Igneous rocks
(ignite = set on fire)
(rocks are mixtures of minerals)
How to form an igneous rock:
(what to do to a mixture of minerals)
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•
•
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Heat the minerals until they
melt into magma, then
cool so the minerals will again
crystallize
Where does heat to melt
rock come from?
• Compression from the original formation of
the Earth
• Radioactive elements inside the Earth
• Compression from plate collisions
• Friction from plate collisions
Layer Details:
Core - density = 10.8 g/cm3; 16% of Earth’s
volume; 31.5% of Earth’s mass; mostly iron
and nickel
A. Inner core - solid - 2,600 km diameter
B. Outer core - liquid - 2,200 km thick
Mantle - half as dense as core; 82% of
Earth’s volume; 68% of Earth’s mass
(Layers continued):
• Asthenosphere - upper part of
mantle; 200 km thick; 65-100 km
below surface; plastic
• Lithosphere - 65-100 km thick; rigid
rock
Differentiation
(= becoming different)
Imaginary Field Trip to
Foundry !
• Felsic (feldspar & silica) = light color,
low density, low melting temperature.
These float to top of magma chamber.
• Mafic ( magnesium & iron (Fe)) = dark,
dense, high melting temperature.
These sink to bottom of magma
chamber.
So, because of differentiation,
different igneous rocks have different
compositions! (They are made of
different minerals.)
• 1.) Light-colored, low density, low
melting temperature minerals,
• 2.) Medium minerals, or
• 3.) Dark, dense, high melting
temperature minerals.
Texture
• For Rocks - texture is judged by sight, not
touch!
• Texture refers to crystal size.
• Magma that cools slowly underground
forms big crystals and course texture.
• Lava that cools quickly at the surface
forms crystals too small to see without a
lens and has fine texture.
Intrusive v.s. Extrusive
• Intrusive igneous rocks cooled slowly
underground and have big, visible
crystals (coarse texture).
• Extrusive igneous rocks cooled
quickly and have small crystals that
you would need a lens to see (fine
texture).
Simplified Version:
Composition
Texture
Rock Names
Extrusive Exceptions:
Porphyry
• If magma starts cooling slowly
underground, but erupts to the surface
before completely solidifying and finishes
cooling quickly at the surface, the resulting
rock will have large crystals surrounded by
smaller crystals. This texture is a
porphyry. Andesite is often a porphyry.
Glassy Texture
• If lava cools instantly, crystals do not
have time to form and volcanic glass
results. This rock is named obsidian.
• Native Americans often made
arrowheads out of obsidian.
Vesicles = Bubbles
• If the lava had lots of dissolved
gasses, it will bubble while cooling
quickly, and the resulting rock will
have a vesicular texture.