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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.

LAVAMolten rock that flows above ground.

Kilauea, HI16-25 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 rateof 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 frommagmas 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 arealso common.

Found in Rumford, Betheland Popham Beachin Maine.

IGNEOUS PROCESSES AND IGNEOUS ROCKS

TEXTURE OF IGNEOUS ROCKS

Magmas or lavas that cool quickly allow little time forcrystals to grow.

Produces small crystals < 2mm.Crystals < 2mm are called APHANITIC OR FINE-

GRAINED.Occurs when igneous rocks cool at or near Earth’s

surface.

IGNEOUS PROCESSES AND IGNEOUS ROCKS

TEXTURE OF IGNEOUS ROCKS

APHANITIC TEXTUREBecause 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 TEXTUREWhen 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 TEXTURETexture 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.