Rocks 6. Metamorphic Rocks Eng. Iqbal Marie Metamorphism occurs - - PowerPoint PPT Presentation

• 6. Metamorphic Rocks

Rocks

• Eng. Iqbal Marie

Engineering Geology

Engineering Geology is backbone of civil engineering

Metamorphism

• ccurs when any previously existing rock, the parent rock ( protolith ) , is

buried in the earth under layers of other rock, at high temperature and pressure to produce changes in texture and crystallization. Metamorphic rock looks quite different from the original rock. Metamorphic textures and minerals are most likely formed over 10 to 20 million years or longer

Metamorphic rocks are produced from

– Igneous rocks – Sedimentary rocks – Other metamorphic rocks

In most cases, the overall chemistry of the metamorphic rock is very similar to that of the parent rock

Metamorphic Environments

• Contact or thermal metamorphism
• Result from a rise in temperature when magma invades a

host rock. Occurs at high temperature and (typically) low

• pressure. Normally affects a small area.

There are three types of metamorphism

• Contact
• Regional
• Dynamic

• Regional metamorphism
• Affects a large area and produces the greatest quantity of

metamorphic rock. Associated with mountain building

• High temperature, high pressure and shear stress.

Fault zone metamorphic rocks are formed as a consequence

• f changes that happens

along fault lines within the Earth's crust.

• Hydrothermal metamorphism
• Chemical alteration caused when hot, ion-rich

fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock

• Heat
• Most important agent
• Two sources of heat

– Contact metamorphism – heat from magma – An increase in temperature with depth due to the geothermal gradient 15-30oC increase per km

• Pressure and differential stress
• Confining pressure applies forces equally in all directions; increases with depth
• Rocks may also be subjected to differential stress which is unequal in different

directions

• Chemically active fluids
• Mainly water with other volatile components
• Aids in re-crystallization of existing minerals
• Sources of fluids

– Pore spaces of sedimentary rocks – Fractures in igneous rocks – Hydrated minerals such as clays and micas

Factors Controlling Metamorphism

If the same protoliths experience different conditions of temperature and pressure, they will yield different metamorphic rocks Basalt Greenschist Eclogite Heat Pressure

Metamorphic Grade: intensity of Metamorphism Low Intermediate High

Low-grade (mild) metamorphism: small changes in texture and/or mineralogy of parent rock (200 to 350 OC )

High-grade (extreme) metamorphism: radical changes in texture and/or mineral composition of the rock

Metamorphism ends when melting begins Melting begins at ~700oC

350 to 550 OC

above 550 OC

• Rock texture changes
• Foliation: Parallel alignment of platy or elongate mineral

grains (mica/amphibole) in a rock caused by directed stress. – slaty cleavage: parallel alignment of microscopic platy minerals (mainly mica). LOW-GRADE METAMORPHISM – phyllitic texture: parallel, but wavy, foliation of fine-grained platy minerals (mainly mica and chlorite) exhibiting a shiny or glossy luster. LOW-GRADE METAMORPHISM – schistosity: parallel to sub-parallel foliation of medium to coarse-grained platy minerals. INTERMEDIATE TO HIGHGRADE

METAMORPHISM

– gneissic layering: discontinuous light and dark layering due to mineral segregation. INTERMEDIATE TO HIGH-GRADE

METAMORPHISM

• Mineralogy changes

– New minerals form that are stable under the new metamorphic conditions

CHANGES DURING METAMORPHISM?

• Foliation. Due to compressive stress and differential stress and/or shearing

forces the mineral grains in a metamorphic rock form parallel layers or bands. New metamorphic minerals crystallize along this foliation Foliation usually formed planes of weakness in metamorphic rock because the rock can be easily break along the foliation planes.

Kinds of foliation

Slaty cleavage Schistosity Gneissic banding

• The rocks which do not have planar patterns of strain or stretching

are termed as non-foliated metamorphic rocks. These rocks are most

• ften deduced from single mineral sedimentary rocks.
• High temperature and pressure erases out the fossils of the

metamorphic rocks.

• Metamorphic rocks do not have pores or openings, and may be

accompanied with visible layers of crystals.

Some main points about metamorphic rocks

Foliated Metamorphic Rocks (Regional

Metamorphism)

• Gneiss – highest grade of metamorphism,

coarse grained; generally banded (segregation bands of light and dark- coloured minerals);

• Schist – medium grade of metamorphism;

sand size, schistocity cleavage.

• Phyllite – low grade of metamorphism; fine

grained (silt-sized);

• Slate – lowest grade of metamorphism; very

fine-grained; mud-sized; smooth surface.

Common metamorphic rocks

Phyllite

slate shale Gneiss Schist

GNEISS

Medium- to coarse-grained Often composed of white or light- colored feldspar-rich layers with bands of dark ferromagnesian minerals

Non-Foliated Metamorphic Rocks

(Contact/Thermal Metamorphism)

• Granulite (granite / acid igneous rocks
• rigin)
• Amphibolite (basic igneous rocks origin)-

basalt ( Amphipole )

• Quarzite (sandstone origin)
• Marble (limestone origin)
• Hornfels (mudstone/shale origin)

Recrystallization – minerals grow and develop an interlocking texture

Marble :Coarse, crystalline

• Parent rock was limestone or dolostone
• Used as a decorative and monument stone
• Exhibits a variety of colors

Quartzite

Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together

• A. Building material:

– Marble which is used for ornamental building stone. – Slate which is used for roofing, flooring, billiard/pool tables, and blackboards

• B. Economic metamorphic minerals include:

– Graphite used in pencils and lubricants. – Garnet and Corundum used as gemstones and abrasives. – Asbestos formerly used as a heat insulator. – Kyanite, Andalusite, Sillimanite (aluminum silicates) are used a raw material in the ceramics industry.

• C. Ore Deposits - result from contact metamorphism where hydrothermal

solutions precipitate ore minerals in surrounding rocks: – Iron and Tin Oxides deposits (hematite, magnetite, and cassiterite) – Precious metal deposits (gold) Importance of Metamorphism valuable mineral and rock resources