Metamorphic Rocks (الصخور المتحولة)

Metamorphism (التحول) involves the transformation of pre- existing (igneous rocks, sedimentary rocks, and metamorphic rocks)

Metamorphism ( التحول ) Metamorphism ( التحول ) The transformation of rock by temperature and pressure The transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Metamorphic rocks are produced by transformation of: Igneous, sedimentary and metamorphic rocks Igneous, sedimentary and metamorphic rocks Occurs at depths greater than that of lithification (Diagenesis). Occurs at depths greater than that of lithification (Diagenesis).

Metamorphism ( التحول ) Metamorphism progresses from low to high grades Metamorphism progresses from low to high grades Rocks remain solid during metamorphism Rocks remain solid during metamorphism Mineralogical, chemical, and physical changes that occur in solid rocks (without melting). 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) Temperature (heat) (الحرارة) Pressure (الضغط) Pressure (الضغط) Chemically active fluids (السوائل النشطة كيميائيا) Chemically active fluids (السوائل النشطة كيميائيا) Time (الوقت) Time (الوقت)

Agents of Metamorphism Temperature (Heat) ( الحرارة كعامل للتحول ) Temperature (Heat) ( الحرارة كعامل للتحول ) Most important agent Most important agent Temperature increases with depth at a rate known as the geothermal gradient ( التدرج الحراري ). Temperature increases with depth at a rate known as the geothermal gradient ( التدرج الحراري ). In the upper crust, this increase in temperature averages about 30 o C per km In the upper crust, this increase in temperature averages about 30 o C per km Rock can be heated by burial or by nearby igneous intrusion. 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. 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. An intrusion may be shallow, resulting in low pressure, or deep, resulting in high pressure. Heat drives recrystallization - creates new, stable minerals Heat drives recrystallization - creates new, stable minerals

Rocks may also be subjected to high temperatures if they are carried deep within Earth Temperature (heat)

Approximate temperatures in the crust and mantle

Temperature (heat) Below about 200 o C, most minerals are stable (little or no metamorphism) Below about 200 o C, most minerals are stable (little or no metamorphism) Low-grade metamorphism is the result of metamorphic processes that occur at temperatures from about 200 o C to 500 o C, and at relatively low pressures. Low-grade metamorphism is the result of metamorphic processes that occur at temperatures from about 200 o C to 500 o C, and at relatively low pressures. High-grade metamorphism is the result of metamorphic processes at high temperatures (above 500 o C), and at high pressure. High-grade metamorphism is the result of metamorphic processes at high temperatures (above 500 o C), and at high pressure. Above 600 o C, some minerals begin to melt (transition to igneous rocks) Above 600 o C, some minerals begin to melt (transition to igneous rocks)

Temperature (heat)

Temperature, Pressure and Rock Type

Agents of Metamorphism Pressure (stress) (الضغط كعامل للتحول) Pressure (stress) (الضغط كعامل للتحول) Increases with depth Increases with depth Stress ( القص او الضغط ) is applied pressure that results in deformation in a solid, and the development of new textures Stress ( القص او الضغط ) is applied pressure that results in deformation in a solid, and the development of new textures Pressure can be applied equally in all directions or differentially Pressure can be applied equally in all directions or differentially 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 الاتجاهات متساوي) 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 الاتجاهات متساوي) Differential stress: pressure is greater in one direction than in another (الضغط في جميع الاتجاهات غير متساوي) Differential stress: pressure is greater in one direction than in another (الضغط في جميع الاتجاهات غير متساوي) Texture ( النسيج ) is controlled by differential versus uniform stress. Texture ( النسيج ) is controlled by differential versus uniform stress.

Temperature, Pressure and Rock Type

Pressure, like temperature, also increases with depth Pressure

Pressure

Pressure Pressure

Chemical activity and Metamorphism ( النشاط الكيميائي و التحول ) Chemically active fluids (water containing ions in solution) enhance the metamorphism process ( السوائل النشطة كيميائيا ) Chemically active fluids (water containing ions in solution) enhance the metamorphism process ( السوائل النشطة كيميائيا ) Some water is contained in the pore spaces ( الفراغات البينية ) of sedimentary rocks Some water is contained in the pore spaces ( الفراغات البينية ) of sedimentary rocks In addition, many minerals contain water within their crystalline structures ( البنية الهيكلية للمعادن ) 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. 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 الايونات ) Water that surrounds the crystals acts as a catalyst ( عامل مساعد ) by aid ion migration ( انتقال الايونات ) In some cases, the minerals recrystallize to form more stable configurations In some cases, the minerals recrystallize to form more stable configurations In other cases, ion exchange ( تبادل الايونات ) among minerals results in the formation of completely new minerals In other cases, ion exchange ( تبادل الايونات ) among minerals results in the formation of completely new minerals

Role of Time in Metamorphism ( الوقت و التحول ) In general, sizes of the mineral grains increases with time ( حجم الحبيبات يزداد مع الوقت ) In general, sizes of the mineral grains increases with time ( حجم الحبيبات يزداد مع الوقت ) Coarse-grained rocks ( الصخور ذات الحبيبات الخشنة ) are the products of long sustained metamorphic conditions (possibly over millions of years) at high temperatures and pressures. Coarse-grained rocks ( الصخور ذات الحبيبات الخشنة ) are the products of long sustained metamorphic conditions (possibly over millions of years) at high temperatures and pressures. Fine-grained rocks ( الصخور ذات الحبيبات الناعمة ) are products of lower temperatures, lower pressures or, in some cases, short reaction times. Fine-grained rocks ( الصخور ذات الحبيبات الناعمة ) are 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 ) At the lower end, metamorphism occurs in sedimentary and igneous rocks that are subjected to temperatures greater than about 200 o C, 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 200 درجة مئوية ) At the lower end, metamorphism occurs in sedimentary and igneous rocks that are subjected to temperatures greater than about 200 o C, 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 200 درجة مئوية ) At the upper end, metamorphism ceases to occur at temperatures that melt rock ( تنتهي عملية التحول عند ذوبان الصخور ) 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 ) Contact metamorphism ( تحول التماس ) – from a rise in temperature within host rock Contact metamorphism ( تحول التماس ) – from a rise in temperature within host rock Hydrothermal metamorphism – chemical alterations from hot, ion-rich water ( التحول بواسطة الماء للساخن ) Hydrothermal metamorphism – chemical alterations from hot, ion-rich water ( التحول بواسطة الماء للساخن ) Regional metamorphism ( التحول الشامل ) -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock 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 ) Occurs in rocks around a magma body Occurs in rocks around a magma body High temperature (heat from magma) “bakes” the rock around the magma creating an alternation High temperature (heat from magma) “bakes” the rock around the magma creating an alternation High temperature & fluid primary agents, little to no pressure (occurs at shallow depths in the crust) High temperature & fluid primary agents, little to no pressure (occurs at shallow depths in the crust) Metamorphism is restricted to a zone of alteration ( نطاق التحول ) surrounding the intrusion, called a metamorphic aureole ( الاذينة ) Metamorphism is restricted to a zone of alteration ( نطاق التحول ) surrounding the intrusion, called a metamorphic aureole ( الاذينة ) Aureoles reach more than 100 m in thickness 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 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 Regional metamorphism ( التحول الشامل ) Large scale – large volume of rock is affected Large scale – large volume of rock is affected Associated with convergent plate margins and mountain building ( تصاحب عمليات بناء الجبال ) Associated with convergent plate margins and mountain building ( تصاحب عمليات بناء الجبال ) Folding and faulting increase thickness of the crust Folding and faulting increase thickness of the crust Occurs over a range of temperatures and pressures Occurs over a range of temperatures and pressures Fluids are also present Fluids are also present Low grade to high grade metamorphism ( التحول الخفيف الى التحول الشديد ) Low grade to high grade metamorphism ( التحول الخفيف الى التحول الشديد ) Will have zones of contact metamorphism 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 ( متراصة ) and 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