F2 key idea 3 (metamorphism) Flashcards
(24 cards)
metamorphic grade
measure of the intensity of meta morphism
contact metamorphism
effected by increasing temperature (pressure not sig.)
often produces a metamorphic aureole
minerals are not aligned
regional
affects larger areas
cause by low to high temperatures and pressures at convergent plate margins
pressure is significant to minerals have preferred alignment (foliated)
slaty cleavage
formed by low grade regional metamorphism
platy minerals recrystallise perpendicular to direction of stress so that the rock splits into thin sheets
schistosity
in medium to coarse grained crystals
formed by the preferred alignment of flat/ tubular minerals
perpendicular to direction of stress
gneissose banding
segregation of dark and light coloured minerals into layers or bands
light band is usually granoblastic (granular) while the dark band usually shows shistosity
granoblastic
contain interlocking equidimensional crystals
porphyroblastic
large crystals such as garnet grow in the metamorphic rock
may be after the groundmass has developed and may distort the groundmass
or they can form at the same time as the groundmass
stress
used to describe the forces applied to a rock by earth movements or the mass of overlying rocks
strain
used to describe the deformation caused by applying forces
change in volume or shape
incompetence - thickness with vary (plasticity)
competent - same thickness (brittle)
factors effecting stress and strain
- the higher the temperature, more plastic
- strength of the rock increases with confining pressure, higher the depth, more difficult to deform
3.if pressure is applied for short period of time, behave more brittle. over extended period, more plastic
tension
these forces usually result in the fracturing of rocks and crustal extension
displacement on either side is describes as a fault
no displacement is described as a joint
compressional
can result in either fracturing or folding
shortening of the earths crust
shear
deformation of rocks in one plane, usually horizontally, parallel to the surface, causing sliding
dipping beds
the more perpendicular the dip, the narrower the outcrop
more gentle dip means wider surface outcrop
shape of the land (topography) has a major influence on the outcrop pattern
dip-slip and strike-slip
dip-slip
normal - hanging wall downthrown
reverse/ thrust - hanging wall upthrown
strike-slip
dextral- moved to the right
sinistral- moved to the left
thrust faults
type of reverse fault where the fault dip is <45 degrees
tear fault
result of shearing forces applied to the rocks
smaller scale
transform faults
associated with plate margins and often described as conservative plate margins
common at right angles to mid ocean ridges
result from different rates of movement within a plate
slickensides
scratches found on fault planes
left by the grinding of pieces of rock along the fault plane as the fault moves
two surfaces are left with linear grooves and ridges parallel to the direction of movement (striations)
show direction of last movement
fault gouge
in some faults the grinding of the rock along the fault plane can be extreme
producing fault gouge consisting of very fine particles of rock
these are then fused together due to friction, heat
unconformity
- deposition of rock beds, with oldest at the bottom and the youngest at the top
- beds of rock are tilted by earth’s movements
- erosion of the rock beds. may be a time gap of thousands or millions of years
- deposition of younger overlying beds uncomformably on the underlying series (different dip)
fragments of the underlying beds may form conglomerate
this would be the first bed of the younger rocks (basal conglomerate) - uplift and erosion expose the unconformity
often all the rocks will be folded again
isoclinal fold
limbs are parallel that are nearly vertical and very tight
stereo nets
the closer together the clusters, the more open the fold
the more defined the clusters, the more pointed the fold
the the clusters are not parallel to one another, the fold is plunging
