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IGNEOUS PROCESSES AND IGNEOUS ROCKS
IGNEOUS ROCKS
Cooled and crystallized from molten rock.
Can occur at surface of the Earth or at depth.
MAGMA (Hot, liquid magma)
Molten rock within the Earth.
May be completely liquid.
Commonly contains liquid, solid
crystals and dissolved gases.
LAVA
Molten rock that flows above ground.
Kilauea, HI
16-25 February, 2003
IGNEOUS PROCESSES AND IGNEOUS ROCKS
MELTING ROCKS AND CRYSTALLIZING MAGMA
When rocks melt, bonds between ions are broken.
Crystalline solid no longer exists.
Hot liquid contains still solid fragments.
Minerals melt at different temperatures.
Melting causes changes in magma composition.
Cooling of hot liquid allows crystallization of minerals.
Bonds are re-established.
Crystalline structure grows until edges touch
adjacent crystals.
Minerals crystallize at different temperatures.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
IGNEOUS ROCKS
Classification of Igneous Rocks uses
TEXTURE and COMPOSITION.
TEXTURE OF IGNEOUS ROCKS
TEXTURE is the appearance of the size, shape and
arrangement of the mineral grains in the rock.
TEXTURE is directly influenced by the cooling rate
of the magma or lava.
Slow cooling yields large crystals.
Rapid cooling yields small or no crystals.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
TEXTURE is directly related to cooling rate.
Slow cooling produces large crystals.
Slow cooling take 100’s to 1000’s of years.
Large crystals are > 2mm.
Called PHANERITIC or COARSE-GRAINED.
Occurs when igneous rocks
cool beneath Earth’s
surface.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
PHANERITIC TEXTURE
Because the Earth is a poor conductor of heat, rocks
with phaneritic texture cool very slowly.
Slow cooling takes place within the Earth.
Rocks with phaneritic texture are called
INTRUSIVE or PLUTONIC rocks.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
Some igneous rocks form at low temperatures from
magmas that have high water content.
Very large crystals are produced in this process.
These rocks are called PEGMATITES.
PEGMATITES usually
contain quartz,
feldspar and micas.
Beryl and tourmaline are
also common.
Found in Rumford, Bethel
and Popham Beach
in Maine.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
Magmas or lavas that cool quickly allow little time for
crystals to grow.
Produces small crystals < 2mm.
Crystals < 2mm are called APHANITIC OR FINEGRAINED.
Occurs when igneous rocks cool at or near Earth’s
surface.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
APHANITIC TEXTURE
Because air and water are good conductors of heat,
rocks with aphanitic texture cool very quickly.
Rapid cooling takes place at or near the Earth’s
surface.
Rocks with aphanitic texture are called
EXTRUSIVE or VOLCANIC rocks.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
GLASSY TEXTURE
When lava cools extremely fast, i.e., instantaneously,
crystals have no time to form.
Usually occurs when lava enters atmosphere or water.
Ions are frozen in place randomly.
Texture is termed GLASSY.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
TEXTURE OF IGNEOUS ROCKS
PORPHYRITIC TEXTURE
Texture reflects two stage cooling history of the rock.
Minerals begin to form slowly beneath Earth’s surface,
resulting in large crystals (plutonic stage).
Then eruption occurs allowing remaining magma or
lava to cool quickly resulting in smaller crystals
(volcanic stage).
PHENOCRYST
GROUNDMASS
IGNEOUS PROCESSES AND IGNEOUS ROCKS
IGNEOUS COMPOSITIONS
Magmas typically contain O, Si, Al, Fe, Mg, Ca, Na,
K and S.
Influences composition of igneous rocks.
TEMPERATURE, PRESSURE AND WATER
CONTENT also determine magma composition.
IGNEOUS PROCESSES AND IGNEOUS ROCKS
IGNEOUS COMPOSITIONS
Gases are a major constituent in lava and magmas.
Include :
Water vapor (H2O)
Carbon dioxide (CO2)
Sulfur dioxide (SO2)
Silicate minerals are a major constituent.
Classification of igneous rocks also based
on SILICA CONTENT of the rock.