2. What Rocks Tell UsWhat Rocks Tell Us
Rock Type How Classified What it Tells Us
Igneous
Composition Tectonic Setting
Texture Cooling History
Sedimentary
Chemical
Composition
Surface
Environment
Grain Size
Energy of
Environment
Metamorphic
Composition Original Rock Type
Mineral Makeup
Temperature,
Pressure
Texture Degree of Change
3. General characteristicsGeneral characteristics
of magmaof magma
• Igneous rocks form as molten rockIgneous rocks form as molten rock
cools and solidifiescools and solidifies
• General characteristics ofGeneral characteristics of magma:magma:
• Parent material of igneous rocksParent material of igneous rocks
• Forms from partial melting of rocksForms from partial melting of rocks
• Magma at surface is calledMagma at surface is called lavalava
4. General characteristicsGeneral characteristics
of magmaof magma
• The nature of magmaThe nature of magma
• Consists of three components:Consists of three components:
– Liquid portionLiquid portion == meltmelt
– SolidsSolids, if any, are silicate minerals, if any, are silicate minerals
– VolatilesVolatiles = dissolved gases in the melt,= dissolved gases in the melt,
including water vapor (Hincluding water vapor (H22O), carbon dioxideO), carbon dioxide
(CO(CO22), and sulfur dioxide (SO), and sulfur dioxide (SO22))
5. General characteristicsGeneral characteristics
of magmaof magma
• Crystallization of magmaCrystallization of magma
• Cooling of magma results in theCooling of magma results in the
systematic arrangement of ions intosystematic arrangement of ions into
orderly patternsorderly patterns
• The silicate minerals resulting fromThe silicate minerals resulting from
crystallization form in a predictablecrystallization form in a predictable
orderorder
• Texture - size and arrangement ofTexture - size and arrangement of
mineral grainsmineral grains
6. Igneous texturesIgneous textures
• TextureTexture is used to describe theis used to describe the
overall appearance of a rock basedoverall appearance of a rock based
on the size, shape, and arrangementon the size, shape, and arrangement
of interlocking mineralsof interlocking minerals
• Factors affecting crystal sizeFactors affecting crystal size
• Rate of coolingRate of cooling
– Slow rate = fewer but larger crystalsSlow rate = fewer but larger crystals
– Fast rate = many small crystalsFast rate = many small crystals
– Very fast rate forms glassVery fast rate forms glass
7. Igneous texturesIgneous textures
• Types of igneous texturesTypes of igneous textures
• AphaniticAphanitic (fine-grained) texture(fine-grained) texture
– Microscopic crystalsMicroscopic crystals
– May containMay contain vesiclesvesicles (holes from gas(holes from gas
bubbles)bubbles)
• PhaneriticPhaneritic (coarse-grained) texture(coarse-grained) texture
– Slow coolingSlow cooling
– Large, visible crystalsLarge, visible crystals
10. Igneous texturesIgneous textures
• Types of igneous texturesTypes of igneous textures
• PorphyriticPorphyritic texturetexture
– Minerals form at different temperaturesMinerals form at different temperatures
– Large crystals (Large crystals (phenocrystsphenocrysts) are embedded) are embedded
in a matrix of smaller crystals (in a matrix of smaller crystals (groundmassgroundmass))
• GlassyGlassy texturetexture
– Very rapid cooling of lavaVery rapid cooling of lava
– Resulting rock is calledResulting rock is called obsidianobsidian
13. Igneous compositionsIgneous compositions
• Silica contentSilica content as an indicator ofas an indicator of
compositioncomposition
• Exhibits a considerable range in theExhibits a considerable range in the
crustcrust
– 45% to 70%45% to 70%
• Silica contentSilica content influences magmainfluences magma
behaviorbehavior
• Granitic magmas = high silica contentGranitic magmas = high silica content
and viscousand viscous
• Basaltic magmas = much lower silicaBasaltic magmas = much lower silica
content and more fluid-like behaviorcontent and more fluid-like behavior
14. Igneous compositionsIgneous compositions
• Naming igneous rocks – granitic rocksNaming igneous rocks – granitic rocks
• GraniteGranite
– PhaneriticPhaneritic
– Over 25% quartz.Over 25% quartz.
– Very abundant - often associated withVery abundant - often associated with
mountain buildingmountain building
– The term granite includes a wide range ofThe term granite includes a wide range of
mineral compositionsmineral compositions
22. Igneous compositionsIgneous compositions
• Naming igneous rocks – mafic rocksNaming igneous rocks – mafic rocks
• GabbroGabbro
– Intrusive equivalent of basaltIntrusive equivalent of basalt
– Much of the Earth's oceanic crust is made ofMuch of the Earth's oceanic crust is made of
gabbrogabbro
– Phaneritic texture consisting of pyroxenePhaneritic texture consisting of pyroxene
and calcium-rich plagioclaseand calcium-rich plagioclase
– Significant % of the oceanic crustSignificant % of the oceanic crust
