Earth Sciences: Igneous Rocks (Rock Lab Exam 1)

Rock

Any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet

Intrusive

Plutonic, when magma cools slowly beneath the surface of the earth, you can see the mineral crystals

Crystalline Rocks

Rocks that are phaneritic and aphantic

Extrusive

Volcanic, when lava cools on the surface of the earth, you can't see the mineral crystals

Phaneritic Texture

visible crystals develop as magma cools slowly under the surface of the earth, coarse grained

Aphantic Texture

microscopic crystals develop as lava cools quickly on the surface of the earth, fine grained

Porphytic Texture

large crystals (phenocrysts) in fine matrix (groundmass), formed from slow cooling followed by fast cooling

Vesicular Texture

Aphantic, gas bubbles (vesicles) created when gas was escaping as lava cooled

Felsic

Feldspar + Silica, 65% silicate, non ferromagnesian silicate, light color, pink/orange, formed in a convergent boundary

Intermediate/Andesitic

53-65% silicate, roughly equal amounts felsic and mafic, light grey

Mafic

42 - 52% silicate, ferromagnesian silicate and calcium feldspar, dark grey,/black, likely to be divergent or hot spot, found in the ocean and on volcanic islands

Ultramafic

45%+ silicate, olivine

How to identify aphantic and porphyritic rocks

Using colors

How to identify phaneritic rocks

Using Minerals

Volcanic glasses

Form from very fast cooling of gas and silica rich lava, it freezes while the lava is airborne or in contact with water/ice

Rocks are pumice and Obsidian

Pyroclastic

Formed from fragments of tephra ejected from a volcano during explosive eruptions

Quartz

clear, grayish color

conchoidal fracture

hardness >5.5

Orthoclase (potassium feldspar)

pink,

sometimes right-angled cleavage planes are visible

Albite (Na-rich plagioclase feldspar)

opque white/light grey, cream yellow

sometimes right-angled cleavage planes with twinning striations are visible

Labradorite (Na-Ca plagioclase feldspar)

dark grey to black

cleavage planeswith obvious twinning striations

blue-green shimmer

Biotite/Muscovite Mica

Flaky, use finger nail to flake some off, if you cannot it's not muscovite, it shimmers

Horneblende (amphibole)

black, elongated shape with horizontal streaks

Augite (pyroxene)

black, stubby shape