chapter 6 rocks 6.2 igneous rocks
Post on 08-Jan-2018
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Igneous Rock FormationIgneous Rock Formation• Form from the cooling
of molten rock – Extrusive (volcanic)-
form at or near the surface from lava
– Intrusive (plutonic) – form under the surface from magma
Igneous Rock FormationIgneous Rock Formation• Igneous rocks are
classified by mineral composition (mafic or felsic) and texture
• Texture – is the size, shape
and arrangement of grains (crystals)
TextureTexture
• Grain or crystal size depends on:– Rate of cooling
• Fast = small• Slow = large
– Dissolved gasses• More = larger
– Order of solidification
The Starting MaterialThe Starting Material• Magma is Classified as
Felsic, Mafic or and intermediate
• Felsic – Light colored– High silica magma with
abundance of quartz and feldspar.
– Thick slow flowing in molten state.
– Most intrusive rocks. – Acidic
The Starting MaterialThe Starting Material• Mafic
– Dark colored, – Low silica
ferromagnesian magma
– More fluid than felsic magma.
– Most extrusive igneous rocks.
Underground MagmaUnderground Magma• Magma can cool slow
or fast depending on where cooling occurs– Intrusive rocks cool
very slowly underground
– Result of slow cooling• Coarse (large) grains
At the SurfaceAt the Surface• Magma pouring onto
the surface is called lava– Extrusive igneous
rocks cool rapidly– Microscopic grains
or none at all
At the SurfaceAt the Surface• Porphyry
– Rocks that cool both underground and at the surface
– Results in large and small grains
Igneous familiesIgneous families
• Grouped according to mineral composition
• Texture is determined where it cools – intrusive or extrusive– Each family can have coarse grained, fine
grained or glassy textures– Also can have vesicles
• Vesicles are gas cavities within the rock
Igneous FamiliesIgneous Families• Granite family
– Felsic magma– Light colored– Quartz and Feldspar
• Coarse– Granite
• Fine– Rhyolite
• Glassy– Pumice and
Obsidian
• Gabbro Family– Mafic magma– Dark colored– Pyroxene and Olivine
• Coarse– Gabbro
• Fine– Basalt
• Vesicular– Scoria
IGNEOUS INTRUSIONSIGNEOUS INTRUSIONS• PLUTONS – A ROCK MASS THAT FORMS
WHEN MAGMA COOLS UNDER GROUND– DIKES– SILLS– LACOLITHS– VOLCANIC NECKS– BATHOLITHS – LARGEST OF ALL
PLUTONS, FORM THE CORES OF MOST MOUNTAINS
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