermediate extrusive, felsic extrusive --
chemically similar but different textures
high SiO2 content - ANSWER✔✔-makes magma more viscous (thicker)
volatiles in magma - ANSWER✔✔-chemicals in magma in gas form include H2O, CO2 and H2S
higher viscosity - ANSWER✔✔-causes eruptions to be more explosive
pillow lavas - ANSWER✔✔-contain a chill margarine meaning they cool slower on the inside -- they
indicate there was water at the time of the eruption
composite/stratovolcanoes - ANSWER✔✔-large steep sided volcanos, explosive, contain high silica
content
pyroclastic flows - ANSWER✔✔-rapidly moving clouds of ash mixed with hot, suffocating gases
nuee ardente - ANSWER✔✔-Blast of gas associated with high viscosity
landslides - ANSWER✔✔-This happened at mount st Helen -- a massive earthquake cause the top
of the mountain to slide off which then caused it to erupt
lahaars - ANSWER✔✔-Volcanic mud flow
ash - ANSWER✔✔-Small rock particles that are shot into the atmosphere and rain down
general global ramifications - ANSWER✔✔-Ash blocks out the sun and can change the climate
FOR STUDY PURPOSES ONLY COPYRIGHT © 2025 ALL RIGHTS RESERVED 2
