Rock types Igneous Sedimentary Metamorphic

Differences in the rock textures Igneous – isometric

Differences in the rock textures Sedimentary rocks have layers, bedding, strata

Differences in the rock textures Metamorphic rocks – are banded, and foliated

metamorphic rocks Re crystallization of sedimentary or igneous rocks or even other metamorphic rocks In solid state (not liquid)

Metamorphism Due to: –High pressure –High temperature –Liquid in pores

Metamorphic temperatures Lower boundary – ca 150 Celcius Upper boundary – ca 1200 Celcius – Above this temperature the rock mass will melt and form igneous rocks

Result of metamorphism new minerals grow minerals deform and rotate re crystallization of course minerals rocks become either: –stronger but more brittle –or –weaker and anisotropic

?What is the difference between metamorphism, diagenesis and chemical weathering? metamorphism – entails high temperature and high pressure diagenesis – entails a bonding and hardening of a sediment relatively near the earths surface weathering – entails the disruption and alteration of a solid rock at or near the earth surface

high pressure

Thin section of mica schist

Types of metamorphic rocks Regionala metamorphism - high pressure Contact metamorphism – high temperature due to igneous intrusion

Regionala metamorphism

Contact metamorf

Contact metamorphism

Textur FoliatedFoliated – preferred direction of minerals or banding Non foliatedNon foliated – no preferred direction

metamorphic textures mosaic texture mosaic texture – interlocking equigranular grains (fig 8.1) porphoblstic texture porphoblstic texture – larger minerals in a fine matrix (fig 8.2) folitation folitation – anisotropy, strong directional structure which effects the properties of the rock so they are different in different directions

Foliation types fracture cleavagefracture cleavage – strong rock between fractures, evenly spaced, low grade metamorphism slaty cleavageslaty cleavage – parallel to mineral plates, often mica bedding cleavage – when slaty cleavage coincides with bedding schistosityschistosity – is foliation developed by non-random orientation of macroscopic minerals –lepidoblastic schistosity –lepidoblastic schistosity – flat mineral plates parallel to on another –nemotablastic schistosity –nemotablastic schistosity – elongate minerals in one plane but not parallel to one another in the plane

Foliation types fracture cleavagefracture cleavage – strong rock between fractures, evenly spaced, low grade metamorphism slaty cleavageslaty cleavage – parallel to mineral plates, often mica bedding cleavage – when slaty cleavage coincides with bedding schistosityschistosity – is foliation developed by non-random orientation of macroscopic minerals –lepidoblastic schistosity –lepidoblastic schistosity – flat mineral plates parallel to on another –nemotablastic schistosity –nemotablastic schistosity – elongate minerals in one plane but not parallel to one another in the plane

Foliation types fracture cleavagefracture cleavage – strong rock between fractures, evenly spaced, low grade metamorphism slaty cleavageslaty cleavage – parallel to mineral plates, often mica bedding cleavage – when slaty cleavage coincides with bedding schistosityschistosity – is foliation developed by non-random orientation of macroscopic minerals –lepidoblastic schistosity –lepidoblastic schistosity – flat mineral plates parallel to on another –nemotablastic schistosity –nemotablastic schistosity – elongate minerals in one plane but not parallel to one another in the plane

Preferred directions

banding and other structures bandsbands – segregation of different minerals in distinct bands augenaugen – german for eye – refers to the growth porphyroblasts, large crystals, in the otherwise finer matrix around which the banding is bent so an eye shape is formed boudinageboudinage –French for sausage – is a term for a structure where one layer, which is more brittle than the surrounding layers, is broken into segments around which the other more plastic rock fill in the voids. This forms a series of boudin. (fig 8.4) lineationslineations – a linear structure, deferrers from foliation in that it is a sub texture of the main metamorphic texture. All linear structure are oriented parallel

banding and other structures bandsbands – segregation of different minerals in distinct bands augenaugen – german for eye – refers to the growth porphyroblasts, large crystals, in the otherwise finer matrix around which the banding is bent so an eye shape is formed boudinageboudinage –French for sausage – is a term for a structure where one layer, which is more brittle than the surrounding layers, is broken into segments around which the other more plastic rock fill in the voids. This forms a series of boudin. (fig 8.4) lineationslineations – a linear structure, deferrers from foliation in that it is a sub texture of the main metamorphic texture. All linear structure are oriented parallel

banding and other structures bandsbands – segregation of different minerals in distinct bands augenaugen – german for eye – refers to the growth porphyroblasts, large crystals, in the otherwise finer matrix around which the banding is bent so an eye shape is formed boudinageboudinage –French for sausage – is a term for a structure where one layer, which is more brittle than the surrounding layers, is broken into segments around which the other more plastic rock fill in the voids. This forms a series of boudin. (fig 8.4) lineationslineations – a linear structure, deferrers from foliation in that it is a sub texture of the main metamorphic texture. All linear structure are oriented parallel

banding and other structures bandsbands – segregation of different minerals in distinct bands augenaugen – german for eye – refers to the growth porphyroblasts, large crystals, in the otherwise finer matrix around which the banding is bent so an eye shape is formed boudinageboudinage –French for sausage – is a term for a structure where one layer, which is more brittle than the surrounding layers, is broken into segments around which the other more plastic rock fill in the voids. This forms a series of boudin. (fig 8.4) lineationslineations – a linear structure, deferrers from foliation in that it is a sub texture of the main metamorphic texture. All linear structure are oriented parallel

Most common metamorphic rock types

Metamorphic grade Pressure and temperature together results in changes such as the growth of minerals and textures Indicator minerals – show which temperature and pressure the rock has undergone

Indication minerals

Mineral associations - facies

Weathering of metamorphic rocks Wide range of weathering products –gneiss, granulites and other quartz rich rocks – sandy residual soils smilar to granitic soils –slate and phyllite – miaceous, silty residual soils –marble – red clay –foliation – bladed outcrops tombstone – rock head separated by completely decomposed material –banded – banded saprolite with clay between bands –weathering depth – dependent upon spacing of fractures, typical 6 to 24 m depth

joints sets of 4 or more are common joints coated with clay or silt intersections isolate potentially removable blocks (fig 8.12)

foliation shears persistent shear zones parallel to the plane of foliation, cm to m in size tensile and shear strengths are considerably less than in all other directions, strength anisotropy characterized by finely fractured or crushed rock two end members –impervious –impervious - plastic clay seam common, alteration to chlorite or kaolinite replacing feldspars and biotite –pervious –pervious - shear zone of crushed rock

Engineering and metamorphic rocks

Exploration irregular extent recognition of rock types foliation directions important weathering zones detection geophysical methods drilling (difficult in quartzite)

slope stability Landslides common due to extremely weak shear strengths of platy minerals valley form asymetrical –gentle - dip slip slopes – slab slide –steep – slopes with creep, toppling, landslides

surface excavation blasting often required landslide risk – block theory slab slides toppling

foundations fresh rock – usually good weathered rock –compressible sandy silty soil –pile difficult saprolite – protect from drying – loose of fabric slope failures – common along the plain of foliation

Case histories Several in the book Read – do you understand the descriptions of the rock and their physical nature?