definition
formed deep within the earth by transformation of pre-existing rocks by heat, pressure, or both
- change textures/new minerals due to deformation/reorientation of mineral components, recrystallization of minerals into large grains, and chemical recombination
- come from both igneous and sedimentary
Slate
- fine grained, foliated, homogeneous
- formed from fine-grained mudstone
- highly developed cleavage planes
- metamorphic grade= low from regional metamorphism
phyllite
- from continued action of stresses upon slate
- minerals=quartz mica and chlorite
- metamorphism degree is between slate and mica schist
- larger minerals than slate
- metamorphic grade= low from regional metamorphism
regional metamorphism
A type of metamorphism in which the mineralogy and texture of rocks are changed over a wide area by deep burial and heating associated with the large-scale forces of plate tectonics
Schist
- originally clay and mudstone
- minerals are micas, chlorite, and hornblende
- flaky
- metamorphic grade=medium and also from regional metamorphism
hornfels
- from thermal metamorphism of sedimentary rocks
- contains feldspar, biotite, and quartz
- metamorphic grade= high due to thermal metamorphism
Quartzites
- originally sandstone changed by heat/pressure
- hard, resistant to chem weathering, and often forms ridges and hilltops
- doesn’t accomodate vegitation
Gneiss
- from regional metamorphism of igneous or sed rocks
- medium to coarse foliated and largely recrystallized minerals
- minerals: feldspar, quartz, micas, or hornblende, and garnet
- metamorphic grade=high
Marble
-from recrystallization of limestone in regional metamorphism or contact environments
-mostly calcite
-marble is also used to indicate stones that can reach levels of high polish
metamorphic grade=varies
foliations
- indicates the mineral showing compositional banding
- basically realignment of particles due to stress creating planar fabric
- fabric forms at right angles to stress
recrystallization
- hardening of material
- strengthens final rock structure
Causes of metamorphism
- pressure from sinking deeper, continental collisions,
- temp from sinking into warmer layers, igneous hot lava (contact/thermal metamorphism), steam from hot vents
anisotropic
having a physical property that has a different value when measured in different directions
typical meta rocks in HK
- foliated: phylite and schist
- non-foliated: marble, hornfels, and skarn
marble in HK
- from carboniferous period
- info is found from drillhole cores
- Yuen Long Formation: 600 mthick, UCS=50 MPa-140 MPa, silicified by contact metamorphism from granite intrusion
- Ma On Shan Formation: relatively pure marble, strongly foliated, min thickness of 200 m
- Tolo Harbour formation: carbonate rocks, occur as isolated blocks, up to 350 m thick, dark grey/pale grey/bluish grey/cream colored marble
Engineering consideration
- affects engineering consideration by changing constituent minerals or by foliation
- foliation results in large anisotropic strength differences, usually lowers shear strength
- marble can have chemically worn cavities like limestone
rock strength and resistance to weathering
type grade strength rtoweather slate/ low small weak phylite schist/ marble hornfels/ quartzites/ gneiss/ Marble high large high
