Igneous Rocks

Question 1

Aphanitic

Answer 1

• fine crystals (only visible with microscope)
• forms from rapid cooling close/at the surface
• may have vesicular texture

Question 2

Phaneritic

Answer 2

• course crystals
• Slow cooling magma formed deep within the Earth’s crust.
• can be identified with naked eye
• crystals roughly equal in size

Question 3

Porphyritic/porphyry

Answer 3

• large crystals surrounded by smaller crystals
• Large crystals called phenocrysts form in magma at depth.
• Magma erupts and Rapid cooling within produces fine grained crystal groundmass

Question 4

Glassy

Answer 4

• no crystals because of instantaneous cooling

Question 5

Pyroclastic

Answer 5

• Not crystals but individual rock fragments
• Rock fragments from violent volcanic eruptions combine to form an igneous rock.

Question 6

Pegmatitic

Answer 6

• very course crystals larger than 1cm diameter
  -Formed in the fluid-rich, late stages of magma crystallisation.
• can contain rare and valuable minerals

Question 7

Ultra-mafic

Answer 7

• less than %45 silica
• olivine and pyroxene minerals
• calcium rich plagioclase feldspar
• aphanitic eg komatite
• phaneritic eg peridotite

Question 8

Mafic

Answer 8

• 45-52% silica
• Pyroxene, calcium-rich plagioclase feldspar minerals
• also some Amphibole, Olivine
• aphanitic eg basalt
• Phaneritic eg gabbro

Question 9

Intermediate

Answer 9

• 52-70% silica
• Amphibole, sodium and calcium-rich plagioclase feldspar minerals
• some Pyroxene and biotite
• apharatic andesite
• phaneratic diorite

Question 10

Felsic

Answer 10

• 70+% silica
• Quartz, potassium feldspar, sodium-rich plagioclase feldspar minerals
• some Amphibole
  Muscovite, Biotite
• apharatic eg Rhyolite
• phaneratic eg Granite