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
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
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
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
