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Rocks Notes
Igneous Rocks
-All start as igneous rocks
-Intrusive igneous rocks – rocks created when magma cools
underground (plutons)
-Extrusive igneous rocks – rocks created when magma cools on the
surface (volcanoes, etc.)
Igneous rock identification
Composition: Color
-Igneous rocks are classified based upon chemical composition,
however most of the time color is a good indication of rock
type and can be used for identification.
-Color used to be used more widely for ID in what was called
the Color Index
Felsic –Light in color (White, light gray, tan or pink)
-Rick in Silica (SiO2)
Intermdiate – Intermdeiate color
-Equal amount of light and dark minerals
Mafic – Dark in color (Black to brown)
-Poor in silica (SiO2) but rich in Fe and Mg
Exceptions
-Obsidian is felsic in composition but is dark black or
brown in color
-Dunite is felsic in composition but light green or
greenish yellow in color
Texture
-Texture in igneous rocks does not refer to the roughness or
smoothness, but the crystal size
-Pegmatic – large crystals >2-3 cm
-Phaneritic – crystals are large enough to see but smaller than
2 cm, entire rock is composed of crystals
-Aphanitic – fine grained, crystals are too small to see
-Porphorytic – made of crystals of 2 different sizes
-Large crystals are called phenocrysts and smaller
crystals are not seen and are called groundmass
-Glassy – made entirely of glass, few if any crystals can be seen
-Vesicular – lava solidifies before gas can escape leaving holes
and bubbles in the rock
-Pyroclastic – made of volcanic fragments (ash, lapilli or
bombs)
Sedimentary Rocks
-Rocks that are composed of sediment
-Sediment is deposited by wind or water
-Classification is based on texture and make-up
-Texture represents the origin or make-up of sedimentary rocks
-Texture types
1) Clastic sedimentary rocks – contains clasts (small pieces of rocks
or minerals)
-Gravel-sized particles (>2mm) – conglomerate
-Sand-sized particles (1/16 mm – 2 mm) – sandstone
-Clay and silt-sized particles (<1/16 mm) – siltstone, shale
2) Chemical sedimentary rocks
-Sedimentary rocks made of a particular mineral
-4 important types
1) Quartz – white to clear, hardness = 7 (Hard)
2) Gypsum – clear, hardness = 2 (soft, scratched by
fingernail)
3) Halite – clear, common table salt, hardness = 2.5
4) Calcite – hardness of 3 (reacts with HCl)
3) Biological sedimentary rocks
-Formed of biological sediment or affected by biological
activity
-Ex.
-Coal – made of decomposed plant material
-Coquina – made of shell fragments (calcite)
-Fossil limestone – limestone containing fossils
Metamorphic Rocks
-Rocks that have been changed by heat and or pressure through a process
called metamorphism
-Parent Rock – original rock before metamorphism
-Types of Metamorphism
1) Regional metamorphism – when high temperature and pressure
affect large regions of Earth’s crust.
2) Contact metamorphism – when molten rocks come into contact
with a solid rock
3) Hydrothermal metamorphism – when very hot water reacts with
rock to changes its chemistry and mineral structure
-Common near igneous intrusions and volcanoes
-Texture
-Foliated – rocks with wavy layers or bands of minerals
-Pressure during metamorphism causes minerals with flat or
needle like crystals to arrange with their long axis
perpendicular to the pressure pg. 136
-Nonfoliated – rocks without layers or bands of minerals