Igneous Rocks. Characteristics of magma Igneous rocks form as molten rock cools and solidifies Characteristics of magma (molten rock) Parent material.
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Characteristics of magma
Igneous rocks form as molten rock cools and solidifies
Characteristics of magma (molten rock)• Parent material of igneous rocks• Forms from partial melting of rocks inside
the Earth• Magma that reaches the surface is lava
through a volcano or crack (fissure)
Characteristics of magma
Molten rock (magma) originates in the mantle andcrust as a result of partial melting of solid rock
Characteristics of magma
General Characteristic of magma• Rocks formed from lava at the surface are
classified as extrusive, or volcanic rocks• Rocks formed from magma that
crystallizes at depth are termed intrusive, or plutonic rocks
Intrusive Igneous Rock
Most magma never reaches the surface. Rather it crystallizes at depth to produce intrusive or plutonic igneous rocks
Intrusive Igneous Rock
Although intrusive igneous rocks form at depth, upliftingand erosion often expose them at the surface
Characteristics of magma
The nature of magma• Consists of three components:
– A liquid portion, called melt, that is composed of mobile ions
– Solids, if any, are silicate minerals that have already crystallized from the melt
– Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
Characteristics of magma
Crystallization of magma• Cooling of magma results in the
systematic arrangement of ions into orderly patterns
• The silicate minerals resulting from crystallization form in a predictable order
Crystallization of minerals in magma bodies
Bowen’s reaction series and the composition of igneous rocks
• N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points
• A listing of the most common silicate minerals in igneous rocks
• Different minerals crystallize from magmas at different temperatures
Bowen’s reaction series
Bowen’s reaction series also lists the minerals in the order in which they melt with increasing temperatures
Minerals higher on the reaction series crystallize before minerals lower on the reaction series
Bowen’s reaction series has two branches. They are:Discontinuous reaction series, from olivine to biotite
Continuous reaction series, from Ca plagioclase to Na plagioclase
Bowen’s reaction series
The discontinuous reaction series involves the dark-colored ferromagnesian minerals:
OlivinePyroxeneAmphiboleBiotite
Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure
Bowen’s reaction series
The continuous reaction series involves the plagioclase feldsparsPlagioclase feldspars consists of two end members:
Albite or Na plagioclase, the sodium end memberAnorthite or Ca plagioclase, the calcium end member
The series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure
Bowen’s reaction series
Bowen’s reaction series help us to understand why certain minerals tend to occur together in igneous rocks
For example, basalt and gabbro tend to contain olivine, pyroxene, and calcium-rich plagioclase feldspar
These are all minerals which crystallize at high temperatures
Bowen’s reaction series
Bowen’s reaction series help us to understand why certain minerals do Not occur together in igneous rocks
For example, olivine and quartz are unlikely to occur in the same igneous rock, because olivine is a high temperature, and quartz is a low temperature mineral
Zoned feldspar (plagioclase) showing change in composition with time in magma chamber
(calcic in core to sodic at rim)
Processes responsible for changing a magma’s composition ( تغير عن المسؤولة العمليات
الماجما ( تركيب
Crystal Settling ( البلورات (ترسيبAssimilation ( في خارجية مواد ادخال
النارية (الحشوةMagma Mixing ( الماجما (اختالط
Magmatic Differentiation( الصخور من انواع عدة (تكوين
The formation of several different rock types from one initial magma, through separation of earlier-formed crystals, causing the magma to evolve to become more silica-rich
Magmatic DifferentiationCrystal Settling: As a magma cools, the early-formed crystals may settle to the bottom of the magma chamber. This would produce a rock type at the bottom of the magma chamber that is dominated by early-formed minerals such as olivine, pyroxene, and calcic plagioclase
Crystallized minerals have a density greater than the magma and settle to the bottom due to gravity
Because Fe and Mg are first removed, melt becomes rich in SiO2, Na, and K
Magmatic DifferentiationAssimilation: Magma reacts with the “country rock” ( مضيف which is adjacent to the magma (صخرchamber ( الصهارة (غرفة
Magma composition is altered according to the composition of the assimilated country rockInclusions are rocksincompletely melted chunks of country rock
Magmatic DifferentiationMagma Mixing: Magmas of different compositions are mixed together
Resulting magma is of a composition intermediate between the parents
Characteristics of magma ( خصائص(الماجما
Crystallization of magma ( الماجما (تبلور• Texture in igneous rocks is determined by the
size and arrangement of mineral grains• Igneous rocks are typically classified by both
( على يعتمد النارية الصخور :(تصنيف– Texture ( النسيج) – Mineral composition ( المعدني (التركيب
Igneous texturesالنارية ) الصخور (انسجة
Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals ( متداخلة (معادنFactors affecting crystal size ( التي العوامل
البلورات حجم على (تؤثر• Rate of cooling ( التبريد (معدل
– Slow rate promotes the growth of fewer but larger crystals (thousands to millions of years)
Igneous texturesالنارية ) الصخور (انسجة
Factors affecting crystal size• Rate of cooling ( التبريد (معدل
– Fast rate forms many small crystals (hours, days, weeks)
– Very fast rate forms glass (seconds, minutes)
• Amount of silica (SiO2) present ( نسبة(السليكا
• Amount of dissolved gases ( الغازات (كمية
Types of Igneous textures( النارية الصخور انسجة (انواع
Types of igneous textures• Aphanitic (fine-grained) texture ( دقيقة نارية صخور(النسيج
– Rapid rate of cooling of lava or magma
– Microscopic crystals
– May contain vesicles ( الغازات تسرب عن الناتجة holes) (الفتحاتfrom gas bubbles in the lava or magma)
– Very porous; may resemble a sponge; low density; may float on water
• Phaneritic (coarse-grained) texture ( خشنة نارية صخور(النسيج
– Slow cooling
– Crystals can be identified without a microscope
Igneous textures
Types of igneous textures• Porphyritic texture ( بورفيرية نارية صخور(النسيج
– Minerals form at different temperatures as well as differing cooling rates
– Large crystals, called phenocrysts ( كبيرة ,(حبيباتare embedded in a matrix of smaller crystals, called the groundmass ( الصخور من ناعمة حبيبات(النارية
• Glassy texture ( النسيج زجاجية نارية (صخور– Very rapid cooling of molten rock– Resulting rock is called obsidian
Types of Igneous textures
Types of igneous textures• Pyroclastic texture ( نارية فتاتيه نارية صخور(النسيج
– Various fragments ejected during a violent volcanic eruption– Textures often appear to more similar to sedimentary rocks
• Pegmatitic texture ( متباينة نارية صخور(النسيج
– Exceptionally coarse grained (many over 2 cm)– Form in late stages of crystallization of granitic magmas– Granite pegmatite or pegmatitic granite
Igneous Compositions ( التركيب(المدني
Igneous rocks are composed primarily of silicate minerals that include:
• Dark (or ferromagnesian) silicates
( اللون داكنة سليكاتية (معادن– Olivine
– Pyroxene
– Amphibole
– Biotite mica
Igneous Compositions ( المدني (التركيب
Igneous rocks are composed primarily of light silicate minerals that include:
( اللون ناصعة سليكاتية (معادن– Quartz
– Muscovite mica
– Feldspars
Igneous compositions ( المدني (التركيب
Granitic versus basaltic compositions• Granitic composition
– Composed of light-colored silicates
– Designated as being felsic (feldspar and silica) in composition
– Contains high amounts of silica (SiO2)
– Major constituents of continental crust
Igneous compositions
Naming igneous rocks – granitic (felsic) rocks
• Granite– Phaneritic– Over 65 percent quartz, about 25 percent or
more feldspar– Abundant and often associated with
mountain building– The term granite covers a wide range of
mineral compositions
Igneous compositions
Naming igneous rocks – granitic (felsic) rocks
• Rhyolite– May contain glass fragments and vesicles
– Aphanitic texture
– Less common and less voluminous than granite
– Phenocrysts can include quartz and feldspar
Igneous compositions
Granitic versus basaltic compositions• Basaltic composition
– Composed of dark silicates and calcium-rich feldspar
– Designated as being mafic (magnesium and ferrum, for iron) in composition
– More dense than granitic rocks
– Comprise the ocean floor as well as many volcanic islands
Igneous compositions
Naming igneous rocks – basaltic (mafic) rocks
• Basalt– Volcanic origin
– Aphanitic texture
– Composed mainly of pyroxene and calcium-rich plagioclase feldspar
– Most common extrusive igneous rock
Basaltic rocks are rich in dark minerals and contain about 50% silica. Basalt makes up the ocean floor as well as lava flows and volcanic structures.
Basalt
Igneous compositions
Naming igneous rocks – basaltic (mafic) rocks
• Gabbro– Intrusive equivalent of basalt
– Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
– Makes up a significant percentage of the oceanic crust
Igneous compositions
Other compositional groups• Intermediate (or andesitic) composition
– Contain at least 25 percent dark silicate minerals
– Associated with explosive volcanic activity
• Ultramafic composition– Rare composition that is high in magnesium
and iron– Composed entirely of ferromagnesian
silicates
Igneous compositions
Naming igneous rocks – intermediate rocks
• Andesite– Volcanic origin
– Aphanitic texture
– Often resembles rhyolite
Igneous compositions
Naming igneous rocks – intermediate rocks
• Diorite– Plutonic equivalent of andesite
– Coarse grained
– Intrusive
– Composed mainly of intermediate feldspar and amphibole
Igneous compositions
Silica content influences a magma’s behavior
• Granitic magma– High silica content
– Extremely viscous
– Liquid exists at temperatures as low as 700oC
– Goes Boom when it erupts
Igneous compositions
Silica content influences a magma’s behavior
• Basaltic magma– Much lower silica content
– Fluid-like behavior
– Crystallizes at higher temperatures
Igneous compositions
Naming igneous rocks – granitic (felsic) rocks. Extrusive products can include:
• Obsidian– Dark colored– Glassy texture
• Pumice– Volcanic– Glassy texture– Frothy appearance with numerous voids
(extrusive foam)
Igneous compositions
Naming igneous rocks – pyroclastic rocks• Composed of fragments ejected during a
volcanic eruption• Varieties
– Tuff – ash-sized fragments (less than 2 mm)
– Volcanic breccia – particles larger than ash (greater than 2 mm)
FORMS OF INTRUSIVE IGNEOUS ROCKS( النارية الصخور (اشكال
• Commonly observed forms of Plutonic (Subsurface igneous bodies) rocks observed in the field are: dykes, sills, laccoliths, batholiths and Stock.
• Based on the attitudes of the associated country rocks the forms are called either as Concordant or Discordant.
• Concordant: Oriented parallel to surrounding layered rocks. Formed from magma that is injected between the layers of the surrounding rock.
• Discordant:Oriented at an angle to surrounding layered rocks. Magmas cut through the existing layering
Dykes
• They are discordant• Cut across the bedding of the rocks in which
they intrude• Vertical to steeply inclined and sheetlike body• Thickness vary widely from less than 1 cm to 1
km • Injected through fractures, and joints
SILLS
Sills are relatively thin tabular sheetlike body that penetrates parallel to the bedding planes.
Laterally it may extends for 100s of km and up to 10 km in width.
Mostly sills are made up of gabbros, dolerites and basalts
LACCOLITHS: It is a concordant body, with flat bottom and convex upward. It is dome shaped.
When viscous magma is injected rapidly along the bedding, as it cannot spreads it pushes up the overlying layers and keep on piling up.
It causes folding of the overlying rock layers.
Their occurrence is commonly associated with the mountain-building process
More than 100 km2
These are either granites or granodiorites in composition
BATHOLITHS: are the largest kind of plutons, irregular in shape and occupies large area.
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