Download Classification of Igneous Rock

Classification of Igneous
Rock
Classification
• Classification of igneous
rocks is based on
mineral composition,
crystal size, and texture.
• Review Vocabulary
• fractional crystallization
– a sequential process
during which early
formed crystals are
removed from the melt
and do not react with
the remaining magma
Mineral Composition
Intrusive Rock
• Magma cools and
crystallizes below Earth’s
surface (plutonic)
Extrusive Rock
• Magma that cools and
crystallizes on Earth’s
surface
Mineral Composition
• Rock type can be determined by estimating
relative percentages of minerals in the rocks
Texture
• Texture refers to the size, shape, and
distribution of the crystals or grains that make
up a rock
• Lava cools quickly
– Small crystal formation
– Volcanic glass
• Lava cools slowly below Earths surface
– Forms large crystals
Aphanitic Texture
• Fine-grained texture
– Rapid rate of cooling of lava
or magma
– Microscopic crystals
– May contain vesicles
(holes from gas bubbles)
Phaneritic Texture
• Coarse-grained texture
– Slow cooling
– Crystal can be identified
without a microscope
Porphyritic Texture
• Large, well formed
crystals
• Surrounded by finergrained crystals or the
same minerals or
different minerals
Glassy Texture
• Glassy
– Very rapid cooling
– Resulting rock is called
obsidian
Vesicular Rocks
• Spongy appearance that
results from gas
bubbles trapped in lave
Igneous Composition
Dark Silicates
• Olivine
• Biotite mica
Light Silicates
• Quartz
• Muscovite mica
• Flespars
Mineral Composition
Basaltic Rocks
• Dark-colored
• Lower silca content
• Contain mostly plagioclase
and proxene
• Comprise the ocean floor as
well as many volcanic
islands
• More dense then granitic
rocks
Granitic Rocks
• Light-colored
– Light silicates
• High silca content
• Designated as felsic
– Feldspar and silica
• Major constituents of
continental crust
Mineral Composition
• Rocks that have a
composition of minerals
that is somewhere
between basaltic and
granitic are called
intermediate rocks.