Transcript Metamorphic Rocks

Metamorphic Rocks
(‫)الصخور المتحولة‬
Metamorphism (‫ )التحول‬involves the transformation of preexisting (igneous rocks, sedimentary rocks, and
metamorphic rocks)
Metamorphism (‫)التحول‬
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The transformation of rock by temperature and
pressure
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Metamorphic rocks are produced by transformation of:
 Igneous, sedimentary and metamorphic rocks
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Occurs at depths greater than that of lithification
(Diagenesis).
Metamorphism (‫)التحول‬
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Metamorphism progresses from low to high grades
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Rocks remain solid during metamorphism
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Mineralogical, chemical, and physical changes
that occur in solid rocks (without melting).
Temperature, Pressure and Rock Type
Agents of Metamorphism (‫)عوامل التحول‬
 Temperature
(heat) (‫)الحرارة‬
 Pressure (‫)الضغط‬
 Chemically active fluids (‫)السوائل النشطة كيميائيا‬
 Time (‫)الوقت‬
Agents of Metamorphism
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Temperature (Heat) (‫)الحرارة كعامل للتحول‬
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Most important agent
Temperature increases with depth at a rate known as the
geothermal gradient (‫)التدرج الحراري‬.
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In the upper crust, this increase in temperature averages about 30 oC
per km
Rock can be heated by burial or by nearby igneous intrusion.
Burial is inevitably accompanied by an increase in pressure due to
the weight of the overlying rocks.
An intrusion may be shallow, resulting in low pressure, or deep,
resulting in high pressure.
Heat drives recrystallization - creates new, stable minerals
Temperature (heat)
Rocks may also be subjected to high temperatures
if they are carried deep within Earth
Approximate temperatures in the
crust and mantle
Temperature (heat)
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Below about 200 oC, most minerals are stable (little
or no metamorphism)
Low-grade metamorphism is the result of
metamorphic processes that occur at temperatures
from about 200oC to 500oC, and at relatively low
pressures.
High-grade metamorphism is the result of
metamorphic processes at high temperatures
(above 500oC), and at high pressure.
Above 600oC, some minerals begin to melt (transition
to igneous rocks)
Temperature (heat)
Temperature, Pressure and Rock Type
Agents of Metamorphism
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Pressure (stress) (‫)الضغط كعامل للتحول‬
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Increases with depth
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Stress (‫ )القص او الضغط‬is applied pressure that results in deformation in a
solid, and the development of new textures
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Pressure can be applied equally in all directions or differentially
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The confining pressure (Uniform stress) is applied equally in all the
directions and causes a reduction in volume, thereby generating a more
compact (dense) rock (‫)الضغط في جميع االتجاهات متساوي‬
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Differential stress: pressure is greater in one direction than in another
(‫)الضغط في جميع االتجاهات غير متساوي‬
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Texture (‫ )النسيج‬is controlled by differential versus uniform stress.
Temperature, Pressure and Rock Type
Pressure
Pressure, like temperature, also increases with depth
Pressure
Pressure
Chemical activity and Metamorphism
(‫)النشاط الكيميائي و التحول‬
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Chemically active fluids (water containing ions in solution)
enhance the metamorphism process (‫)السوائل النشطة كيميائيا‬
Some water is contained in the pore spaces (‫ )الفراغات البينية‬of
sedimentary rocks
In addition, many minerals contain water within their
crystalline structures (‫)البنية الهيكلية للمعادن‬
When deep burial occurs, water is forced out of the mineral
structures and is then available to aid in chemical reactions.
Water that surrounds the crystals acts as a catalyst (‫)عامل مساعد‬
by aid ion migration (‫)انتقال االيونات‬
In some cases, the minerals recrystallize to form more stable
configurations
In other cases, ion exchange (‫ )تبادل االيونات‬among minerals
results in the formation of completely new minerals
Role of Time in Metamorphism
(‫)الوقت و التحول‬
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In general, sizes of the mineral grains increases
with time (‫)حجم الحبيبات يزداد مع الوقت‬
Coarse-grained rocks (‫ )الصخور ذات الحبيبات الخشنة‬are
the products of long sustained metamorphic
conditions (possibly over millions of years) at high
temperatures and pressures.
Fine-grained rocks (‫ )الصخور ذات الحبيبات الناعمة‬are
products of lower temperatures, lower pressures
or, in some cases, short reaction times.
The Upper And Lower Limits Of Metamorphism
(‫)الحدود العليا و السفلى للتحول‬
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At the lower end, metamorphism occurs in
sedimentary and igneous rocks that are subjected to
temperatures greater than about 200oC, usually under
pressures of hundreds of atmospheres, caused by the
weight of a few thousand meters of overlying rock
( ‫تبدأ عملية التحول عندما تتعرض الصخور الى درجات حرارة اعلى من‬
‫ درجة مئوية‬200)
At the upper end, metamorphism ceases to occur at
temperatures that melt rock ( ‫تنتهي عملية التحول عند ذوبان‬
‫)الصخور‬
Types of metamorphic settings
(‫)تواجد الصخور المتحولة‬
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Contact metamorphism (‫ – )تحول التماس‬from a rise in
temperature within host rock
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Hydrothermal metamorphism – chemical alterations
from hot, ion-rich water (‫)التحول بواسطة الماء للساخن‬
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Regional metamorphism (‫ )التحول الشامل‬-- Occurs in the
cores of mountain belts and makes great volumes of
metamorphic rock
Types of Metamorphism
Contact metamorphism (‫)تحول التماس‬
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Occurs in rocks around a magma body
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High temperature (heat from magma) “bakes” the rock around the
magma creating an alternation
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High temperature & fluid primary agents, little to no pressure (occurs
at shallow depths in the crust)
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Metamorphism is restricted to a zone of alteration (‫)نطاق التحول‬
surrounding the intrusion, called a metamorphic aureole (‫)االذينة‬
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Aureoles reach more than 100 m in thickness
The metamorphic rocks produced from contact metamorphism are
often a fine-grained rocks that show no foliation, called a hornfels
Contact Metamorphism
Contact metamorphism
Produced mostly by local heat source
Types of Metamorphism
Regional metamorphism ( ‫)التحول الشامل‬
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Large scale – large volume of rock is affected
Associated with convergent plate margins and mountain
building (‫)تصاحب عمليات بناء الجبال‬
Folding and faulting increase thickness of the crust
Occurs over a range of temperatures and pressures
Fluids are also present
Low grade to high grade metamorphism ( ‫التحول الخفيف الى التحول‬
‫)الشديد‬
Will have zones of contact metamorphism
Regional Metamorphism
Textural and Mineralogical Changes
(‫)التغيرات النسيجية و المعدنية‬
The degree of metamorphism is reflected in the rock’s texture
and/or mineral composition
Textural and Mineralogical Changes
When rocks are subjected to low-grade metamorphism (‫)التحول الخفيف‬, they
become more compact (‫ )متراصة‬and thus more dense (‫)اكثر كثافة‬
Textural and Mineralogical Changes
Under more extreme pressures and temperatures, some minerals recrystallize to
form larger crystals
Textural and Mineralogical Changes
During metamorphism the crystals of some minerals, such as micas which have a sheet
and hornblends which has an elongated structure, will recrystallize with a preferred
orientation